No full-text available
To read the full-text of this research,
you can request a copy directly from the authors.
Accretion Disks in Active Galactic Nuclei: Gas Supply Driven by Star Formation
Abstract
Self-gravitating accretion disks collapse to star-forming (SF) regions extending to the inner edge of the dusty torus in active galactic nuclei (AGNs). A full set of equations including feedback of star formation is given to describe the dynamics of the regions. We explore the role of supernova explosions (SNexp), which act to excite turbulent viscosity, in the transportation of angular momentum in the regions within 1 pc scale. We find that accretion disks with typical rates in AGNs can be driven by SNexp in the regions and metals are produced spontaneously. The present model predicts a metallicity-luminosity relationship consistent with that observed in AGNs. As relics of SF regions, a ring (or belt) consisting of old stars remains for every episode of supermassive black hole activity. We suggest that multiple stellar rings with random directions interact and form a nuclear star cluster after episodes driven by star formation.
... Shortly, star formation in the outer part of the disk is believed to be an inevitable consequence (Kolykhalov & Syunyaev 1980;Loose et al. 1982;Shlosman & Begelman 1987;Collin & Zahn 1999;Goodman 2003;Milosavljević & Loeb 2004;Collin & Zahn 2008). Wang et al. (2010) showed that turbulence driven by SN explosion plays a key role in the outward transportation of angular momentum; meanwhile, metals are produced consequently. In this context, a quantitative explanation for the relation between metallicity and luminosity/Eddington ratio becomes feasible. ...
... Many thanks are given to RM campaigns (Kaspi et al. 2000;Peterson et al. 2004;Du et al. 2014;Shen et al. 2015;U et al 2022;Shen et al. 2023), showing that the typical radii of AGN BLRs are of the order of 10 3 ∼ 10 5 R g (where R g is the gravitational radius of the central SMBH; see Figure 6 in Du et al. 2016), which is in good agreement with the selfgravitating regions (Goodman 2003;Sirko & Goodman 2003). The idea of star formation is extended to connect with the BLR metallicity by Wang et al. (2010Wang et al. ( , 2011Wang et al. ( , 2012 for properties of metallicity with SMBH masses, accretion rates, and luminosity (see also Qi et al. 2022;Fan & Wu 2023). This is also supported by evidence from the correlation between accretion rates and star formation rates (Chen et al. 2009;Zhuang & Ho 2020). ...
... Once one EMC candidate is confirmed by joint observations of LIGO and EMC observations, this will provide a new clue to understand undergoing physics of AGN disks connecting metal productions, emissions of accretion disks, and gravitational waves. This is the third paper of the series based on Wang et al. (2010). The objective of this series is to explore whether self-gravity, star formation, and the evolution of stellar populations in a gaseous disk could jointly govern the formation of AGN structures, which are known as torus (10 5 R g ), BLRs (∼10 3−5 R g ), and the central part of accretion disks emitting optical-ultraviolet and X-rays (10 2 R g ).The first two papers focus on the effects of star formation (inside the BLRs) on metallicity gradients of the BLRs (Wang et al. 2011, Paper I) and episodic appearances of the BLRs (Wang et al. 2012, Paper II), respectively. ...
Strong iron lines are a common feature of the optical spectra of active galactic nuclei (AGNs) and quasars from z ∼ 6−7 to the local universe, and [Fe/Mg] ratios do not show cosmic evolution. During active episodes, accretion disks surrounding supermassive black holes (SMBHs) inevitably form stars in the self-gravitating part, and these stars accrete with high accretion rates. In this paper, we investigate the population evolution of accretion-modified stars (AMSs) to produce iron and magnesium in AGNs. The AMSs, as a new type of star, are allowed to have any metallicity but without significant loss from stellar winds, since the winds are choked by the dense medium of the disks and return to the core stars. Mass functions of the AMS population show a pile-up or cutoff pile-up shape in top-heavy or top-dominant forms if the stellar winds are strong, consistent with the narrow range of supernovae (SNe) explosions driven by the known pair-instability. This provides an efficient way to produce metals. Meanwhile, SN explosions support an inflated disk as a dusty torus. Furthermore, the evolving top-heavy initial mass functions lead to bright luminosity in infrared bands in dusty regions. This contributes a new component in infrared bands, which is independent of the emissions from the central part of accretion disks, appearing as a long-term trending of the NIR continuum compared to optical variations. Moreover, the model can be further tested through reverberation mapping of emission lines, including LIGO/LISA detections of gravitational waves and signatures from spatially resolved observations of GRAVITY+/VLTI.
... It is well-known that SSDs are gravitationally unstable and prone to be self-gravitated at distances larger than several thousand Schwarzschild radii (Toomre 1964;Paczynski 1978;Shlosman & Begelman 1987;Thompson et al. 2005). Fragmentations due to the gravitational instability have been proposed as a mechanism for star formation in AGNs (Cheng & Wang 1999;Collin & Zahn 1999;Nayakshin 2006;Collin & Zahn 2008;Kawakatu & Wada 2008;Wada et al. 2009;Wang et al. 2010Wang et al. , 2011Wang et al. , 2012, e.g., to explain the stars in the Galactic center (Levin & Beloborodov 2003;Nayakshin & Sunyaev 2005;Levin 2007;Mapelli et al. 2012;Davies & Lin 2020). The evolution of massive stars may lead to heavy-element enrichment near Sgr A * , which has indeed been detected in the spectra of some stars within 0.5 pc of the Galactic center (Do et al. 2018). ...
... The evolution of massive stars may lead to heavy-element enrichment near Sgr A * , which has indeed been detected in the spectra of some stars within 0.5 pc of the Galactic center (Do et al. 2018). Based on supernovae (SNe) in the accretion disk, Wang et al. (2010) found that it can roughly reproduce the observed positive metallicity-luminosity correlation. The stellar evolution in the disk environments may be much different from that in host galaxies (e.g., the so-called "accretionmodified stars (AMS)" in Wang et al. 2021b) and potentially increase the supernova rate and the number of compact objects in the nuclei of galaxies Dittmann et al. 2021;Jermyn et al. 2021Jermyn et al. , 2022. ...
... The gravitational instability in the outer part of the AGN disk can trigger star formation, and it is also a candidate region for producing metals (e.g., Wang et al. 2010Wang et al. , 2011Wang et al. , 2012Śniegowska et al. 2021). We explore metal enrichment by considering the contribution of stars and SNe in the outer part of the SSD. ...
Recent observations show that the metallicity Z BLR of the broad-line region (BLR) in active galactic nuclei (AGNs) is solar to supersolar, which is positively correlated with the mass of supermassive black holes ( M BH ) and does not evolve with the redshift up to z ∼ 7. We revisit the M BH − Z BLR correlation with more AGNs with M BH ∼ 10 6–8 M ⊙ and find that the positive correlation becomes flat in the low-mass range. It is known that the outer part of accretion disks is gravitationally unstable and can fragment into stars. Considering the star formation and supernovae in the outer AGN disk, we calculate the metal enrichment and find that the positive M BH − Z BLR correlation can be roughly reproduced if the stellar mass distribution is “top heavy.” We find that the observed BLR size is more or less similar to the self-gravity radius of the AGN disk, which suggests that the BLR may be closely correlated with the underlying accretion process.
... It is well-known that SSD is gravitationally unstable and prone to be self-gravitated at distances larger than several thousand Schwarzschild radii (Toomre 1964;Paczynski 1978;Shlosman & Begelman 1987;Goodman 2003;Thompson et al. 2005). Fragmentations due to the gravitational instability have been proposed as a mechanism for star formation in AGNs (Cheng & Wang 1999;Collin & Zahn 1999;Nayakshin 2006;Collin & Zahn 2008;Wang et al. 2010Wang et al. , 2011Wang et al. , 2012Kawakatu & Wada 2008;Wada et al. 2009), e.g., to explain the stars in the Galactic center (Levin & Beloborodov 2003;Nayakshin & Sunyaev 2005;Levin 2007;Davies & Lin 2020;Mapelli et al. 2012). The evolution of massive stars may lead to heavy-element enrichment near Sgr A*, which has indeed been detected in the spectra of some stars within 0.5 pc of the Galactic center (Do et al. 2018). ...
... The evolution of massive stars may lead to heavy-element enrichment near Sgr A*, which has indeed been detected in the spectra of some stars within 0.5 pc of the Galactic center (Do et al. 2018). Based on SNe in the accretion disk, Wang et al. (2010) found that it can roughly reproduce the observed positive metallicity-luminosity correlation. The stellar evolution in the disk environments may be much different from that in host galaxies (e.g., so-called "accretion-modified stars(AMS)" in Wang et al. 2021b) and potentially increase supernova rate and the number of compact objects in the nuclei of galaxies Dittmann et al. 2021;Jermyn et al. 2021Jermyn et al. , 2022. ...
... The gravitational instability in the outer part of the AGN disk can trigger star formation, where is also a candidate region for producing the metals (e.g., Wang et al. 2010Wang et al. , 2011Wang et al. , 2012Śniegowska et al. 2021). We explore metal enrichment by considering the contribution of stars and SNe in the outer part of the SSD. ...
Recent observations show that the metallicity of the broad line region ($Z_{\rm BLR}$) in active galactic nuclei (AGNs) is solar-to-supersolar, which is positively correlated with the mass of supermassive black holes ($M_{\rm BH}$) and does not evolve with redshift up to $z \sim 7$. We revisit the $M_{\rm BH}-Z_{\rm BLR}$ correlation with more AGNs with $M_{\rm BH}\sim 10^{6-8} M_{\odot}$ and find that the positive correlation become flat in low-mass range. It is known that outer part of accretion disks is gravitationally unstable and can fragment into stars. Considering the star formation and supernovae (SNe) in the outer AGN disk, we calculate the metal enrichment and find that positive $M_{\rm BH}-Z_{\rm BLR}$ correlation can be roughly reproduced if the stellar mass distribution is ``top-heavy". We find that the observed BLR size is more or less similar to the self-gravity radius of the AGN disk, which suggests that the BLR may be closely correlated with the underlying accretion process.
... Star formation is unavoidable in these regions because of self-gravity (Paczyński 1978;Kolykhalov & Sunyaev 1980;Shlosman & Begelman 1989;Collin & Zahn 1999;Goodman 2003;Goodman & Tan 2004;Collin & Zahn 2008), producing compact stellar remnants from the rapid evolution of massive stars (Artymowicz et al. 1993;Cheng & Wang 1999;Cantiello et al. 2021;Grishin et al. 2021;Moranchel-Basurto et al. 2021;Wang et al. 2021). Stellar evolution rapidly releases metals into the outer parts of the selfgravitating (SG) disk (Wang et al. 2010(Wang et al. , 2011(Wang et al. , 2012, offering an explanation for the supersolar metallicities observed in AGNs across cosmic time (Hamann & Ferland 1999;Warner et al. 2003;Nagao et al. 2006;Shin et al. 2013;Du & Wang 2014). Interestingly, quasiperiodic ejections have been found in normal galaxies by eROSITA (Arcodia et al. 2021), implying that stellarmass black holes (BHs) do reside around SMBHs in galactic centers. ...
... Compact objects will be formed through rapid evolution of massive stars in SMBH disks, which originate either from captures of stars from nuclear star clusters (Artymowicz et al. 1993;Cheng & Wang 1999;Cantiello et al. 2021) or star formation in the SG disks (Collin & Zahn 1999, 2008Goodman 2003;Wang et al. 2010Wang et al. , 2011Wang et al. , 2012. In this paper, we focus on AMS BHs, whose properties depend on the mass density of the SMBH disks. ...
The recent advanced LIGO/Virgo detections of gravitational waves (GWs) from stellar binary black hole (BBH) mergers, in particular GW190521, which is potentially associated with a quasar, have stimulated renewed interest in active galactic nuclei as factories of merging BBHs. Compact objects evolving from massive stars are unavoidably enshrouded by a massive envelope to form accretion-modified stars (AMSs) in the dense gaseous environment of a supermassive black hole (SMBH) accretion disk. We show that most AMSs form binaries due to gravitational interaction with each other during radial migration in the SMBH disk, forming BBHs inside the AMS. When a BBH is born, its orbit is initially governed by the tidal torque of the SMBH. Bondi accretion onto a BBH at a hyper-Eddington rate naturally develops and then controls the evolution of its orbits. We find that Bondi accretion leads to efficient removal of the orbital angular momentum of the binary, whose final merger produces a GW burst. Meanwhile, the Blandford-Znajek mechanism pumps the spin energy of the merged BH to produce an electromagnetic counterpart (EMC). Moreover, hyper-Eddington accretion onto the BBH develops powerful outflows and triggers a Bondi explosion, which manifests itself as an EMC of the GW burst, depending on the viscosity of the accretion flow. Thermal emission from the Bondi sphere appears as one of the EMCs. The BBHs radiate GWs with frequencies of ∼102 Hz, which are accessible to LIGO. © 2021. The American Astronomical Society. All rights reserved.
... It has been suggested that the outer regions of accretion disks are gravitationally unstable and could trigger fast star formation (e.g., Paczynski 1978;Kolykhalov & Syunyaev 1980;Chen et al. 2023). The broad emission lines of AGNs indicate that the vicinities of SMBHs are metal-rich (e.g., several times the solar abundances) and seem to be independent of redshifts (Hamann & Ferland 1992, 1999Nagao et al. 2006;Xu et al. 2018;Wang et al. 2022), which is widely explained by rapid and intense star formation and evolution in AGN disk (Artymowicz et al. 1993;Wang et al. 2010Wang et al. , 2011Wang et al. , 2012Fan & Wu 2023;Wang et al. 2023). Paumard et al. (2006) suggested that in situ star formation in the accretion disk is in good agreement with the disk structure and stellar features of the stars in the CWD (see also Levin & Beloborodov 2003;Nayakshin et al. 2007;Bonnell & Rice 2008;Mapelli et al. 2012). ...
