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Density of angular momentum vectors of gas in run B1, in the plane defined by cos θ and φ (see equation A2). Top panel: t = 0 . 5 Myr. Bottom panel: t = 1 . 5 Myr. Only gas particles on bound orbits around the SMBH with semi-major axis 0 . 1 ≤ a/ pc ≤ 10 are shown. The reference system is defined by the total angular momentum of the stellar disc at time t , as described in Appendix A. Note that the zero point of φ and θ is not exactly the same between the two panels. The colour map is logarithmic.
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The Galactic centre (GC) is a crowded environment: observations have revealed
the presence of (molecular, atomic and ionized) gas, of a cusp of late-type
stars, and of ~100 early-type stars, about half of which lying in one or
possibly two discs. In this paper, we study the perturbations exerted on a thin
stellar disc (with outer radius ~0.4 pc) by...
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... A young coeval population of massive Wolf-Rayet stars, luminous blue variables and O-type stars are observed in one or two warped disks in the 0.03-0.5 pc vicinity of Sgr A å (Schödel et al. 2005; Bartko et al. 2009Bartko et al. , 2010Lu et al. 2009;Yelda et al. 2014;Schödel et al. 2018;Gallego-Cano et al. 2018;Ali et al. 2020); some of these massive stars form a tightly bound group (e.g., complex IRS 13E) (Schödel et al. 2005), while old low-mass stars are observed to be spherically distributed (Feldmeier et al. 2014;Schödel et al. 2018;Gallego-Cano et al. 2018). Two main formation channels have been proposed to explain the observed distribution (Tremaine et al. 1975;Milosavljević & Merritt 2001;Hansen & Milosavljevi 2003;Kim et al. 2004;Levin et al. 2005;Antonini et al. 2012;Antonini 2013;Alig et al. 2013;Mapelli et al. 2013;Gnedin et al. 2014;Antonini 2014;Arca-Sedda & Capuzzo-Dolcetta 2014a;Antonini et al. 2015;Arca-Sedda et al. 2015, 2018Tsatsi et al. 2017;Arca-Sedda & Gualandris 2018;Trani et al. 2018;Mastrobuono-Battisti et al. 2019;Schödel et al. 2020;Arca-Sedda et al. 2020;Do et al. 2020): the observed distribution is either (i) a remnant of a previous star-forming gas disk; or (ii) the stars have been delivered via massive compact star clusters from the surrounding regions of the Galaxy. ...
We investigate the dynamical evolution of an intermediate-mass black hole (IMBH) in a nuclear star cluster hosting a supermassive black hole (SMBH) and both a spherical and a flattened disk-like distribution of stellar-mass objects. We use a direct N-body (φ GPU) and an orbit-averaged (N-ring) numerical integrator to simulate the orbital evolution of stars and the IMBH. We find that the IMBH's orbit gradually aligns with the stellar disk if their mutual initial inclination is less than 90 . If it is larger than 90 , i.e., counter-rotating, the IMBH does not align. Initially, the rate of orbital reorientation increases linearly with the ratio of the mass of the IMBH over the SMBH mass, and it is orders of magnitude faster than ordinary (i.e., Chandrasekhar) dynamical friction, particularly for high SMBH masses. The semimajor axes of the IMBH and the stars are approximately conserved. This suggests that the alignment is predominantly driven by orbit-averaged gravitational torques of the stars, a process that may be called resonant dynamical friction. The stellar disk is warped by the IMBH, and ultimately increases its thickness. This process may offer a test for the viability of IMBH candidates in the Galactic Center. Resonant dynamical friction is not limited to IMBHs; any object much more massive than disk particles may ultimately align with the disk. This may have implications for the formation and evolution of black hole disks in dense stellar systems and gravitational wave source populations for LIGO, VIRGO, KAGRA, and LISA. © 2021. The American Astronomical Society. All rights reserved..