The presence of young stars, aged around several million years and situated within the range of ∼0.04–1 pc from our Galactic center raises a question about their origins and dynamical evolutions. Their kinematics provide an opportunity to explore their formation or possible subsequent dynamical evolution. If Sagittarius A* was active in the past as suggested by several observations, the accretion disk may have a significant impact on the dynamics of stars in the Galactic center. The drag force exerted on stars during star–disk interaction could lead some of them to sink into the accretion disk, and these embedded stars will rapidly migrate inward and eventually be disrupted within ∼10 ⁵ yr. This could roughly explain the absence of stars within 2.5 × 10 ⁴ R g (∼1000 au). Additionally, Kozai–Lidov oscillations, induced by the gravitational perturbation of the disk, could contribute to the bimodal distribution of S-star inclinations and drive a majority of stars into high-eccentricity orbits.
... Star formation has been suggested by many authors for different purposes since the early attempts of Kolykhalov & Sunyaev (1980) in light of the self-gravity of outer parts of the AGN accretion disks (Paczyński 1978). Consequently, it may be an efficient way of fueling gas to galactic centers, triggering the activity of SMBHs (Shlosman & Begelman 1989;Thompson et al. 2005;Wang et al. 2010). Spectral energy distributions (SEDs) of AGNs could be revised by star formation (Goodman 2003;Goodman & Tan 2004;Thompson et al. 2005) as well as the origins of high metallicity (Collin & Zahn 1999, 2008Wang et al. 2011Wang et al. , 2012bWang et al. , 2023Grishin et al. 2021;Fan & Wu 2023) observed in AGN broad-line regions (Hamann & Ferland 1999;Warner et al. 2003;Nagao et al. 2006;Shin et al. 2013;Du & Wang 2014). ...
In this paper, we investigate the astrophysical processes of stellar-mass black holes (sMBHs) embedded in advection-dominated accretion flows (ADAFs) of supermassive black holes (SMBHs) in low-luminosity active galactic nuclei. The sMBH is undergoing Bondi accretion at a rate lower than the SMBH. Outflows from the sMBH-ADAF dynamically interact with their surroundings and form a cavity inside the SMBH-ADAF, thereby quenching the accretion onto the sMBH. Rejuvenation of the Bondi accretion is rapidly done by turbulence. These processes give rise to quasi-periodic episodes of sMBH activities and create flickerings from relativistic jets developed by the Blandford–Znajek mechanism if the sMBH is maximally rotating. Accumulating successive sMBH-outflows trigger a viscous instability of the SMBH-ADAF, leading to a flare following a series of flickerings. Recently, the similarity of near-infrared flare’s orbits has been found by GRAVITY/VLTI astrometric observations of Sgr A ∗ : their loci during the last 4 yr consist of a ring in agreement with the well-determined SMBH mass. We apply the present model to Sgr A*, which shows quasi-periodic flickerings. An sMBH of ∼40 M ⊙ is preferred orbiting around the central SMBH of Sgr A* from fitting radio to X-ray continuum. Such an extreme mass ratio inspiraling provides an excellent laboratory for LISA/Taiji/Tianqin detection of mHz gravitational waves with strains of ∼10 ⁻¹⁷ , as well as their polarization.
... There are several observations supporting this picture. The high metallicity of quasars is presumably related to frequent explosive phenomena of COs and stars in AGN disks (Artymowicz et al. 1993;Wang et al. 2010;Xu et al. 2018;Toyouchi et al. 2022;. The existence of young stars (Genzel et al. 2003;Levin & Beloborodov 2003) and clusters (Milosavljević & Loeb 2004) around Sgr A * , as well as the high metallicity component of NSCs (Antonini et al. 2015;Do et al. 2020;Neumayer et al. 2020;Fahrion et al. 2021), imply that stars, and hence COs, form in situ in AGN disks. ...
Stellar-mass black holes (BHs) are predicted to be embedded in the disks of active galactic nuclei (AGNs) due to gravitational drag and in situ star formation. However, clear evidence for AGN disk-embedded BHs is currently lacking. Here, as possible electromagnetic signatures of these BHs, we investigate breakout emission from shocks emerging around Blandford–Znajek jets launched from accreting BHs in AGN disks. We assume that most of the highly super-Eddington flow reaches the BH and produces a strong jet, and the jet produces feedback that shuts off accretion and thus leads to episodic flaring. These assumptions, while poorly understood at present, yield observable consequences that can probe the presence of AGN-embedded BHs as well as the accretion process itself. They predict a breakout emission characterized by luminous thermal emission in the X-ray bands and bright broadband nonthermal emission from the infrared to the gamma-ray bands. The flare duration depends on the BH’s distance r from the central supermassive BH, varying between 10 ³ –10 ⁶ s for r ∼ 0.01–1 pc. This emission can be discovered by current and future infrared, optical, and X-ray wide-field surveys and monitoring campaigns of nearby AGNs.
... There are several observations supporting this picture. The high metallicity of quasars is presumably related to frequent explosive phenomena of COs and stars in AGN disks (Artymowicz et al. 1993;Wang et al. 2010;Xu et al. 2018;Wang et al. 2021c;Toyouchi et al. 2021). The existence of young stars (Genzel et al. 2003;Levin & Beloborodov 2003) and clusters (Milosavljević & Loeb 2004) around Sgr A*, as well as the high metallicity component of NSCs (Antonini et al. 2015;Do et al. 2020;Neumayer et al. 2020;Fahrion et al. 2021) imply that stars, and hence COs, form in-situ in AGN disks. ...
Stellar-mass black holes (BHs) are predicted to be embedded in the disks of active galactic nuclei (AGN) due to gravitational drag and in-situ star formation. However, clear evidence for AGN disk-embedded BHs is currently lacking. Here, as possible electromagnetic signatures of these BHs, we investigate breakout emission from shocks emerging around Blandford-Znajek jets launched from accreting BHs in AGN disks. We assume that the majority of the highly super-Eddington flow reaches the BH, produces a strong jet, and the jet produces feedback that shuts off accretion and thus leads to episodic flaring. While these assumptions are highly uncertain at present, they predict a breakout emission characterized by luminous thermal emission in the X-ray bands, and bright, broadband non-thermal emission from the infrared to the gamma-ray bands. The flare duration depends on the BH's distance $r$ from the central supermassive BH, varying between $10^3-10^6$ s for $r \sim 0.01-1$ pc. This emission can be discovered by current and future infrared, optical, and X-ray wide-field surveys and monitoring campaigns of nearby AGNs.
... Therefore, by comparing AGN luminosity ( L AGN ) and the metallicity, it is possible to study the origin and evolution of SMBHs and their host galaxies (e.g. Wang et al. 2010 ). Previous studies have shown a distinct relation between the metallicity from the narrowline region ( Z NLR ) and the L AGN (e.g. ...
We present oxygen abundances relative to hydrogen (O/H) in the narrow line regions (NLRs) gas phases of Seyferts 1 (Sy 1s) and Seyferts 2 (Sy 2s) Active Galactic Nuclei (AGNs). We used fluxes of the optical narrow emission line intensities [3 500 < λ(Å) < 7 000] of 561 Seyfert nuclei in the local universe (z ≲ 0.31) from the second catalog and data release (DR2) of the BAT AGN Spectroscopic Survey, which focuses on the Swift-BAT hard X-ray (≳ 10 keV) detected AGNs. We derived O/H from relative intensities of the emission lines via the strong-line methods. We find that the AGN O/H abundances are related to their hosts stellar masses and that they follow a downward redshift evolution. The derived O/H together with the hard X-ray luminosity (LX) were used to study the X-ray luminosity-metallicity (LX-ZNLR) relation for the first time in Seyfert galaxies. In contrast to the broad-line focused (LX-ZBLR) studies, we find that the LX-ZNLR exhibit significant anti-correlations with the Eddington ratio (λEdd) and these correlations vary with redshifts. This result indicates that the low-luminous AGNs are more actively undergoing Interstellar Medium (ISM) enrichment through star formation in comparison with the more luminous X-ray sources. Our results suggest that the AGN is somehow driving the galaxy chemical enrichment, as a result of the inflow of pristine gas that is diluting the metal rich gas, together with a recent cessation on the circumnuclear star-formation.
... Therefore, by comparing AGN luminosity ( AGN ) and the metallicity, it is possible to study the origin and evolution of SMBHs and their host galaxies (e.g. Wang et al. 2010). Previous studies have shown a distinct relation between the metallicity from the narrowline region ( NLR ) and the AGN (e.g. ...
We present oxygen abundances relative to hydrogen (O/H) in the narrow line regions (NLRs) gas phases of Seyferts 1 (Sy 1s) and Seyferts 2 (Sy 2s) Active Galactic Nuclei (AGNs). We used fluxes of the optical narrow emission line intensities [$3\,500<\lambda($\AA$)<7\,000$] of 561 Seyfert nuclei in the local universe ($z\lesssim0.31$) from the second catalog and data release (DR2) of the BAT AGN Spectroscopic Survey, which focuses on the \textit{Swift}-BAT hard X-ray ($\gtrsim10$ keV) detected AGNs. We derived O/H from relative intensities of the emission lines via the strong-line methods. We find that the AGN O/H abundances are related to their hosts stellar masses and that they follow a downward redshift evolution. The derived O/H together with the hard X-ray luminosity ($L_{\rm X}$) were used to study the X-ray luminosity-metallicity ($L_{\rm X}$-$Z_{\rm NLR}$) relation for the first time in Seyfert galaxies. In contrast to the broad-line focused ($L_{\rm X}$-$Z_{\rm BLR}$) studies, we find that the $L_{\rm X}$-$Z_{\rm NLR}$ exhibit significant anti-correlations with the Eddington ratio ($\lambda_{\rm Edd}$) and these correlations vary with redshifts. This result indicates that the low-luminous AGNs are more actively undergoing Interstellar Medium (ISM) enrichment through star formation in comparison with the more luminous X-ray sources. Our results suggest that the AGN is somehow driving the galaxy chemical enrichment, as a result of the inflow of pristine gas that is diluting the metal rich gas, together with a recent cessation on the circumnuclear star-formation.
... We collect 24 quasars at z 5.7 from Jiang et al. (2007) and Wang et al. (2022) and utilize the theoretical relation = + Z Z log 1.0 1.33 log N v C iv ( ) to convert the lineflux ratios N v/C iv to metallicities Z (Hamann et al. 2002;Wang et al. 2010). These results are shown in Figure 2, which imply that quasar BLRs were enriched in metals at z ∼ 6, and that the enrichment of BLR metal abundances must have happened much earlier and/or much faster. ...
Recent observations of quasars show high line-flux ratios in their broad emission lines and the ratios appear to be independent of redshift up to z ≳ 6, which indicates that the broad-line regions of these early quasars are surprisingly metal-rich. Here, we revisit the chemical evolution of high-redshift quasars by adding a new ingredient, i.e., the neutrino-dominated accretion flows (NDAFs) with outflows, on top of the conventional core-collapse supernovae (CCSNe). In the presence of the chemical contribution from NDAFs with outflows, the total metal mass (i.e., the summation of the conventional CCSN and NDAFs with outflows) per CCSN depends weakly upon the mass of the progenitor star if the mass is in the range of ∼25–55 M ⊙ . We model the chemical evolution by adopting a improved open-box model with three typical initial mass functions (IMFs). We find that, with the additional chemical contribution from NDAFs with outflows, the quasar metallicity can be enriched more rapidly in the very early universe ( z ∼ 10) and reaches a higher saturation than the no-NDAF case at z ∼ 8, after which they evolve slowly with redshift. The quasar metallicity can reach ∼20 Z ⊙ ( Z ⊙ denotes the metallicity of the Sun, ∼20% of which is produced by NDAF outflows) at z ∼ 8 for the “top-heavy” IMF model in Toyouchi et al., which readily explains the quasar observations on the supersolar metal abundance and redshift-independent evolution.
... We collect 24 quasars at z 5.7 from Jiang et al. (2007) and Wang et al. (2022) and utilize the theoretical relation log Z/Z ⊙ = 1.0 + 1.33 log(N v/C iv) to convert the line-flux ratios N v/C iv to metallicities Z (Hamann et al. 2002;Wang et al. 2010). These results are shown in Figure 2, which imply that quasar BLRs were enriched in metals at z ∼ 6, and that the enrichment of BLR metal abundances must have happened much earlier and/or much faster. ...
Recent observations of quasars show high line-flux ratios in their broad emission lines and the ratios appear to be independent of redshift up to $z \gtrsim 6$, which indicate that the broad-line regions of these early quasars are surprisingly metal-rich. Here, we revisit the chemical evolution of high-redshift quasars by adding a new ingredient, i.e., the neutrino-dominated accretion flows (NDAFs) with outflows, on top of the conventional core-collapse supernovae (CCSNe). In the presence of the chemical contribution from NDAFs with outflows, the total metal mass (i.e., the summation of the conventional CCSN and NDAFs with outflows) per CCSN depends weakly upon the mass of the progenitor star if the mass is in the range of $\sim 25-55~M_{\odot}$. We model the chemical evolution by adopting a improved open-box model with three typical initial mass functions (IMFs). We find that, with the additional chemical contribution from NDAFs with outflows, the quasar metallicity can be enriched more rapidly in the very early Universe ($z \sim 10$) and reaches higher saturation than the no-NDAF case at $z \sim 8$, after which they evolve slowly with redshift. The quasar metallicity can reach $\sim 20~Z_{\odot}$ ($Z_\odot$ denotes the metallicity of the Sun; and $\sim 20\%$ of which is produced by NDAF outflows) at $z \sim 8$ for the ``top-heavy'' IMF model in \citet{Toyouchi2022}, which readily explains the quasar observations on the super-solar metal abundance and redshift-independent evolution.
... On one hand, the COs produced outside the accretion disc can be captured into it due to repeated collisions with the disc (Norman & Silk 1983;Syer et al. 1991;Artymowicz et al. 1993;MacLeod & Lin 2020;Davies & Lin 2020). On the other, massive stars can form in the outer, gravitationally unstable part of the accretion disc and end up as COs (Kolykhalov & Syunyaev 1980;Shlosman & Begelman 1989;Goodman & Tan 2004;Levin 2007; Wang et al. 2010). Once in the accretion disc, COs could form binaries efficiently by giving away excessive energy and angular momentum to the surrounding gas (McKernan et al. 2012;Secunda et al. 2019;Tagawa et al. 2020). ...
Many black holes (BHs) detected by the Laser Interferometer Gravitational-wave Observatory (LIGO) and the Virgo detectors are multiple times more massive than those in X-ray binaries. One possibility is that some BBHs merge within a few Schwarzschild radii of a supermassive black hole (SMBH), such that the gravitational waves (GWs) are highly redshifted, causing the mass inferred from GW signals to appear higher than the real mass. The difficulty of this scenario lies in the delivery of BBH to such a small distance to a SMBH. Here we revisit the theoretical models for the migration of compact objects (COs) in the accretion discs of active galactic nuclei (AGNs). We find that when the accretion rate is high so that the disc is best described by the slim disc model, the COs in the disc could migrate to a radius close to the innermost stable circular orbit (ISCO) and be trapped there for the remaining lifetime of the AGN. The exact trapping radius coincides with the transition region between the sub- and super-Keplerian rotation of the slim disc. We call this region "the last migration trap" because inside it COs can no longer be trapped for a long time. We pinpoint the parameter space which could induce such a trap and we estimate that the last migration trap contributes a few per cent of the LIGO/Virgo events. Our result implies that a couple of BBHs discovered by LIGO/Virgo could have smaller intrinsic masses.
... Compact objects (neutron stars and stellar black holes) may exist in accretion disks of active galactic nuclei (AGNs) and quasars. On the one hand, pioneering ideas of selfgravitating accretion disks in AGNs (Paczyński 1978;Kolykhalov & Sunyaev 1980;Shlosman & Begelman 1989) have received increasing attention (Collin & Zahn 1999Goodman 2003;Goodman & Tan 2004;Wang et al. 2010Wang et al. , 2011Wang et al. , 2012, as they offer a possible explanation for the super-solar metallicities inferred in AGNs and quasars (Hamann & Ferland 1999;Warner et al. 2003;Nagao et al. 2006;Shin et al. 2013;Du & Wang 2014). The high metallicities can be naturally linked to star formation in selfgravitating disks, which inevitably produces compact objects from supernova explosions. ...
Compact objects are expected to exist in the accretion disks of supermassive black holes (SMBHs) in active galactic nuclei (AGNs), and in the presence of such a dense environment (∼1014 cm-3), they will form a new kind of stellar population denoted as accretion-modified stars (AMSs). This hypothesis is supported by recent LIGO/Virgo detection of the mergers of very high-mass stellar binary black holes (BHs). We show that the AMSs will be trapped by the SMBH disk within a typical AGN lifetime. In the context of SMBH disks, the rates of Bondi accretion onto BHs are ∼109 L Edd/c 2, where L Edd is the Eddington luminosity and c is the speed of light. Outflows developed from the hyper-Eddington accretion strongly impact the Bondi sphere and induce episodic accretion. We show that the hyper-Eddington accretion will be halted after an accretion interval of t a ∼ 105 m 1 s, where m 1 = m •/10M o˙ is the BH mass. The kinetic energy of the outflows accumulated during t a is equivalent to 10 supernovae driving an explosion of the Bondi sphere and developing blast waves. We demonstrate that a synchrotron flare from relativistic electrons accelerated by the blast waves peaks in the soft X-ray band (∼0.1 keV), significantly contributing to the radio, optical, UV, and soft X-ray emission of typical radio-quiet quasars. External inverse Compton scattering of the electrons peaks around 40 GeV and is detectable through Fermi-LAT. The flare, decaying with t -6/5 with a few months, will appear as a slowly varying transient. The flares, occurring at a rate of a few per year in radio-quiet quasars, provide a new mechanism for explaining AGN variability. © 2021. The American Astronomical Society. All rights reserved..
... The strong correlation between accretion rates ˙ M and Fe has been explored by Panda et al. (2018Panda et al. ( , 2019aPanda et al. ( , 2019b), but still remains open. Accretion flows to the central BHs supplied by either star formation from torus ( Wang et al. 2010), or ...
The radius–luminosity ( ) relationship of active galactic nuclei (AGNs) established by the reverberation mapping (RM) observations has been widely used as a single-epoch black hole mass estimator in the research of large AGN samples. However, the recent RM campaigns discovered that the AGNs with high-accretion rates show shorter time lags by factors of a few comparing with the predictions from the relationship. The explanation of the shortened time lags has not been finalized yet. We collect eight different single-epoch spectral properties to investigate how the shortening of the time lags correlates with those properties and to determine the origin of the shortened lags. We find that the flux ratio between Fe ii and H β emission lines shows the most prominent correlation, thus confirming that accretion rate is the main driver for the shortened lags. In addition, we establish a new scaling relation including the relative strength of Fe ii emission. This new scaling relation can provide less biased estimates of the black hole mass and accretion rate from the single-epoch spectra of AGNs.
... These models predict that sub-parsec region of a quasar disc is gravitationally stable, but beyond a characteristic radius where its location depends on the input parameters of the model, the disc fragments into clumps which their subsequent evolution is under intense debate (e.g., Goodman & Tan 2004;Levin 2007;McKernan et al. 2012;Inayoshi & Haiman 2014). On a larger scale, however, these models are not appropriate and in more advanced models not only ongoing intense star formation, stars, and their gravitational interactions with the gas component should be considered, but also angular momentum transport occurs by different mechanisms such as global spiral waves, gravitational star-gas interactions and even supernova explosions (e.g., Thompson et al. 2005;Wang et al. 2010;Krumholz & Burkert 2010;Hopkins & Quataert 2011;Muñoz & Furlanetto 2012;Inayoshi & Haiman 2014. ...
Self-gravitating accretion discs in a gravitoturbulent state, including radiation and gas pressures, are studied using a set of new analytical solutions. While the Toomre parameter of the disc remains close to its critical value for the onset of gravitational instability, the dimensionless stress parameter is uniquely determined from the thermal energy reservoir of the disc and its cooling rate. Our solutions are applicable to the accretion discs with dynamically important radiation pressure like in the quasars discs. We show that physical quantities of a gravitoturbulent disc in the presence of radiation are significantly modified compared to solutions with only gas pressure. We show that the dimensionless stress parameter is an increasing function of the radial distance so that its steepness strongly depends on the accretion rate. In a disc without radiation its slope is 4.5, however, we show that in the presence of radiation, it varies between 2 and 4.5 depending on the accretion rate and the central mass. As for the surface density, we find a shallower profile with an exponent -2 in a disc with sub-Eddington accretion rate compared to a similar disc, but without radiation, where its surface density slope is -3 independent of the accretion rate. We then investigate gravitational stability of the disc when the stress parameter reaches to its critical value. In order to self-consistently determine the fragmentation boundary, however, it is shown that the critical value of the stress parameter is a power-law function of the ratio of gas pressure and the total pressure and its exponent is around 1.7. We also estimate the maximum mass of the central black hole using our analytical solutions.
The postmerger gravitational-wave (GW) signal of a binary neutron star (BNS) merger is expected to contain valuable information that could shed light on the equation of state (EOS) of NSs, the properties of the matter produced during the merger, as well as the nature of any potential intermediate merger product such as hypermassive or supramassive NSs. However, the postmerger lies in the high frequency regime (≳ 1000 Hz) where current LIGO-Virgo detectors are insensitive. While proposed detectors such as NEMO, Cosmic Explorer and Einstein Telescope could potentially detect the postmerger for BNSs within $\mathcal {O}(10~\mathrm{Mpc})$, such events are likely to be rare. In this work, we speculate on the possibility of detecting the postmerger from BNSs coalescing in the vicinity of supermassive black holes (SMBH). The redshift produced by the gravitational field of the SMBH, as well as the BNS’s proper motion around the SMBH, could effectively “stretch” the postmerger signal into the band of the detectors. We demonstrate, using a phenomenological model, that such BNS coalescences would enable constraints on the peak of the postmerger signal that would otherwise have not been possible, provided the degree of redshifting due to the SMBH can be independently acquired. Further, using numerical simulations of binary neutron stars, we show how such mergers would improve EOS model selection using the postmerger signal. We discuss the mechanisms that might deliver such events, and the limitations of this work.
Many black holes (BHs) detected by the Laser Interferometer Gravitational-wave Observatory (LIGO) and the Virgo detectors are multiple times more massive than those in X-ray binaries. One possibility is that some BBHs merge within a few Schwarzschild radii of a supermassive black hole (SMBH), so that the gravitational waves (GWs) are highly redshifted, causing the mass inferred from GW signals to appear higher than the real mass. The difficulty of this scenario lies in the delivery of BBH to such a small distance to a SMBH. Here we revisit the theoretical models for the migration of compact objects (COs) in the accretion discs of active galactic nuclei (AGNs). We find that when the accretion rate is high so that the disc is best described by the slim disc model, the COs in the disc could migrate to a radius close to the innermost stable circular orbit (ISCO) and be trapped there for the remaining lifetime of the AGN. The exact trapping radius coincides with the transition region between the sub- and super-Keplerian rotation of the slim disc. We call this region “the last migration trap” because further inward, COs can no longer be trapped for a long time. We pinpoint the parameter space which could induce such a trap and we estimate that the last migration trap contributes a few per cent of the LIGO/Virgo events. Our result implies that a couple of BBHs discovered by LIGO/Virgo so far could have smaller intrinsic masses.
Supernova (SN) explosions can potentially affect the structure and evolution of circumnuclear disks in active galactic nuclei (AGN). Some previous studies have suggested that a relatively low rate of SN explosions can provide an effective value of alpha viscosity between 0.1 and 1 in AGN accretion disks within a 1 pc scale. In order to test this possibility, we provide some analytic scalings of the evolution of an SN remnant embedded in a differentially rotating smooth disk. We calibrate our estimates using three-dimensional hydrodynamical simulations where the gas is modeled as adiabatic with index γ . Our simulations are suited to include the fact that a fraction of the momentum injected by the SN escapes from the disk into the corona. Based on these results, we calculate the contribution of SN explosions to the effective alpha viscosity, denoted by α SNe , in a model AGN accretion disk, where accretion is driven by the local viscosity α . We find that for AGN galaxies with a central black hole of and a disk with viscosity α = 0.1, the contribution of SN explosions may be as large as , provided that . On the other hand, in the momentum conservation limit, which is valid when the push by the internal pressure of the SN remnant is negligible, we find .
The properties of the dusty tori in active galactic nuclei (AGNs) have been investigated in detail, mainly focusing on the geometry and components; however, the kinematics of the torus is still not clear. The narrow iron K$\alpha$ line at 6.4 keV is thought to be produced by the X-ray reflection from the torus. Thus, the velocity-resolved reverberation mapping of it is able to constrain the kinematics of the torus. Such effort is limited by the spectral resolution of current CCD detectors and should be possible with the microcalorimeter on the next generation X-ray satellite. In this paper, we first construct the response functions of the torus under a uniform inflow, a Keplerian rotation, and a uniform outflow. Then the energy-dependent light curve of the narrow iron K$\alpha$ line is simulated according to the performance of the X-ray Integral Field Unit in Athena. Finally, the energy-dependent cross-correlation function is calculated to reveal the kinematic signal. According to our results, one hundred observations with 5 ks exposure of each are sufficient to distinguish the above three velocity fields. Although the real geometry and velocity field of the torus could be more complex than we assumed, the present result proves the feasibility of the velocity-resolved reverberation mapping of the narrow iron K$\alpha$ line. The combination of the dynamics of the torus with those of the broad line region and the host galaxy is instructive for the understanding of the feeding and feedback process of AGNs.
The infrared spectral energy distributions (SEDs) of $z\gtrsim 5$ quasars can
be reproduced by combining a low-metallicity galaxy template with a standard
AGN template. The host galaxy is represented by Haro 11, a compact, moderately
low metallicity, star-bursting galaxy that shares typical features of high-$z$
galaxies. For the vast majority of $z\gtrsim 5$ quasars, the AGN contribution
is well modeled by a standard empirical template with the contamination of star
formation in the infrared subtracted. Together, these two templates can
separate the contributions from the host galaxy and the AGN even in the case of
limited data points, given that this model has only two free parameters. Using
this method, we re-analyze 69 $z\gtrsim 5$ quasars with extensive Herschel
observations, and derive their AGN luminosities $L_{\rm AGN}$ in a range $\sim
(0.78-27.4) \times10^{13}\, L_{\odot}$, the infrared luminosities from star
formation $L_{\rm SF,IR} \sim (<1.5-25.7)\times10^{12}\, L_{\odot}$, and the
corresponding star formation rates ${\rm SFR}\sim (<290-2650)\, M_\odot/{\rm
yr}$. The average infrared luminosity from star formation and the average total
AGN luminosity of the $z\gtrsim5$ quasar sample follows the correlation defined
by quasars at $z < 2.6$. We assume these quasar host galaxies maintain a
constant average SFR ($\sim620\, M_\odot/{\rm yr}$) during their mass assembly
and estimate the stellar mass that could form till $z\sim5-6$ to be $\langle
M_* \rangle \sim(3-5)\times10^{11} M_\odot$. Combining with the black hole (BH)
mass measurements, this stellar mass is adequate to establish a BH-galaxy mass
ratio $M_{\rm BH}/M_{*}$ at 0.1-1%, consistent with the local relation.
This review describes recent developments related to the unified model of
active galactic nuclei (AGN). It focuses on new ideas about the origin and
properties of the central obscurer (torus), and the connection with its
surrounding. The review does not address radio unification. AGN tori must be
clumpy but the uncertainties about their properties are still large. Todays
most promising models involve disk winds of various types and hydrodynamical
simulations that link the large scale galactic disk to the inner accretion
flow. IR studies greatly improved the understanding of the spectral energy
distribution of AGNs but they are hindered by various selection effects. X-ray
samples are more complete. A basic relationship which is still unexplained is
the dependence of the torus covering factor on luminosity. There is also much
confusion regarding "real type-II AGNs" that do not fit into a simple
unification scheme. The most impressive recent results are due to IR
interferometry, which is not in accord with most torus models, and the accurate
mapping of central ionization cones. AGN unification may not apply to merging
systems and is possibly restricted to secularly evolving galaxies.