... IRS 13E complex) (Schödel et al. 2005); while old low-mass stars are observed to be spherically distributed (Feldmeier et al. 2014;Schödel et al. 2018;Gallego-Cano et al. 2018). Two main formation channels have been proposed to explain the observed distribution (Tremaine et al. 1975;Milosavljević & Merritt 2001;Hansen & Milosavljevi 2003;Kim et al. 2004;Levin et al. 2005;Antonini et al. 2012;Antonini 2013;Alig et al. 2013;Mapelli et al. 2013;Gnedin et al. 2014;Antonini 2014; Arca-Sedda & Capuzzo-Dolcetta 2014a; Antonini et al. 2015;Arca-Sedda et al. 2015Tsatsi et al. 2017;Arca-Sedda & Gualandris 2018a;Trani et al. 2018;Mastrobuono-Battisti et al. 2019;Schödel et al. 2020;Arca Sedda et al. 2020;Do et al. 2020): the observed distribution is either (i) a remnant of a previous star-forming gas disk; or (ii) the stars have been delivered via massive compact star clusters from the surrounding regions of the Galaxy. ...
We investigate the dynamical evolution of an intermediate mass black hole (IMBH) in a nuclear star cluster hosting a supermassive black hole (SMBH) and both a spherical and a flattened disk-like distribution of stellar-mass objects. We use a direct N-body ($\varphi$GPU) and an orbit-averaged (N-ring) numerical integrator to simulate the orbital evolution of stars and the IMBH. We find that the IMBH's orbit gradually aligns with the stellar disk if their mutual initial inclination is less than 90 degrees. If it is larger than 90 degrees, i.e. counterrotating, the IMBH does not align. Initially, the rate of orbital reorientation increases linearly with the mass of the IMBH and it is orders of magnitude faster than ordinary (i.e. Chandrasekhar) dynamical friction. The semimajor axes of the IMBH and the stars are approximately conserved. This suggests that the alignment is predominantly driven by orbit-averaged gravitational torques of the stars, a process which may be called resonant dynamical friction. The stellar disk is warped by the IMBH, and ultimately increases its thickness. This process may offer a test for the viability of IMBH candidates in the Galactic Center. Resonant dynamical friction is not limited to IMBHs; any object much more massive than disk particles may ultimately align with the disk. This may have implications for the formation and evolution of black hole disks in dense stellar systems and gravitational wave source populations for LIGO, VIRGO, KAGRA, and LISA.
... The origin of this disk could have been a massive molecular cloud with the radius of ∼ 15 pc and the impact parameter of ∼ 26 pc, which was tidally disrupted, spiraled in and subsequently formed an eccentric disk (Mapelli et al. 2012) where stars formed. It cannot be excluded that the stellar disk, which previously extended below 0.04 pc where the S cluster is located now, was perturbed by an in-fall of another massive molecular cloud that formed a disk or a ring with an inclination of β with respect to the stellar disk (Mapelli et al. 2013;Trani et al. 2016). The influx of gas that lead to their formation also induced the perturbances in the young S-cluster resulting in vertical resonances that relaxed to the structure seen today. ...
... The influx of gas that lead to their formation also induced the perturbances in the young S-cluster resulting in vertical resonances that relaxed to the structure seen today. According to the numerical simulations of Mapelli et al. (2013) and Trani et al. (2016), the precession driven by the gas disk in the inner 0.5 pc on the stellar disk can significantly increase the stellar inclinations within a few million years, which leads to the disk tilting and/or warping. Since the precession is faster for outer disk parts, T prec ∝ a −3/2 , the S cluster could in principle represent a "primordial" disk part with two perpendicular streamers that have warped to form the CW disk at larger distances. ...