Warped accretion disks have attracted intensive attention because of their
critical role on shaping the spin of supermassive massive black holes (SMBHs)
through the Bardeen-Petterson effect, a general relativistic effect that leads
to final alignments or anti-alignments between black holes and warped accretion
disks. We study such alignment processes by explicitly taking into account the
finite sizes of accretion disks and the episodic lifetimes of AGNs that
delineate the duration of gas fueling onto accretion disks. We employ an
approximate global model to simulate the evolution of accretion disks, allowing
to determine the gravitomagnetic torque that drives the alignments in a quite
simple way. We then track down the evolutionary paths for mass and spin of
black holes both in a single activity episode and over a series of episodes.
Given with randomly and isotropically oriented gas fueling over episodes, we
calculate the spin evolution with different episodic lifetimes and find that it
is quite sensitive to the lifetimes. We therefore propose that spin
distribution of SMBHs can place constraints on the episodic lifetimes of AGNs
and vice versa. Applications of our results on the observed spin distributions
of SMBHs and the observed episodic lifetimes of AGNs are discussed, although
both the measurements at present are yet ambiguous to draw a firm conclusion.
Our prescription can be easily incorporated into semi-analytic models for black
hole growth and spin evolution.
We report the identification of an unusual absorption line system in the
quasar SDSS J080248.18$+$551328.9 and present a detailed study of the system,
incorporating follow-up optical and NIR spectroscopy. A few tens of absorption
lines are detected, including He I*, Fe II* and Ni II* that arise from
metastable or excited levels, as well as resonant lines in Mg I, Mg II, Fe II,
Mn II, and Ca II. All of the isolated absorption lines show the same profile of
width $\Delta v\sim 1,500$km s$^{-1}$ centered at a common redshift as that of
the quasar emission lines, such as [O II], [S II], and hydrogen Paschen and
Balmer series. With narrow Balmer lines, strong optical Fe II multiplets, and
weak [O III] doublets, its emission line spectrum is typical for that of a
narrow-line Seyfert 1 galaxy (NLS1). We have derived reliable measurements of
the gas-phase column densities of the absorbing ions/levels. Photoionization
modeling indicates that the absorber has a density of $n_{\rm H} \sim
(1.0-2.5)\times 10^5~ {\rm cm}^{-3}$ and a column density of $N_{\rm H} \sim
(1.0-3.2)\times 10^{21} \sim {\rm cm}^{-2}$, and is located at $R\sim100-250$
pc from the central super-massive black hole. The location of the absorber, the
symmetric profile of the absorption lines, and the coincidence of the
absorption and emission line centroid jointly suggest that the absorption gas
is originated from the host galaxy and is plausibly accelerated by stellar
processes, such as stellar winds \zhy{and/or} supernova explosions. The
implications for the detection of such a peculiar absorption line system in an
NLS1 are discussed in the context of co-evolution between super-massive black
hole growth and host galaxy build-up.
This work follows Lykins et al. discussion of classic plasma cooling function at low density and solar metallicity. Here,
we focus on how the cooling function changes over a wide range of density (nH <1012 cm−3) and metallicity (Z < 30 Z⊙). We find that high densities enhance the ionization of elements such as hydrogen and helium until they reach local thermodynamic
equilibrium. By charge transfer, the metallicity changes the ionization of hydrogen when it is partially ionized. We describe
the total cooling function as a sum of four parts: those due to H&He, the heavy elements, electron–electron bremsstrahlung
and grains. For the first three parts, we provide a low-density limit cooling function, a density dependence function, and
a metallicity-dependent function. These functions are given with numerical tables and analytical fit functions. We discuss
grain cooling only in the interstellar medium case. We then obtain a total cooling function that depends on density, metallicity
and temperature. As expected, collisional de-excitation suppresses the heavy elements cooling. Finally, we provide a function
giving the electron fraction, which can be used to convert the cooling function into a cooling rate.
The metallicity of active galactic nuclei (AGNs), which can be measured by emission line ratios in their broad- and narrow-line
regions (BLRs and NLRs), provides invaluable information about the physical connection between the different components of
AGNs. From the archival data bases of the International Ultraviolet Explorer, the Hubble Space Telescope and the Sloan Digital Sky Survey, we have assembled the largest sample available of AGNs which have adequate spectra in both
the optical and ultraviolet bands to measure the narrow-line ratio [N ii]/Hα and also, in the same objects, the broad-line N v/C iv ratio. These permit the measurement of the metallicities in the NLRs and BLRs in the same objects. We find that neither the
BLR nor the NLR metallicity correlate with black hole masses or Eddington ratios, but there is a strong correlation between
NLR and BLR metallicities. This metallicity correlation implies that outflows from BLRs carry metal-rich gas to NLRs at characteristic
radial distances of ∼1.0 kpc. This chemical connection provides evidence for a kinetic feedback of the outflows to their hosts.
Metals transported into the NLR enhance the cooling of the ISM in this region, leading to local star formation after the AGNs
turn to narrow-line low-ionization nuclear emission-line regions. This post-AGN star formation is predicted to be observable
as an excess continuum emission from the host galaxies in the near-infrared and ultraviolet, which needs to be further explored.
Recent X-ray observations of Mrk 766 suggest that broad emission-line region clouds cross our line of sight and produce variable
X-ray absorption. Here we investigate what optical/ultraviolet (UV) spectroscopic features would be produced by such ‘intervening
broad-line region (BLR) clouds' (IBC) crossing our line of sight to the accretion disc, the source of the optical/UV continuum.
Although the emission spectrum produced by intervening clouds is identical to the standard BLR model, they may produce absorption
features on the optical or UV continuum. Single clouds will have little effect on the optical/UV spectrum because BLR clouds
are likely to be much smaller than the accretion disc. This is unlike the X-ray case, where the radiation source is considerably
smaller. However, an ensemble of intervening clouds will produce spectroscopic features in the far-ultraviolet including a
strong depression between the Lyman limit and Lyα. The amount of the depression will indicate the line-of-sight covering factor
of clouds, an unknown quantity that is important for the ionization of the intergalactic medium and the energy budget of active
galactic nuclei (AGNs). Comparison with observations suggests that the Spectral Energy Distribution (SED) of Mrk 766 may be
affected by IBC that may exist in most of AGNs.
We investigate the alignment processes of spinning black holes and their
surrounding warped accretion disks in a frame of two different types of feeding
at the outer boundaries. We consider (1) fixed flows in which gas is
continually fed with a preferred angular momentum, and (2) free flows in which
there is no gas supply and the disks diffuse freely at their outer edges. As
expected, we find that for the cases of fixed flows the black hole disk systems
always end up aligning on timescales of several 1e6 yr, irrespective of the
initial inclinations. If the initial inclination angles are larger than pi/2,
the black hole accretion transits from retrograde to prograde fashion, and the
accreted mass onto the black holes during these two phases is comparable. On
the other hand, for the cases of free flows, both alignments and
anti-alignments can occur, depending on the initial inclinations and the ratios
of the angular momentum of the disks to that of the black holes. In such cases,
the disks will be consumed within timescales of 1e6 yr by black holes accreting
at the Eddington limit. We propose that there is a close connection between the
black hole spin and the lifetime for which the feeding persists, which
determines the observable episodic lifetimes of active galactic nuclei. We
conclude that careful inclusion of the disk feeding at the outer boundaries is
crucial for modeling the evolution of the black hole spin.
It has been suggested that the high metallicity generally observed in active galactic nuclei (AGNs) and quasars originates from ongoing star formation in the self-gravitating part of accretion disks around supermassive black holes (SMBHs). We designate this region as the star-forming (SF) disk, in which metals are produced from supernova explosions (SNexp) while at the same time inflows are driven by SNexp-excited turbulent viscosity to accrete onto the SMBHs. In this paper, an equation of metallicity governed by SNexp and radial advection is established to describe the metal distribution and evolution in the SF disk. We find that the metal abundance is enriched at different rates at different positions in the disk, and that a metallicity gradient is set up that evolves for steady-state AGNs. Metallicity as an integrated physical parameter can be used as a probe of the SF disk age during one episode of SMBH activity. In the SF disk, evaporation of molecular clouds heated by SNexp blast waves unavoidably forms hot gas. This heating is eventually balanced by the cooling of the hot gas, but we show that the hot gas will escape from the SF disk before being cooled, and diffuse into the broad-line regions (BLRs) forming with a typical rate of ~1 M ☉ yr–1. The diffusion of hot gas from an SF disk depends on ongoing star formation, leading to the metallicity gradients in BLR observed in AGNs. We discuss this and other observable consequences of this scenario.
Correlations are investigated of the outflow strength of quasars, as measured
by the blueshift and asymmetry index (BAI) of the CIV line (Wang et al. 2011),
with intensities and ratios of broad emission lines, based on composite quasar
spectra built from the Sloan Digital Sky Survey. We find that most of the line
ratios of other ions to CIV prominently increases with BAI. These behaviors can
be well understood in the context of increasing metallicity with BAI. The
strength of dominant coolant, CIV line, decreases and weak collisionally
excited lines increase with gas metallicity as a result of the competition
between different line coolants. Using SiIV+OIV]/CIV as an indicator of gas
metallicity, we present, for the first time, a strong correlation between the
metallicitiy and the outflow strength of quasars over a wide range of 1.7 to
6.9 times solar abundance. Our result implies that the metallicity plays an
important role in the formation of quasar outflows, likely via affecting
outflow acceleration. This effect may have a profound impact on galaxy
evolution via momentum feedback and chemical enrichment.
(abridged) We study the consequence of star formation (SF) in an self-gravity
dominated accretion disk in quasars. The warm skins of the SF disk are governed
by the radiation from the inner part of the accretion disk to form Compton
atmosphere (CAS). The CAS are undergoing four phases to form broad line
regions. Phase I is the duration of pure accumulation supplied by the SF disk.
During phase II clouds begin to form due to line cooling and sink to the SF
disk. Phase III is a period of preventing clouds from sinking to the SF disk
through dynamic interaction between clouds and the CAS. Finally, phase IV is an
inevitable collapse of the entire CAS through line cooling. This CAS evolution
drives the episodic appearance of BLRs. Geometry and dynamics of BLRs can be
self-consistently derived from the thermal instability of the CAS during phases
II and III by linear analysis. The metallicity gradient of SF disk gives rise
to different properties of clouds from outer to inner part of BLRs. We find
that clouds have column density N_H < 10^22cm^{-2} in the metal-rich regions
whereas they have N_H > 10^22 cm^{-2} in the metal-poor regions. The metal-rich
clouds compose the high ionization line (HIL) regions whereas the metal-poor
clouds are in low ionization line (LIL) regions. Metal-rich clouds in HIL
regions will be blown away by radiation pressure, forming the observed
outflows. The LIL regions are episodic due to the mass cycle of clouds with the
CAS in response to continuous injection by the SF disk, giving rise to
different types of AGNs. Based on SDSS quasar spectra, we identify a spectral
sequence in light of emission line equivalent width from Phase I to IV. A key
phase in the episodic appearance of the BLRs is bright type II AGNs with no or
only weak BLRs. We discuss observational implications and tests of the
theoretical predictions of this model.
Galactic X-ray emission is a manifestation of various high-energy phenomena
and processes. The brightest X-ray sources are typically accretion-powered
objects: active galactic nuclei and low- or high-mass X-ray binaries. Such
objects with X-ray luminosities of > 10^{37} ergs/s can now be detected
individually in nearby galaxies. The contributions from fainter discrete
sources (including cataclysmic variables, active binaries, young stellar
objects, and supernova remnants) are well correlated with the star formation
rate or stellar mass of galaxies. The study of discrete X-ray sources is
essential to our understanding of stellar evolution, dynamics, and end-products
as well as accretion physics. With the subtraction of the discrete source
contributions, one can further map out truly diffuse X-ray emission, which can
be used to trace the feedback from active galactic nuclei, as well as from
stars, both young and old, in the form of stellar winds and supernovae. The
X-ray emission efficiency, however, is only about 1% of the energy input rate
of the stellar feedback alone. The bulk of the feedback energy is most likely
gone with outflows into large-scale galactic halos. Much is yet to be
investigated to comprehend the role of such outflows in regulating the
ecosystem, hence the evolution of galaxies. Even the mechanism of the diffuse
X-ray emission remains quite uncertain. A substantial fraction of the emission
cannot arise directly from optically-thin thermal plasma, as commonly assumed,
and most likely originates in its charge exchange with neutral gas. These
uncertainties underscore our poor understanding of the feedback and its
interplay with the galaxy evolution.
Active galactic nuclei (AGNs) are characterized by a clear correlation between luminosity and metallicity (L_AGN-Z_AGN relation). The origin of this correlation is not clear. It may result from a relation between the black hole mass (M_BH) and metallicity, or from a relation between the accretion rate (L/L_Edd) and metallicity. To investigate the origin of the L_AGN-Z_AGN relation, we use optical spectra of 2383 quasars at 2.3 < z < 3.0 from the Sloan Digital Sky Survey. By using this data set, we have constructed composite spectra of 33 subsamples in intervals of both M_BH and L/L_Edd. From these composite spectra we measure emission-line flux ratios that are sensitive to the metallicity of the broad line region (BLR); specifically, NV/CIV, NV/HeII, (SiIV+OIV])/CIV, and AlIII/CIV. We find that there is a significant correlation between M_BH and Z_BLR as inferred from all four metallicity-sensitive emission-line flux ratios. This result strongly suggests that the observed L_AGN-Z_AGN relation is mostly a consequence of the M_BH-Z_AGN relation. The relation between M_BH and Z_BLR is likely a consequence of both the M_BH-M_bul relation and of the mass-metallicity relation in the host galaxy. We also find that L/L_Edd correlates with the emission line flux ratios involving NV (more specifically, NV/CIV and NV/HeII), while it does not correlate with the other two metallicity sensitive emission line flux ratios, i.e., (SiIV+OIV])/CIV and AlIII/CIV. These correlations indicate that the emission-line flux ratios involving NV depend on both metallicity and relative abundance of nitrogen. We suggest that the relation between L/L_Edd and those line ratios involving nitrogen, is caused by a delay of the black hole accretion rate relative to the onset of nuclear star formation of about 10^8 years, which is the timescale required for the nitrogen enrichment. Comment: Accepted for publication in A&A, 16 pages, 6 figures
(Abridged) High redshift galaxies are undergoing intensive evolution of dynamical structure and morphologies. We incorporate the feedback into the dynamical equations through mass dropout and angular momentum transportation driven by the SNexp-excited turbulent viscosity. We numerically solve the equations and show that there can be intensive evolution of structure of the gaseous disk. Secular evolution of the disk shows interesting characteristics that are 1) high viscosity excited by SNexp can efficiently transport the gas from 10kpc to $\sim 1$kpc forming a stellar disk whereas a stellar ring forms for the case with low viscosity; 2) starbursts trigger SMBH activity with a lag $\sim 10^8$yr depending on star formation rates, prompting the joint evolution of SMBHs and bulges; 3) the velocity dispersion is as high as $\sim 100~\kms$ in the gaseous disk. In order to compare the present models with the observed dynamical structure and images, we use the incident continuum from the simple stellar synthesis (GALAXEV) and CLOUDY to calculate emission line ratios of H$\alpha$, H$\beta$, $\OIII$ and $\NII$, and H$\alpha$ brightness of gas photoionized by young massive stars formed on the disks. The models can produce the main features of emission from star forming galaxies and the observed relation between turbulent velocity and the H$\alpha$ brightness. We successfully apply the present model to BX 389 and BX 482 observed in SINS high$-z$ sample, which are bulge and disk-dominated, respectively. High viscosity excited by SNexp is able to efficiently transport the gas into a bulge to maintain high star formation rates, or, to form a stellar ring close enough to the bulge so that it immigrates into the bulge of its host galaxy. This leads to a fast growing bulge. Implications and future work of the present models have been extensively discussed for galaxy formation. Comment: Accepted by ApJ; 22 page in emulateapj, 16 color figures
It has been suggested for quite a long time that galaxy mergers trigger activities of supermassive black holes (SMBHs) on the grounds of imaging observations of individual galaxies. To quantitatively examine this hypothesis, we calculate quasar luminosity functions (LFs) by manipulating the observed galaxy LFs (z {approx}< 2) and theoretical merger rates from semi-analytical formulations. We find that the model reproduces the observed quasar LFs provided that the mass ratio (q) of the secondary galaxy to the newly formed one changes with cosmic time. The results show that the fraction of major mergers decreases from f{sub maj} {approx} 0.2 at z {approx} 2 to f{sub maj} -> 0 at z {approx} 0. As a consequence, the newly formed SMBHs from major mergers at z {approx} 2 may acquire a maximal spin due to the orbital angular momentum of the merging holes. Subsequently, random accretion led by minor mergers rapidly drives the SMBHs to spin down. Such an evolutionary trend of the SMBH spins is consistent with the fact that radiative efficiency of accreting SMBHs strongly declines with cosmic time, reported by Wang et al. This suggests that minor mergers are important in triggering activities of SMBHs at low redshift, while major mergers may dominate at high redshift.
Gas in galactic disks is collected by gravitational instabilities into giant atomic-molecular complexes, but only the inner, molecular parts of these structures are able to collapse to form stars. Determining what controls the ratio of atomic-to-molecular hydrogen in complexes is, therefore, a significant problem in star formation and galactic evolution. In this paper, we use the model of H2 formation, dissociation, and shielding developed in the previous paper in this series to make theoretical predictions for atomic-to-molecular ratios as a function of galactic properties. We find that the molecular fraction in a galaxy is determined primarily by its column density and secondarily by its metallicity, and is to a good approximation independent of the strength of the interstellar radiation field. We show that the column of atomic hydrogen required to shield a molecular region against dissociation is ~10 M sun pc-2 at solar metallicity. We compare our model to data from recent surveys of the Milky Way and of nearby galaxies, and show that the both the primary dependence of molecular fraction on column density and the secondary dependence on metallicity that we predict are in good agreement with observed galaxy properties.
Broad Fe II emission is a prominent feature of the optical and ultraviolet spectra of quasars. We report on a systematical investigation of optical Fe II emission in a large sample of 4037 z < 0.8 quasars selected from the Sloan Digital Sky Survey. We have developed and tested a detailed line-fitting technique, taking into account the complex continuum and narrow and broad emission-line spectra. Our primary goal is to quantify the velocity broadening and velocity shift of the Fe II spectrum in order to constrain the location of the Fe II-emitting region and its relation to the broad-line region. We find that the majority of quasars show Fe II emission that is redshifted, typically by ~400 km s−1, but up to 2000 km s−1, with respect to the systemic velocity of the narrow-line region or of the conventional broad-line region as traced by the Hβ line. Moreover, the line width of Fe II is significantly narrower than that of the broad component of Hβ. We show that the magnitude of the Fe II redshift correlates inversely with the Eddington ratio, and that there is a tendency for sources with redshifted Fe II emission to show red asymmetry in the Hβ line. These characteristics strongly suggest that Fe II originates from a location different from, and most likely exterior to, the region that produces most of Hβ. The Fe II-emitting zone traces a portion of the broad-line region of intermediate velocities whose dynamics may be dominated by infall.
Imaging surveys with the Hubble Space Telescope (HST) have shown that ≈50%-80% of low- and intermediate-luminosity galaxies contain a compact stellar nucleus at their center, regardless of host galaxy morphological type. We combine HST imaging for early-type galaxies from the ACS Virgo Cluster Survey with ground-based long-slit spectra from KPNO to show that the masses of compact stellar nuclei in Virgo Cluster galaxies obey a tight correlation with the masses of the host galaxies. The same correlation is obeyed by the supermassive black holes (SBHs) found in predominantly massive galaxies. The compact stellar nuclei in the Local Group galaxies M33 and NGC 205 are also found to fall along this same scaling relation. These results indicate that a generic by-product of galaxy formation is the creation of a central massive object (CMO)—either an SBH or a compact stellar nucleus—that contains a mean fraction, ≈0.2%, of the total galactic mass. In galaxies with masses greater than gal ~ a few × 1010 ☉, SBHs appear to be the dominant mode of CMO formation.
We report on a systematic investigation of the H β and Fe II emission lines in a sample of 568 quasars within z < 0.8 selected from the Sloan Digital Sky Survey. The conventional broad H β emission line can be decomposed into two components—one with intermediate velocity width and another with very broad width. The velocity shift and equivalent width of the intermediate-width component do not correlate with those of the very broad component of H β , but its velocity shift and width do resemble Fe II. Moreover, the width of the very broad component is roughly 2.5 times that of the intermediate-width component. These characteristics strongly suggest the existence of an intermediate-line region, whose kinematics seem to be dominated by infall, located at the outer portion of the broad-line region.
We study the nuclear star clusters (NCs) in spiral galaxies of various Hubble types using spectra obtained with the STIS on board the Hubble Space Telescope (HST). We observed the nuclear clusters in 40 galaxies, selected from two previous HST WFPC2 imaging surveys. At a spatial resolution of ~02 the spectra provide a better separation of cluster light from underlying galaxy light than is possible with ground-based spectra. Approximately half of the spectra have a sufficiently high signal-to-noise ratio for detailed stellar population analysis. For the other half we only measure the continuum slope, as quantified by the B - V color. To infer the star formation history, metallicity, and dust extinction, we fit weighted superpositions of single-age stellar population templates to the high signal-to-noise ratio spectra. We use the results to determine the luminosity-weighted age, mass-to-light ratio, and masses of the clusters. Approximately half of the sample clusters contain a population younger than 1 Gyr. The luminosity-weighted ages range from 10 Myr to 10 Gyr. The stellar populations of NCs are generally best fit as a mixture of populations of different ages. This indicates that NCs did not form in a single event, but that instead they had additional star formation long after the oldest stars formed. On average, the sample clusters in late-type spirals have a younger luminosity-weighted mean age than those in early-type spirals ( = 8.37 ± 0.25 vs. 9.23 ± 0.21). The average mass-weighted ages are older by ~0.7 dex, indicating that there often is an underlying older population that does not contribute much light but does contain most of the mass. The average cluster masses are smaller in late-type spirals than in early-type spirals ( = 6.25 ± 0.21 vs. 7.63 ± 0.24) and exceed the masses typical of globular clusters. The cluster mass correlates loosely with total galaxy luminosity. It correlates more strongly with both the Hubble type of the host galaxy and the luminosity of its bulge. The latter correlation has the same slope as the well-known correlation between supermassive black hole mass and bulge luminosity. The properties of both nuclear clusters and black holes in the centers of spiral galaxies are therefore intimately connected to the properties of the host galaxy, and in particular its bulge component. Plausible formation scenarios have to account for this. We discuss various possible selection biases in our results, but conclude that none of them can explain the differences seen between clusters in early- and late-type spirals. The inability to infer spectroscopically the populations of faint clusters does introduce a bias toward younger ages, but not necessarily toward higher masses.
We present new observations of the nuclear star cluster in the central parsec of the Galaxy with the adaptive optics assisted, integral field spectrograph SINFONI on the ESO/VLT. Our work allows the spectroscopic detection of early- and late-type stars to mK ≥ 16, more than 2 mag deeper than our previous data sets. Our observations result in a total sample of 177 bona fide early-type stars. We find that most of these Wolf Rayet (WR), O-, and B-stars reside in two strongly warped disks between 08 and 12'' from Sgr A*, as well as a central compact concentration (the S-star cluster) centered on Sgr A*. The later type B-stars (mK >15) in the radial interval between 08 and 12'' seem to be in a more isotropic distribution outside the disks. The observed dearth of late-type stars in the central few arcseconds is puzzling, even when allowing for stellar collisions. The stellar mass function of the disk stars is extremely top heavy with a best-fit power law of dN/dm m –0.45± 0.3. WR/O-stars were formed in situ in a single star formation event ~6 Myr ago, this mass function probably reflects the initial mass function (IMF). The mass functions of the S-stars inside 08 and of the early-type stars at distances beyond 12'' are compatible with a standard Salpeter/Kroupa IMF (best-fit power law of dN/dm m –2.15± 0.3).
We present an atlas of the central regions of 75 Seyfert galaxies imaged in the near-UV with the Advanced Camera for Surveys of the Hubble Space Telescope at an average resolution of ~10 pc. These data complement archival high-resolution data from the Hubble Space Telescope at optical and near-IR wavelengths, creating an extremely valuable data set for astronomers with a broad range of scientific interests. Our goal is to investigate the nature of the near-UV light in these objects, its relation to the circumnuclear starburst phenomenon, and the connection of this to the evolution and growth of the galaxy bulge and central black hole. In this paper we describe the near-UV morphology of the objects and characterize the near-UV emission. We estimate the size and the luminosity of the emitting regions and extract the luminosity profile. We also determine the presence of unresolved compact nuclei. In addition, the circumnuclear stellar cluster population is identified, and the contribution of the stellar clusters to the total light, at this wavelength, is estimated. The size of the sample allows us to draw robust statistical conclusions. We find that Seyfert 1 galaxies (Sy1's) are completely dominated by their bright and compact nuclei, which remains pointlike at this resolution, while we find almost no unresolved nuclei in Seyfert 2 galaxies (Sy2's). The Seyfert types 1 and 2 are quite segregated in an asymmetry versus compactness plot. Stellar clusters are found somewhat more frequently in Sy2's (in ~70% of the galaxies) than in Sy1's (~57%), and contribute more to the total light in Sy2's, but these two differences seem to be mostly due to the large contribution of the compact nuclei in Sy1's, as the luminosity distribution of the clusters is similar in both Seyfert types.
We report the definite spectroscopic identification of 40 OB supergiants, giants, and main-sequence stars in the central parsec of the Galaxy. Detection of their absorption lines has become possible with the high spatial and spectral resolution and sensitivity of the adaptive optics integral field spectrometer SPIFFI/SINFONI on the ESO VLT. Several of these OB stars appear to be helium- and nitrogen-rich. Almost all of the 80 massive stars now known in the central parsec (central arcsecond excluded) reside in one of two somewhat thick ( /R 0.14) rotating disks. These stellar disks have fairly sharp inner edges (R 1'') and surface density profiles that scale as R-2. We do not detect any OB stars outside the central 0.5 pc. The majority of the stars in the clockwise system appear to be on almost circular orbits, whereas most of those in the "counterclockwise" disk appear to be on eccentric orbits. Based on its stellar surface density distribution and dynamics, we propose that IRS 13E is an extremely dense cluster (ρcore 3 × 108 M pc-3) that has formed in the counterclockwise disk. The stellar contents of both systems are remarkably similar, indicating a common age of 6 ± 2 Myr. The K-band luminosity function of the massive stars suggests a top-heavy mass function and limits the total stellar mass contained in both disks to 1.5 × 104 M. Our data strongly favor in situ star formation from dense gas accretion disks for the two stellar disks. This conclusion is very clear for the clockwise disk and highly plausible for the counterclockwise system.