We present a detailed analysis of the kinematics of 112 stars that mostly comprise the high velocity S-cluster and orbit the super massive black hole SgrA* at the center of the Milky Way. For 39 of them orbital elements are known, for the remainder we know proper motions. The distribution of inclinations, and proper motion flight directions deviate significantly from a uniform distribution which one expects if the orientation of the orbits are random. Across the central arcseconds the S-cluster stars are arranged in two almost edge on disks that are located at a position angle approximately +-45 o with respect to the Galactic plane. The angular momentum vectors for stars in each disk point in both directions, i.e. the stars in a given disk rotate in opposite ways. The poles of this structure are located only about 25 o from the line of sight. This structure may be the result of a resonance process that started with the formation of the young B-dwarf stars in the cluster about 6 Myr ago. Alternatively, it indicated the presence of a disturber at a distance from the center comparable to the distance of the compact stellar association IRS13.
... another massive molecular cloud that formed a disk or a ring with an inclination of β with respect to the stellar disk (Mapelli et al. 2013;Trani et al. 2016). The influx of gas that led to their formation also induced the perturbances in the young S cluster, resulting in vertical resonances that relaxed to the structure seen today. ...
... The influx of gas that led to their formation also induced the perturbances in the young S cluster, resulting in vertical resonances that relaxed to the structure seen today. According to the numerical simulations of Mapelli et al. (2013) and Trani et al. (2016), the precession driven by the gas disk in the inner 0.5 pc on the stellar disk can significantly increase the stellar inclinations within a few million years, which leads to the disk tilting and/or warping. As the precession is faster for outer disk parts, µ - T a prec 3 2 , the S cluster could in principle represent a "primordial" disk part with two perpendicular streamers that have warped to form the CW disk at larger distances. ...
We present a detailed analysis of the kinematics of 112 stars that mostly comprise the high-velocity S cluster and orbit the supermassive black hole Sgr A* at the center of the Milky Way. For 39 of them, orbital elements are known; for the remainder, we know proper motions. The distribution of the inclinations and the proper motion flight directions deviate significantly from a uniform distribution, which one expects if the orientation of the orbits are random. Across the central arcseconds, the S-cluster stars are arranged in two almost edge-on disks that are located at a position angle approximately ±45° with respect to the Galactic plane. The angular momentum vectors for stars in each disk point in both directions, i.e., the stars in a given disk rotate in opposite ways. The poles of this structure are located only about 25° from the line of sight. This structure may be the result of a resonance process that started with the formation of the young B-dwarf stars in the cluster about 6 Myr ago. Alternatively, it indicated the presence of a disturber at a distance from the center comparable to the distance of the compact stellar association IRS 13.
... Kim & Morris 2003;Kim et al. 2004;Fujii et al. 2008Fujii et al. , 2010Perets et al. 2009;Perets & Gualandris 2010, but see Petts & Gualandris 2017 for the most recent discussion of the issues connected with this scenario), while in the in situ scenarios star formation can be triggered by the disruption of a giant molecular cloud (e.g. Sanders 1998; Bonnell & Rice 2008;Hobbs & Nayakshin 2009;Alig et al. 2011;Mapelli et al. 2012;Alig et al. 2013;Mapelli et al. 2013b). ...
... Kozai-Lidov resonances (Kozai 1962;Lidov 1962) may have a role especially for stars with large semi-major axes; however Kozai cycles are expected to occur for stars misaligned to the CW disc; in addition, the spherical distribution of old stars would damp the cycles even for stars with inclined orbits. The net effect we may expect is a change in the longitude of the ascending node and argument of the pericentre, but again our results would probably be unchanged (Mapelli et al. 2013b;Trani et al. 2016b). Moreover, two-body encounters can significantly affect the distribution of eccentricities and semimajor axes (e.g. ...