The ACS Virgo Cluster Survey is a Hubble Space Telescope program to obtain high-resolution imaging in widely separated bandpasses (F475W ≈ g and F850LP ≈ z) for 100 early-type members of the Virgo Cluster, spanning a range of ≈460 in blue luminosity. We use this large, homogenous data set to examine the innermost structure of these galaxies and to characterize the properties of their compact central nuclei. We present a sharp upward revision in the frequency of nucleation in early-type galaxies brighter than MB ≈ -15 (66% fn 82%) and show that ground-based surveys underestimated the number of nuclei due to surface brightness selection effects, limited sensitivity and poor spatial resolution. We speculate that previously reported claims that nucleated dwarfs are more concentrated toward the center of Virgo than their nonnucleated counterparts may be an artifact of these selection effects. There is no clear evidence from the properties of the nuclei, or from the overall incidence of nucleation, for a change at MB ~ -17.6, the traditional dividing point between dwarf and giant galaxies. There does, however, appear to be a fundamental transition at MB ~ -20.5, in the sense that the brighter, "core-Sérsic" galaxies lack resolved (stellar) nuclei. A search for nuclei that may be offset from the photocenters of their host galaxies reveals only five candidates with displacements of more than 05, all of which are in dwarf galaxies. In each case, however, the evidence suggests that these "nuclei" are, in fact, globular clusters projected close to the galaxy photocenter. Working from a sample of 51 galaxies with prominent nuclei, we find a median half-light radius of rh = 4.2 pc, with the sizes of individual nuclei ranging from 62 pc down to ≤2 pc (i.e., unresolved in our images) in about a half-dozen cases. Excluding these unresolved objects, the nuclei sizes are found to depend on nuclear luminosity according to the relation rh 0.50±0.03. Because the large majority of nuclei are resolved, we can rule out low-level AGNs as an explanation for the central luminosity excess in almost all cases. On average, the nuclei are ≈3.5 mag brighter than a typical globular cluster. Based on their broadband colors, the nuclei appear to have old to intermediate age stellar populations. The colors of the nuclei in galaxies fainter than MB ≈ -17.6 are tightly correlated with their luminosities, and less so with the luminosities of their host galaxies, suggesting that their chemical enrichment histories were governed by local or internal factors. Comparing the nuclei to the "nuclear clusters" found in late-type spiral galaxies reveals a close match in terms of size, luminosity, and overall frequency. A formation mechanism that is rather insensitive to the detailed properties of the host galaxy properties is required to explain this ubiquity and homogeneity. The mean of the frequency function for the nucleus-to-galaxy luminosity ratio in our nucleated galaxies, log η = -2.49 ± 0.09 dex (σ = 0.59 ± 0.10), is indistinguishable from that of the SBH-to-bulge mass ratio, log(•/gal) = -2.61 ± 0.07 dex (σ = 0.45 ± 0.09), calculated in 23 early-type galaxies with detected supermassive black holes (SBHs). We argue that the compact stellar nuclei found in many of our program galaxies are the low-mass counterparts of the SBHs detected in the bright galaxies. If this interpretation is correct, then one should think in terms of "central massive objects"—either SBHs or compact stellar nuclei—that accompany the formation of almost all early-type galaxies and contain a mean fraction ≈0.3% of the total bulge mass. In this view, SBHs would be the dominant formation mode above MB ≈ -20.5.
We classify 329 late-type giants within 1 pc of Sgr A*, using the adaptive optics integral field spectrometer SINFONI on the VLT. These observations represent the deepest spectroscopic data set so far obtained for the Galactic center, reaching a 50% completeness threshold at the approximate magnitude of the helium-burning red clump (KS ~ 15.5 mag). Combining our spectroscopic results with NaCo H and KS photometry, we construct an observed Hertzsprung-Russell diagram, which we quantitatively compare to theoretical distributions of various star formation histories of the inner Galaxy, using a χ2 analysis. Our best-fit model corresponds to continuous star formation over the last 12 Gyr with a top-heavy initial mass function (IMF). The similarity of this IMF to the IMF observed for the most recent epoch of star formation is intriguing and perhaps suggests a connection between recent star formation and the stars formed throughout the history of the Galactic center.
The growth of supermassive black holes (BHs) located at the centers of their host galaxies comes mainly from the accretion of gas, but how to fuel them remains an outstanding unsolved problem in quasar evolution. This issue can be elucidated by quantifying the radiative efficiency parameter (η) as a function of redshift, which also provides constraints on the average spin of the BHs and its possible evolution with time. We derive a formalism to link η with the luminosity density, BH mass density, and duty cycle of quasars, quantities we can estimate from existing quasars, and galaxy survey data. We find that η has a strong cosmological evolution: at z 2, η 0.3, and by z 0 it has decreased by an order of magnitude, to η 0.03. We interpret this trend as evolution in BH spin, and we appeal to episodic, random accretion as the mechanism for reducing the spin. The observation that the fraction of radio-loud quasars decreases with increasing redshift is inconsistent with the popular notion that BH spin is a critical factor for generating strong radio jets. In agreement with previous studies, we show that the derived history of BH accretion closely follows the cosmic history of star formation, consistent with other evidence that BHs and their host galaxies co-evolve.
Quasar (QSO) elemental abundances provide unique probes of high-redshift star formation and galaxy evolution. There is growing evidence from both the emission and intrinsic absorption lines that QSO environments have roughly solar or higher metallicities out to redshifts >4. The range is not well known, but solar to a few times solar metallicity appears to be typical. There is also evidence for higher metallic-ities in more luminous objects and for generally enhanced N/C and Fe/α abundances compared with solar ratios. These results identify QSOs with vigorous, high-redshift star formation—consistent with the early evolution of massive galactic nuclei or dense protogalactic clumps. However, the QSOs offer new constraints. For example, (a) most of the enrichment and star formation must occur before the QSOs "turn on" or become observable, on time scales of 1 Gyr at least at the highest redshifts. (b) The tentative result for enhanced Fe/α suggests that the first local star formation began at least ∼1 Gyr before the QSO epoch. (c) The star formation must ultimately be extensive to reach high metallicities; that is, a substantial fraction of the local gas must be converted into stars and stellar remnants. The exact fraction depends on the shape of the initial mass function (IMF). (d) The highest derived metallicities require IMFs that are weighted slightly more toward massive stars than in the solar neighborhood. (e) High metallicities also require deep gravitational potentials. By analogy with the well-known mass–metallicity relation among low-redshift galaxies, metal-rich QSOs should reside in galaxies (or protogalaxies) that are minimally as massive (or as tightly bound) as our own Milky Way.
The luminosities of the centres of nearby elliptical galaxies are very low compared to models of thin disc accretion on to their black holes at the Bondi rate, typically a few hundredths to a few tenths of a solar mass per year. This has motivated models of inefficiently radiated accretion that invoke weak electron–ion thermal coupling, and/or inhibited accretion rates due to convection or outflows. Here we point out that, even if such processes are operating, a significant fraction of the accreting gas is prevented from reaching the central black hole because it condenses into stars in a gravitationally unstable disc. Star formation occurs inside the Bondi radius (typically ∼100 pc in giant ellipticals), but still relatively far from the black hole in terms of Schwarzschild radii. Star formation depletes and heats the gas disc, eventually leading to a marginally stable, but much reduced, accretion flow to the black hole. We predict the presence of cold (∼100 K), dusty gas discs, containing clustered Hα emission and occasional Type II supernovae, both resulting from the presence of massive stars. Star formation accounts for several features of the M87 system: a thin disc, traced by Hα emission, is observed on scales of about 100 pc, with features reminiscent of spiral arms and dust lanes; the star formation rate inferred from the intensity of Hα emission is consistent with the Bondi accretion rate of the system. Star formation may therefore help to suppress accretion on to the central engines of massive ellipticals. We also discuss some implications for the fuelling of the Galactic Centre and quasars.
The presence of young massive stars orbiting on eccentric rings within a few tenths of a parsec of the supermassive black
hole in the galactic center is challenging for theories of star formation. The high tidal shear from the black hole should
tear apart the molecular clouds that form stars elsewhere in the Galaxy, and transport of stars to the galactic center also
appears unlikely during their lifetimes. We conducted numerical simulations of the infall of a giant molecular cloud that
interacts with the black hole. The transfer of energy during closest approach allows part of the cloud to become bound to
the black hole, forming an eccentric disk that quickly fragments to form stars. Compressional heating due to the black hole
raises the temperature of the gas up to several hundred to several thousand kelvin, ensuring that the fragmentation produces
relatively high stellar masses. These stars retain the eccentricity of the disk and, for a sufficiently massive initial cloud,
produce an extremely top-heavy distribution of stellar masses. This potentially repetitive process may explain the presence
of multiple eccentric rings of young stars in the presence of a supermassive black hole.
The structure of obscuring matter in the environment of active galactic nuclei with associated nuclear starbursts is investigated using 3-D hydrodynamical simulations. Simple analytical estimates suggest that the obscuring matter with energy feedback from supernovae has a torus-like structure with a radius of several tens of parsecs and a scale height of about 10 pc. These estimates are confirmed by the fully non-linear numerical simulations, in which the multi-phase inhomogeneous interstellar matter and its interaction with the supernovae are consistently followed. The globally stable, torus-like structure is highly inhomogeneous and turbulent. To achieve the high column densities (> 10^{24} cm^{-2}) as suggested by observations of some Seyfert 2 galaxies with nuclear starbursts, the viewing angle should be larger than about 70 degree from the pole-on for a 10^8 solar mass massive black hole. Due to the inhomogeneous internal structure of the torus, the observed column density is sensitive to the line-of-sight, and it fluctuates by a factor of order 100. The covering fraction for N > 10^{23} cm^{-2} is about 0.4. The average accretion rate toward R < 1 pc is 0.4 solar mass/yr, which is boosted to twice that in the model without the energy feedback. Comment: ApJL in press (4 pages, 3 figures) A gziped ps file with high resolution figures is available at http://th.nao.ac.jp/~wada/AGN/
The broad emission lines (BELs) of quasars and active galactic nuclei (AGNs) are important diagnostics of the relative abundances and overall metallicity in the gas. Here we present new theoretical predictions for several UV BELs. We focus specifically on the relative nitrogen abundance as a metallicity indicator, based on the expected secondary enrichment of nitrogen at metallicities Z > 0.2 Z_o. Among the lines we consider, N III] 1750/O III] 1664, N V 1240/(C IV 1549 + O VI 1034) and N V/He II 1640 are the most robust diagnostics. We argue, in particular, that the average N V BEL is not dominated by scattered Ly-alpha photons from a broad absorption line wind. We then compare our calculated line ratios with observations from the literature. The results support earlier claims that the gas-phase metallicities near quasars are typically near or several times above the solar value. We conclude that quasar activity is preceded by, or coeval with, an episode of rapid and extensive star formation in the surrounding galactic (or proto-galactic) nuclei. Chemical evolution models of these environments suggest that, to reach Z > Z_o in well-mixed interstellar gas, the star formation must have begun > 10^8 yr before the observed quasar activity. Comment: 15 pages, 5 figures in 6 files, in press with ApJ. This version fixes some minor glitches and includes previously omitted references
New surface photometry of all known elliptical galaxies in the Virgo cluster is combined with published data to derive composite profiles of brightness, ellipticity, position angle, isophote shape, and color over large radius ranges. These provide enough leverage to show that Sérsic log I ∝ r 1/n functions fit the brightness profiles I(r) of nearly all ellipticals remarkably well over large dynamic ranges. Therefore, we can confidently identify departures from these profiles that are diagnostic of galaxy formation. Two kinds of departures are seen at small radii. All 10 of our ellipticals with total absolute magnitudes MVT ≤ –21.66 have cuspy cores—"missing light"—at small radii. Cores are well known and naturally scoured by binary black holes (BHs) formed in dissipationless ("dry") mergers. All 17 ellipticals with –21.54 ≤ MVT ≤ –15.53 do not have cores. We find a new distinct component in these galaxies: all coreless ellipticals in our sample have extra light at the center above the inward extrapolation of the outer Sérsic profile. In large ellipticals, the excess light is spatially resolved and resembles the central components predicted in numerical simulations of mergers of galaxies that contain gas. In the simulations, the gas dissipates, falls toward the center, undergoes a starburst, and builds a compact stellar component that, as in our observations, is distinct from the Sérsic-function main body of the elliptical. But ellipticals with extra light also contain supermassive BHs. We suggest that the starburst has swamped core scouring by binary BHs. That is, we interpret extra light components as a signature of formation in dissipative ("wet") mergers.
Besides extra light, we find three new aspects to the ("E-E") dichotomy into two types of elliptical galaxies. Core galaxies are known to be slowly rotating, to have relatively anisotropic velocity distributions, and to have boxy isophotes. We show that they have Sérsic indices n > 4 uncorrelated with MVT. They also are α-element enhanced, implying short star-formation timescales. And their stellar populations have a variety of ages but mostly are very old. Extra light ellipticals generally rotate rapidly, are more isotropic than core Es, and have disky isophotes. We show that they have n 3 ± 1 almost uncorrelated with MVT and younger and less α-enhanced stellar populations. These are new clues to galaxy formation. We suggest that extra light ellipticals got their low Sérsic indices by forming in relatively few binary mergers, whereas giant ellipticals have n > 4 because they formed in larger numbers of mergers of more galaxies at once plus later heating during hierarchical clustering.
We confirm that core Es contain X-ray-emitting gas whereas extra light Es generally do not. This leads us to suggest why the E-E dichotomy arose. If energy feedback from active galactic nuclei (AGNs) requires a "working surface" of hot gas, then this is present in core galaxies but absent in extra light galaxies. We suggest that AGN energy feedback is a strong function of galaxy mass: it is weak enough in small Es not to prevent merger starbursts but strong enough in giant Es and their progenitors to make dry mergers dry and to protect old stellar populations from late star formation.
Finally, we verify that there is a strong dichotomy between elliptical and spheroidal galaxies. Their properties are consistent with our understanding of their different formation processes: mergers for ellipticals and conversion of late-type galaxies into spheroidals by environmental effects and by energy feedback from supernovae.
In an appendix, we develop machinery to get realistic error estimates for Sérsic parameters even when they are strongly coupled. And we discuss photometric dynamic ranges necessary to get robust results from Sérsic fits.
We examine rest-frame ultraviolet spectra of 70 high redshift quasars (z>3.5) to study the chemical enrichment history of the gas closely related to the quasars, and thereby estimate the epoch of first star formation. The fluxes of several ultraviolet emission lines were investigated within the framework of the most recent photoionization models to estimate the metallicity of the gas associated with the high-z quasars. Standard photoionization parameters and the assumption of secondary nitrogen enrichment indicate an average abundance of Z/Z_sol = 4 to 5 in the line emitting gas. Assuming a time scale of t_evol = 0.5 - 0.8 Gyrs for the chemical enrichment of the gas, the first major star formation for quasars with z>=4 should have started at a redshift of z_f = 6 - 8, corresponding to an age of the universe of several 10^8 yrs (H_o = 65 km/s/Mpc, Omega_M = 0.3, Omega_Lambda = 0.7). We note that this also appears to be the era of re-ionization of the universe. Finally, there is some evidence for a positive luminosity - metallicity relation in this high redshift quasar sample. Comment: 18 pages, 8 figures. accepted for publication in Astrophys.Journal, V589, June 1
We study galaxies that host both nuclear star clusters and active galactic nuclei (AGN) implying the presence of a massive black hole. We select a sample of 176 galaxies with previously detected nuclear star clusters that range from ellipticals to late-type spirals. We search for AGN in this sample using optical spectroscopy and archival radio and X-ray data. We find galaxies of all Hubble types and with a wide range of masses (10^9-11 solar masses) hosting both AGN and nuclear star clusters. From the optical spectra, we classify 10% of the galaxies as AGN and an additional 15% as composite, indicating a mix of AGN and star-formation spectra. The fraction of nucleated galaxies with AGN increases strongly as a function of galaxy and nuclear star cluster mass. For galaxies with both a NC and a black hole, we find that the masses of these two objects are quite similar. However, non-detections of black holes in Local Group nuclear star clusters show that not all clusters host black holes of similar masses. We discuss the implications of our results for the formation of nuclear star clusters and massive black holes. Comment: Accepted for publication in ApJ. Version with high resolution figures available at http://www.cfa.harvard.edu/~aseth/nsc_agn_paper.pdf
We present UV to soft X-ray spectra of 31 radio-quiet quasars, comprising data from the IUE ULDA database and the ROSAT pointed observation phase. 90 % of the sample members show a soft X-ray excess above an underlying hard X-ray power law spectrum. Particularly for the steep X-ray spectrum (alpha_energy > 1.5), low redshift subsample (17 objects) the X-ray spectral power law index is strongly correlated with the optical to X-ray broad-band spectral index alpha_ox, indicating that the main contribution to the soft X-ray and optical emission is due to the same emission component. We model the UV/soft X-ray spectra in terms of thermal emission from a geometrically thin alpha-accretion disk. The structure and radiation field of the disk is calculated self-consistently and Compton scattering is treated by the Kompaneets equation. All relativistic effects on the disk structure and the emergent disk spectrum are included. Satisfactory spectral fits of the UV and soft X-ray continuum spectra are achieved when additional non-thermal hard X-ray and IR power law emission components are taken into account. The UV and soft X-ray spectra are well described by emission resulting from accretion rates in the range ~0.1 to ~0.3 times the Eddington accretion rate. Low mass/low redshift objects are found to accret at < 0.15 Mdot_Eddington. Correlations of the accretion disk parameters with alpha_ox are discussed.
We present a set of low and intermediate mass star yields based on a modeling of the TP--AGB phase which affects the production of nitrogen and carbon. These yields are evaluated by using them in a Galaxy Chemical Evolution model, with which we analyze the evolution of carbon abundances. By comparing the results with those obtained with other yield sets, and with a large amount of observational data, we conclude that the model using these yields combined with those from Woosley & Weaver (1995) for massive stars properly reproduce all the data. The model reproduces well the increase of C/O with increasing O/H abundances. Since these massive star yields do not include winds, it implies that these stellar winds might have a smoother dependence on metallicity than usually assumed and that a significant quantity of carbon proceeds from LIM stars. Comment: 21 pages, 11 figures. To be published in Astronomy and Astrophysics
We present new near infrared spectroscopic measurements of the H_beta region
for a sample of 29 luminous high redshift quasars. We have measured the width
of H_beta in those sources, and added archival H_beta width measurements, to
create a sample of 92 active galactic nuclei (AGNs) for which H_beta width and
rest-frame UV measurements of N V \lambda 1240 and C IV \lambda 1549
emission-lines are available. Our sample spans six orders of magnitude in
luminosity and includes 31 radio-loud AGNs. It also includes 10 narrow-line
Seyfert 1 galaxies and one broad absorption-line quasar. We find that
metallicity, indicated by the N V/C IV line ratio, is primarily correlated with
accretion rate, which is a function of luminosity and H_beta line-width. This
may imply an intimate relation between starburst, responsible for the metal
enrichment of the nuclear gas, and AGN fueling, represented by the accretion
rate. The correlation of metallicity with luminosity, or black hole (BH) mass,
is weaker in contrast with recent results which were based on measurements of
the width of C IV. We argue that using C IV as a proxy to H_beta in estimating
M_BH might be problematic and lead to spurious BH mass and accretion rate
estimates in individual sources. We discuss the potential implications of our
new result in the framework of the starburst-AGN connection and theories of BH
growth.
Accretion onto supermassive black holes (SBHs) is widely believed to be responsible for the phenomena of active galactic nuclei (AGN). It is not known whether the accretion flow is fuelled by mass loss from the dense cluster of stars surrounding the SBH, or enters the nucleus from the galactic interstellar medium. Here the authors adopt the latter view, assuming that most of the incoming fuel forms a thin accretion disk at distances of ≡10 - 103pc from the SBH. Such a disk must be vertically self-gravitating. They analyse some of its thermal and dynamical properties, and conclude that its energetics is likely to be dominated by backscattered AGN radiation. They also discuss the conditions under which Jeans fragmentation of such a disk into weakly interacting cloudlets can be avoided, and give a necessary condition for disk fragmentation to occur.
Many quasars and active galactic nuclei show a bump or excess in the
near-infrared continuum. In this paper, a model is developed to account
for the bump in terms of thermal radiation from dust heated by the
primary optical/ultraviolet continuum source. This model naturally
explains the onset of the bump at about 2 μm, since this wavelength
corresponds to the spectral peak for optically thin emission from
graphite grains at their evaporation temperature (≡1500K).
Infrared spectra are calculated for two cases, one in which the grains
are smoothly distributed and another in which the dust is clumped into
discrete clouds that are optically thick to the ultraviolet continuum.
Continuum spectra of 3C 273 and the "infrared quasar" IRAS 13349+2438
are fitted between ≡0.1 and 100 μm with a multicomponent model.
Detailed calculations of the structure and spectrum of massive
geometrically thin 'bare' accretion disks were obtained, solving the
radiative transfer equations using the Eddington approximation for an
atmosphere with a vertical temperature gradient. The results show alpha
disks to be radiation pressure dominated throughout the region where
self-gravity dominates. In contrast to previous findings, the surface
temperature is found to be close to the effective temperature, even for
regions where electron scattering effects are significant. From the
upper limit on the accretion rate it is inferred that thin disks without
coronas do not have sufficient soft X-ray flux to explain the
observations of bright quasars.
If an isolated black hole is massive enough and if it accretes matter at a fast enough rate, the outer parts of the accretion disk will become self-gravitating and may become unstable, disintegrating into fragments which could form separate stars. The fragmentation would produce around the black hole a ring of gas and stars which would survive even after accretion onto the hole has ceased. The stars in the ring would travel in near-Keplerian orbits, with little velocity dispersion. This torus could have a mass several percent of that of the black hole inside.
Recent efforts to model the processes occurring in Type I and II
supernovae, and the diagnostics used to model the events, are described.
Both types of events release about the same amount of energy and produce
heavy elements. Features which distinguish the two types of supernovae
are summarized. Prominent stages in Type II events are reviewed,
including the presupernova evolution of massive stars, core collapse and
bounce. Processes occurring in a delayed explosion, e.g., failure of the
shock and possible formation of a 'neutrinosphere', are discussed, along
with nucleosynthesis during Type II explosions and features of peculiar
Type II events.
It has long remained an open question as to the geometry of the broad line region (BLR) in active galactic nuclei (AGNs). The reverberation mapping technique which measures the response of the broad emission lines to the ionizing continuum, when combined with multiwavelength continuum fitted by sophisticated accretion disks, provides a way of probing the BLR geometry. We analyze a sample of 35 AGNs, which have been monitored by the reverberation mapping campaign. In view of energy budget, the reverberation-based BH masses are found to be in agreement with those obtained by accretion disk models in two thirds of the present sample while the reverberation mapping methods underestimate the BH masses in about one third of objects, as also suggested by Collin et al. in a recent work. We point out that there are obviously two kinds of BLR geometry, which are strongly dependent on the Eddington ratio, and separated by the value LBol/LEdd~0.1. These results prefer a scenario of the disk and wind configuration of the BLR and identify the Eddington ratio as the physical driver regulating the wind in the BLR.
Motivated by Genzel et al.'s observations of high-redshift star-forming galaxies, containing clumpy and turbulent rings or disks, we build a set of equations describing the dynamical evolution of gaseous disks with inclusion of star formation and its feedback. Transport of angular momentum is due to "turbulent" viscosity induced by supernova explosions in the star formation region. Analytical solutions of the equations are found for the initial cases of a gaseous ring and the integrated form for a gaseous disk, respectively. For a ring with enough low viscosity, it evolves in a slow process of gaseous diffusion and star formation near the initial radius. For a high viscosity, the ring rapidly diffuses in the early phase. The diffusion drives the ring into a region with a low viscosity and starts the second phase undergoing pile-up of gas at a radius following the decreased viscosity torque. The third is a sharply decreasing phase because of star formation consumption of gas and efficient transportation of gas inward forming a stellar disk. We apply the model to two z ~ 2 galaxies BX 482 and BzK 6004, and find that they are undergoing a decline in their star formation activity.
Episodic activity of quasars is driving growth of supermassive black holes (SMBHs) via accretion of baryon gas. In this Letter, we develop a simple method to analyze the duty cycle of quasars up to the redshift z ~ 6 universe from luminosity functions (LFs). We find that the duty cycle below redshift z ~ 2 follows the cosmic history of star formation rate (SFR) density. Beyond z ~ 2, the evolutionary trends of the duty cycle are opposite to that of the cosmic SFR density history, implying the role of feedback from black hole activity. With the duty cycle, we get the net lifetime of quasars (z ≤ 5) about ~109 yr. Based on the local SMBHs, the mean mass of SMBHs is obtained at any redshifts and their seeds are of 105 M☉ at the reionization epoch (zre) of the universe through the conservation of black hole number density in a comoving frame. We find that primordial black holes (~103 M☉) are able to grow up to the seeds via a moderate super-Eddington accretion of ~30 times the critical rate from z = 24 to zre. Highly super-Eddington accretion onto the primordial black holes is not necessary.
Tracing the star formation history in circumnuclear regions (CNRs) is a key step toward understanding the starburst-active galactic nucleus (AGN) connection. However, bright nuclei outshining the entire host galaxy prevent the analysis of the stellar populations of CNRs around type-I AGNs. Obscuration of the nuclei by the central torus provides a unique opportunity to study the stellar populations of AGN host galaxies. We assemble a sample of 10,848 type-II AGNs with a redshift range of 0.03 ≤ z ≤ 0.08 from the Sloan Digital Sky Survey's Data Release 4, and measure the mean specific star formation rates (SSFRs) over the past 100 Myr in the central ~1-2 kpc. We find a tight correlation between the Eddington ratio (λ) of the central black hole (BH) and the mean SSFR, strongly implying that supernova explosions (SNexp) play a role in the transportation of gas to galactic centers. We outline a model for this connection by accounting for the role of SNexp in the dynamics of CNRs. In our model, the viscosity of turbulence excited by SNexp is enhanced, and thus angular momentum can be efficiently transported, driving inflows toward galactic centers. Our model explains the observed relation λ SSFR1.5-2.0, suggesting that AGNs are triggered by SNexp in CNRs.
We consider the structure of marginally Toomre-stable starburst disks under the assumption that radiation pressure on dust grains provides the dominant vertical support against gravity. This assumption is particularly appropriate when the disk is optically thick to its own infrared radiation, as in the central regions of ULIRGs. We argue that because the disk radiates at its Eddington limit (for dust), the "Schmidt law" for star formation changes in the optically thick limit, with the star formation rate per unit area scaling as Σg/κ, where Σg is the gas surface density and κ is the mean opacity of the disk. Our calculations further show that optically thick starburst disks have a characteristic flux, star formation rate per unit area, and dust effective temperature of F ~ 1013 L☉ kpc-2, ~ 103 M☉ yr-1 kpc-2, and Teff ~ 90 K, respectively. We compare our model predictions with observations of ULIRGs and find good agreement. We extend our model of starburst disks from many hundred parsec scales to subparsec scales and address the problem of fueling AGNs. We assume that angular momentum transport proceeds via global torques (e.g., spiral waves, winds, or a central bar) rather than a local viscosity. We consistently account for the radial depletion of gas due to star formation and find a strong bifurcation between two classes of disk models: (1) solutions with a starburst on large scales that consumes all of the gas with little or no fueling of a central AGN and (2) models with an outer large-scale starburst accompanied by a more compact starburst on 1-10 pc scales and a bright central AGN. The luminosity of the latter models is in many cases dominated by the AGN, although these disk solutions exhibit a broad mid- to far-infrared peak from star formation. We show that the vertical thickness of the starburst disk on parsec scales can approach h ~ r, perhaps accounting for the nuclear obscuration in some type 2 AGNs. We also argue that the disk of young stars in the Galactic center may be the remnant of such a compact nuclear starburst.
We numerically model fragmentation of a gravitationally unstable gaseous disc under conditions that may be appropriate for the formation of the young massive stars observed in the central parsec of our Galaxy. In this study, we adopt a simple prescription with a locally constant cooling time. We find that, for cooling times just short enough to induce disc fragmentation, stars form with a top-heavy initial mass function (IMF), as observed in the Galactic Centre (GC). For shorter cooling times, the disc fragments much more vigorously, leading to lower average stellar masses. Thermal feedback associated with gas accretion on to protostars slows down disc fragmentation, as predicted by some analytical models. We also simulate the fragmentation of a gas stream on an eccentric orbit in a combined Sgr A* plus stellar cusp gravitational potential. The stream precesses, self-collides and forms stars with a top-heavy IMF. None of our models produces large enough comoving groups of stars that could account for the observed ‘ministar cluster’ IRS13E in the GC. In all of the gravitationally unstable disc models that we explored, star formation takes place too fast to allow any gas accretion on to the central supermassive black hole. While this can help to explain the quiescence of ‘failed active galactic nucleus’ such as Sgr A*, it poses a challenge for understanding the high gas accretion rates inferred for many quasars.
The outer parts of standard steady-state accretion discs around quasi-stellar objects (QSOs) are prone to self-gravity, and they might be expected to fragment into stars rather than feed the central black hole. Possible solutions to this well-known problem are examined with an emphasis on general dynamic constraints. Irradiation by the QSO is insufficient for stability even if the outer disc is strongly warped. Marginal local gravitational instability enhances viscous transport but extends the stable regions only modestly. Compton cooling in the observed QSO radiation field rules out hot thick discs unless the local accretion rate is vastly super-Eddington. The formation of stars or stellar-mass black holes, and the release of energy in these objects by fusion or accretion, may help to stabilize the remaining gas in an otherwise standard disc. But at fixed mass accretion rate, the energy inputs required for stability increase with radius; beyond a parsec, they approach the total QSO luminosity and are probably unsustainable by stars. Magnetic torques from a wind or corona, and gravitational torques from bars or global spirals, may increase the accretion speed and reduce the density of the disc. But dynamical arguments suggest that the accretion speed is at most sonic, so that instability still sets in beyond about a parsec. Alternatively, the QSO could be fed by stellar collisions in a very dense stellar cluster, but the velocity dispersion would have to be much higher than observed in nearby galactic nuclei containing quiescent black holes. In view of these difficulties, we suggest that QSO discs do not extend beyond a thousand Schwarzschild radii or so. Then they must be frequently replenished with gas of small specific angular momentum.
We consider the effects of supernovae (SNe) on accretion and star formation in a massive gaseous disc in a large primeval
galaxy. The gaseous disc we envisage, roughly 1 kpc in size with ≳108 M⊙ of gas, could have formed as a result of galaxy mergers where tidal interactions removed angular momentum from gas at larger
radius and thereby concentrated it within the central ∼1 kpc region. We find that SNe lead to accretion in the disc at a rate
of roughly 0.1–1 M⊙ yr−1 and induce star formation at a rate of ∼10–100 M⊙ per year which contributes to the formation of a bulge; a part of the stellar velocity dispersion is due to SN shell speed
from which stars are formed and a part due to the repeated action of stochastic gravitational field of SNe remnant network
on stars. The rate of SN in the inner kpc is shown to be self-regulating, and it cycles through phases of low and high activity.
The SN-assisted accretion transports gas from about 1 kpc to within a few pc of the centre. If this accretion were to continue
down to the central black hole then the resulting ratio of black hole mass to the stellar mass in the bulge would be of the
order of ∼ 10−2–10−3, in line with the observed Magorrian relation.
A few dozen young high-mass stars orbit Sgr A* at distances as short as ∼0.1 pc, where star formation should be quenched by
the strong tidal shear from Sgr A*. The puzzling young stellar population is believed to come into existence in one of two
ways: (i) ‘normal’ star formation at several tens of parsecs in a very massive star cluster that then spiralled in; or (ii)
star formation in situ in a massive self-gravitating disc. We propose to constrain these two scenarios via the expected X-ray emission from young
low-mass stars that should have formed alongside the massive stars. To this end we compare the recent Chandra observations of X-ray emission from young stars in the Orion nebula, and the Chandra observations of the Sgr A* field. We show that the cluster spiral-in model is ruled out irrespectively of the initial mass
function (IMF) of the young stars. In addition, for the in situ model, we find that no more than a few thousand low-mass stars could have formed alongside the massive stars. This is more
than a factor of 10 fewer than expected if these stars were formed with the standard IMF as elsewhere in the Galaxy. The young stars in the Galactic Centre are thus
the first solid observational evidence for star formation in active galactic nucleus (AGN) discs and also require the IMF
of these stars to be top-heavy. We briefly consider the implications of these results for AGN in general.
The evolution of discs under the action of viscosity is studied by both similarity solutions and Green's functions. The angular momentum is steadily concentrated onto a small fraction of the mass which orbits at greater and greater radii while the rest is accreted onto the central body.
We assume that the angular momentum excess of a proto-star is initially concentrated onto one-third of the total mass which forms a disc orbiting the new-born star. Viscous dissipation in this disc will cause it to shine with a luminosity greater than the final main sequence star for a period of 10⁵ yr or so. Most of the properties of T Tauri stars can be explained as a consequence of disc evolution. Flares in Flare stars are interpreted as the entry of blobs of an old disc into the late type stellar atmospheres. On this hypothesis flaring activity could be observed in M stars of up to 5 × 10⁸ yr old, and planetary systems will be common. Disc solutions appropriate to dwarf novae and X-ray sources are also given.
We fitted Spitzer/IRS ~2-35 um spectra of 26 luminous QSOs in attempt to
define the main emission components. Our model has three major components: a
clumpy torus, dusty narrow line region (NLR) clouds and a blackbody-like dust.
The models utilize the clumpy torus of Nenkova et al. (2008) and are the first
to allow its consistent check in type-I AGNs. Single torus models and combined
torus-NLR models fail to fit the spectra of most sources but three component
models adequately fit the spectra of all sources. We present torus inclination,
cloud distribution, covering factor and torus mass for all sources and compare
them with bolometric luminosity, black hole mass and accretion rate. The torus
covering factor and mass are found to be correlated with the bolometric
luminosity of the sources. We find that a substantial amount of the ~2-7 um
radiation originates from a hot dust component, which likely situated in the
innermost part of the torus. The luminosity radiated by this component and its
covering factor are comparable to those of the torus. We quantify the emission
by the NLR clouds and estimate their distance from the center. The distances
are ~700 times larger than the dust sublimation radius and the NLR covering
factor is about 0.07. The total covering factor by all components is in good
agreement with the known AGN type-I:type-II ratio.
A mechanism for the formation of the exponential stellar disk distribution in spiral galaxies is proposed. It is assumed that the gas which falls in to form the disk behaves like an accretion disk which evolves on a viscous time scale t(v) and forms stars on a time scale t(star). It is demonstrated that if t(v) roughly equals t(star) the resulting stellar disk has an exponential distribution independent of the disk rotation law and of the assumed viscosity prescription. Why the viscous and star formation time scales might be related is briefly commented on.
The principal properties of the tori of gas which surround Seyfert nuclei are discussed. The internal state of the clouds and their size distribution function are examined, and it is shown that the Jeans mass scale results in clouds which are individually sufficiently opaque to block out the nucleus, and that the balance of processes which controls their size distribution function also forces the covering factor to be of the order of or greater than unity. Where the gravitational potential is dominated by stars, cloud-cloud collisions keep the molecular clouds close to the equatorial plane. Stirring by stellar processes is never strong enough to compete with collisional losses. The position of the inner edge of the torus is determined by a balance between the inward flow of clouds and the rate at which the nuclear continuum can evaporate them.
Fits to the ultraviolet-optical-infrared spectra of 60 quasars and active galactic nuclei (AGNs) are investigated, using black hole accretion disk models. The disk is assumed to be geometrically thin and optically thick. The observed spectrum from a black hole accretion disk, rotating (Kerr) or nonrotating (Schwarzschild), is transformed by the geometric inclination effect, Doppler shifts, gravitational redshifts, and gravitational focusing. These effects alter the locally emitted fluxes and produce harder spectra for an observer at higher inclination angles. The relativistic corrections are much more significant in the Kerr geometry since the inner edge of the disk is much closer to the rotating black hole. A simple inverse relation between the inferred black hole mass and the assumed inclination angle was found in the Kerr case, while the inferred accretion rate (solar masses/yr) remains independent of the angle. In the Schwarzschild case, the ratio of the inferred accretion rate to the black hole mass remains roughly constant when the viewing angle changes. In both geometries, low-redshift Seyfert galaxies have relatively low accretion rates, only a few percent of their Eddington luminosities, while the most luminous quasars are accreting near their Eddington limits.
We study the observational constraints on the growth of massive black holes (BHs) in galactic nuclei. We use the velocity dispersions of early-type galaxies obtained by the SDSS and the relation between BH mass and velocity dispersion to estimate the local BH mass density to be 2.5x10^5 Msun/Mpc^3. We also use the QSO luminosity function from the 2dF Redshift Survey to estimate the BH mass density accreted during optically bright QSO phases. The local BH mass density is consistent with the density accreted during optically bright QSO phases if QSOs have an efficiency 0.1. By studying the continuity equation for the BH mass distribution, including the effect of BH mergers, we find relations between the local BH mass function and the QSO luminosity function. If the BH mass is assumed to be conserved during BH mergers, comparison of the predicted relations with the observations suggests that luminous QSOs (L_{bol}>10^{46} erg/s) have a high efficiency (e.g. 0.2), and the growth of high-mass BHs (>10^8 Msun) comes mainly from accretion during optically bright QSO phases, or that luminous QSOs have a super-Eddington luminosity. If luminous QSOs are not accreting with super-Eddington luminosities and the growth of low-mass BHs also occurs mainly during optically bright QSO phases, less luminous QSOs must accrete with a low efficiency <0.1; alternatively, they may accrete with high efficiency, but a significant fraction should be obscured. We estimate that the mean lifetime of luminous QSOs is (3-13)x10^7 yr, which is comparable to the Salpeter time. We also investigate the case in which total BH mass decreases during BH mergers due to gravitational radiation, and the observations again suggest that BHs in most luminous QSOs are Kerr BHs accreting with an efficiency >0.1.
Observations of star formation rates (SFRs) in galaxies provide vital clues
to the physical nature of the Hubble sequence, and are key probes of the
evolutionary properties of galaxies. The focus of this review is on the broad
patterns in the star formation properties of galaxies along the Hubble
sequence, and their implications for understanding galaxy evolution and the
physical processes that drive the evolution. Star formation in the disks and
nuclear regions of galaxies are reviewed separately, then discussed within a
common interpretive framework. The diagnostic methods used to measure SFRs are
also reviewed, and a self-consistent set of SFR calibrations is presented as an
aid to workers in the field.
We find 9 nuclear cluster candidates in a sample of 14 edge-on, late-type galaxies observed with HST/ACS. These clusters have magnitudes (M_I ~ -11) and sizes (r_eff ~ 3pc) similar to those found in previous studies of face-on, late-type spirals and dE galaxies. However, three of the nuclear clusters are significantly flattened and show evidence for multiple, coincident structural components. The elongations of these three clusters are aligned to within 10 degrees of the galaxies' major axes. Structurally, the flattened clusters are well fit by a combination of a spheroid and a disk or ring. The nuclear cluster disks/rings have F606W-F814W (~V-I) colors 0.3-0.6 magnitudes bluer than the spheroid components, suggesting that the stars in these components have ages < 1 Gyr. In NGC 4244, the nearest of the nuclear clusters, we further constrain the stellar populations and provide a lower limit on the dynamical mass via spectroscopy. We also present tentative evidence that another of the nuclear clusters (in NGC 4206) may also host a supermassive black hole. Based on our observational results we propose an in situ formation mechanism for nuclear clusters in which stars form episodically in compact nuclear disks, and then lose angular momentum or heat vertically to form an older spheroidal structure. We estimate the period between star formation episodes to be 0.5 Gyr and discuss possible mechanisms for tranforming the disk-like components into spheroids. We also note the connection between our objects and massive globular clusters (e.g. $\omega$ Cen), UCDs, and SMBHs. (Abridged)
Quasars are powered by accretion onto supermassive black holes, but the problem of the duty cycle related to the episodic activity of the black holes remains open as one of the major questions of cosmological evolution of quasars. In this Letter, we obtain quasar duty cycles based on analyses of a large sample composed of 10,979 quasars with redshifts $z\le2.1$ from the Sloan Digital Sky Survey (SDSS) Data Release Three. We estimate masses of quasar black holes and obtain their mass function (MF) of the present sample. We then get the duty cycle $\bar{\delta}(z)=10^{-3}\sim 1$ based on the So{\l}tan's argument, implying that black holes are undergoing multiple episodic activity. We find that the duty cycle has a strong evolution. By comparison, we show that evolution of the duty cycle follows the history of cosmic star formation rate (SFR) density in the Universe, providing intriguing evidence for a natural connection between star formation and triggering of black hole activity. Feedback on star formation from black hole activity is briefly discussed. Comment: Accepted by ApJL, 4 pages and 2 Figures
Young massive stars in the central parsec of our Galaxy are best explained by star formation within at least one, and possibly two, massive self-gravitating gaseous discs. With help of numerical simulations, we here consider whether the observed population of young stars could have originated from a large angle collision of two massive gaseous clouds at R approx. 1 parsec from Sgr A*. In all the simulations performed, the post-collision gas flow forms an inner, nearly circular gaseous disc and one or two eccentric outer filaments, consistent with the observations. Furthermore, the radial stellar mass distribution is always very steep, Sigma proportional to R^-2, again consistent with the observations. All of our simulations produce discs that are warped by between 30 to 60 degrees, in accordance with the most recent observations. The 3D velocity structure of the stellar distribution is sensitive to initial conditions (e.g., the impact parameter of the clouds) and gas cooling details. For example, the runs in which the inner disc is fed intermittently with material possessing fluctuating angular momentum result in multiple stellar discs with different orbital orientations, contradicting the observed data. In all the cases the amount of gas accreted by our inner boundary condition is large, enough to allow Sgr A* to radiate near its Eddington limit over approx. 10^5 years. This suggests that a refined model would have physically larger clouds (or a cloud and a disc such as the circumnuclear disc) colliding at a distance of a few parsecs rather than 1 parsec as in our simulations. Comment: 18 pages, 14 figures. Accepted for publication in MNRAS. Minor additions at referee request. Movies of simulations available at http://www.astro.le.ac.uk/~aph11/movies.html
- J Kormendy
Kormendy, J., et al. 2009, ApJS, 182, 216
- I Shlosman
- M C Begelman
Shlosman, I. & Begelman, M. C. 1987, Nature, 329, 810