The Galactic Centre (GC) is a unique place to study the extreme dynamical processes occurring near a super-massive black hole (SMBH). Here we investigate the role of supernova (SN) explosions occurring in massive binary systems lying in a disc-like structure within the innermost parsec. We use a regularized algorithm to simulate 30,000 isolated three-body systems composed of a stellar binary orbiting the SMBH. We start the integration when the primary member undergoes a SN explosion, and we analyze the impact of SN kicks on the orbits of stars and compact remnants. We find that SN explosions scatter the lighter stars in the pair on completely different orbits, with higher eccentricity and inclination. In contrast, stellar-mass black holes (BHs) and massive stars retain memory of the orbit of their progenitor star. Our results suggest that SN kicks are not sufficient to eject BHs from the GC. We thus predict that all BHs that form in situ in the central parsec of our Galaxy remain in the GC, building up a cluster of dark remnants. In addition, the change of NS orbits induced by SNe may partially account for the observed dearth of NSs in the GC. About 40 per cent of remnants stay bound to the stellar companion after the kick; we expect up to 70 per cent of them might become X-ray binaries through Roche-lobe filling. Finally, the eccentricity of some light stars becomes > 0.7 as an effect of the SN kick, producing orbits similar to those of the G1 and G2 dusty objects.
Hydrodynamical simulations of turbulent molecular clouds show that star clusters form from the hierarchical merger of several sub-clumps. We run smoothed-particle hydrodynamics simulations of turbulence-supported molecular clouds with mass ranging from 1700 to 43000 Msun. We study the kinematic evolution of the main cluster that forms in each cloud. We find that the parent gas acquires significant rotation, because of large-scale torques during the process of hierarchical assembly. The stellar component of the embedded star cluster inherits the rotation signature from the parent gas. Only star clusters with final mass < few X 100 Msun do not show any clear indication of rotation. Our simulated star clusters have high ellipticity (~0.4-0.5 at t=4 Myr) and are subvirial (Q_vir<~0.4). The signature of rotation is stronger than radial motions due to subvirial collapse. Our results suggest that rotation is common in embedded massive (>~1000 Msun) star clusters. This might provide a key observational test for the hierarchical assembly scenario.
The Galactic center hosts several hundred early-type stars, about 20% of
which lie in the so-called clockwise disk, while the remaining 80% do not
belong to any disks. The circumnuclear ring (CNR), a ring of molecular gas that
orbits the supermassive black hole (SMBH) with a radius of 1.5 pc, has been
claimed to induce precession and Kozai-Lidov oscillations onto the orbits of
stars in the innermost parsec. We investigate the perturbations exerted by a
gas ring on a nearly-Keplerian stellar disk orbiting a SMBH by means of
combined direct N-body and smoothed particle hydrodynamics simulations. We
simulate the formation of gas rings through the infall and disruption of a
molecular gas cloud, adopting different inclinations between the infalling gas
cloud and the stellar disk. We find that a CNR-like ring is not efficient in
affecting the stellar disk on a timescale of 3 Myr. In contrast, a gas ring in
the innermost 0.5 pc induces precession of the longitude of the ascending node
Omega, significantly affecting the stellar disk inclination. Furthermore, the
combined effect of two-body relaxation and Omega-precession drives the stellar
disk dismembering, displacing the stars from the disk. The impact of precession
on the star orbits is stronger when the stellar disk and the inner gas ring are
nearly coplanar. We speculate that the warm gas in the inner cavity might have
played a major role in the evolution of the clockwise disk.
The Galactic center uniquely provides opportunities to resolve how star clusters form in neutral gas overdensities engulfed in a large-scale accretion flow. We have performed sensitive Green Bank 100m Telescope (GBT), Karl G. Jansky Very Large Array (JVLA), and Submillimeter Array (SMA) mapping observations of molecular gas and thermal dust emission surrounding the Galaxy`s supermassive black hole (SMBH) Sgr A^{\ast}. We resolved several molecular gas streams orbiting the center on {\gtrsim}10 pc scales. Some of these gas streams appear connected to the well-known 2-4 pc scale molecular circumnuclear disk (CND). The CND may be the tidally trapped inner part of the large-scale accretion flow, which incorporates inflow via exterior gas filaments/arms, and ultimately feeds gas toward Sgr A^{\ast}. Our high resolution GBT+JVLA NH_3 images and SMA+JCMT 0.86 mm dust continuum image consistently reveal abundant dense molecular clumps in this region. These gas clumps are characterized by {\gtrsim}100 times higher virial masses than the derived molecular gas masses based on 0.86 mm dust continuum emission. In addition, Class I CH_3OH masers and some H_2O masers are observed to be well associated with the dense clumps. We propose that the resolved gas clumps may be pressurized gas reservoirs for feeding the formation of 1-10 solar-mass stars. These sources may be the most promising candidates for ALMA to probe the process of high-mass star-formation in the Galactic center.
Several thousand solar masses of molecular, atomic and ionized gas lie in the
innermost ~10 pc of our Galaxy. The most relevant structure of molecular gas is
the circumnuclear ring (CNR), a dense and clumpy ring surrounding the
supermassive black hole (SMBH), with a radius of ~2 pc. We propose that the CNR
formed through the tidal disruption of a molecular cloud, and we investigate
this scenario by means of N-body smoothed-particle hydrodynamics simulations.
We ran a grid of simulations with different cloud mass (4X10^4, 1.3X10^5 solar
masses), different initial orbital velocity (v_in=0.2-0.5 v_esc, where v_esc is
the escape velocity from the SMBH), and different impact parameter (b=8, 26
pc). The disruption of the molecular cloud leads to the formation of very dense
and clumpy gas rings, containing most of the initial cloud mass. If the initial
orbital velocity of the cloud is sufficiently low (v_in<0.4 v_esc, for b=26 pc)
or the impact parameter is sufficiently small (b<10 pc, for v_in>0.5 v_esc), at
least two rings form around the SMBH: an inner ring (with radius ~0.4 pc) and
an outer ring (with radius ~2-4 pc). The inner ring forms from low-angular
momentum material that engulfs the SMBH during the first periapsis passage,
while the outer ring forms later, during the subsequent periapsis passages of
the disrupted cloud. The inner and outer rings are misaligned by ~24 degrees,
because they form from different gas streamers, which are affected by the SMBH
gravitational focusing in different ways. The outer ring matches several
properties (mass, rotation velocity, temperature, clumpiness) of the CNR in our
Galactic centre. We speculate that the inner ring might account for the neutral
gas observed in the central cavity.
We present radio and infrared observations indicating on-going star formation
activity inside the $\sim2-5$ pc circumnuclear ring at the Galactic center.
Collectively these measurements suggest a continued disk-based mode of on-going
star formation has taken place near Sgr A* over the last few million years.
First, VLA observations with spatial resolution 2.17$"\times0.81"$ reveal 13
water masers, several of which have multiple velocity components. The presence
of interstellar water masers suggests gas densities that are sufficient for
self-gravity to overcome the tidal shear of the 4$\times10^6$ \msol\, black
hole. Second, SED modeling of stellar sources indicate massive YSO candidates
interior to the molecular ring, supporting in-situ star formation near Sgr A*
and appear to show a distribution similar to that of the counter-rotating disks
of $\sim$100 OB stars orbiting Sgr A*. Some YSO candidates (e.g., IRS~5) have
bow shock structures suggesting that they have have gaseous disks that are
phototoevaporated and photoionized by the strong radiation field. Third, we
detect clumps of SiO (2-1) and (5-4) line emission in the ring based on CARMA
and SMA observations. The FWHM and luminosity of the SiO emission is consistent
with shocked protostellar outflows. Fourth, two linear ionized features with an
extent of $\sim0.8$ pc show blue and redshifted velocities between $+50$ and
$-40$ \kms, suggesting protostellar jet driven outflows with mass loss rates of
$\sim5\times10^{-5}$ solar mass yr$^{-1}$. Finally, we present the imprint of
radio dark clouds at 44 GHz, representing a reservoir of molecular gas that
feeds star formation activity close to Sgr A*.
