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— Upper panel: L-band spectrum of NGC 6240. The 2 nuclei are clearly separated. Lower panel: Integrated emission along the slit. We extracted the spectrum of the faintest nucleus from the cyan shaded region on the right. The background was extracted from the yellow shaded region on the left, symmetric with the source region with respect to the emission peak of the brightest nucleus.
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We present 3-5 μm spectroscopy of the interacting system NGC 6240, revealing the presence of two active galactic nuclei (AGNs). The brightest (southern) nucleus shows up with a starburst-like emission, with a prominent 3.3 μm emission feature. However, the presence of an AGN is revealed by the detection of a broad Brα emission line, with a width of...
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... In particular, the contribution of the brightest nucleus to the emission in the extraction region of the faint nucleus is not negligible. In order to correct for this contamination, we subtracted from the faint nucleus a background extracted from the region symmetric to the extraction region with respect to the emission peak of the bright nucleus (Fig. ...
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Citations
... NGC 6240 is an interacting/merging galaxy hosting two nuclei that are separated by ∼1 5 (Beswick et al. 2001 ) has been reported in this galaxy (Henkel et al. 1984;Hagiwara et al. 2002;Nakai et al. 2002). NGC 6240 has a large far-infrared luminosity ≈ L FIR =10 11-12 L e , which cannot be accounted for by starburst activity alone, but which can be more naturally explained as a buried AGN heating surrounding dusty components (e.g., Depoy et al. 1986;Sanders et al. 1988;Risaliti et al. 2006). Neutral Fe Kα lines at 6.4 keV due to reflection from optically thick material are detected from both nuclei, which is clear evidence for both being AGNs (Komossa et al. 2003). ...
We present the results of observations of 22 GHz H2O maser emission in NGC 6240 and M51 made with the Karl G. Jansky Very Large Array. Two major H2O maser features and several minor features are detected toward the southern nucleus of NGC 6240. These features are redshifted by about 300 km s-1 from the galaxy's systemic velocity and remain unresolved at the synthesized beam size. A combination of our two-epoch observations and published data reveals an apparent correlation between the strength of the maser and the 22 GHz radio continuum emission, implying that the maser excitation relates to the activity of an active galactic nucleus in the southern nucleus rather than star-forming activity. The star-forming galaxy M51 hosts H2O maser emission in the center of the galaxy; however, the origin of the maser has been an open question. We report the first detection of 22 GHz nuclear radio continuum emission in M51. The continuum emission is co-located with the maser position, which indicates that the maser arises from nuclear active galactic nucleus-activity and not from star-forming activity in the galaxy. © 2015. The American Astronomical Society. All rights reserved.
... NGC 6240 is a well known double-merging system 107 Mpc far away. Here the Xray imaging and spectroscopy performed with Chandra (Komossa et al. 2003) revealed a Compton Thick double AGN, that was confirmed at all wavelengths, plus a SB covering a projected ∼ 2 kpc scale (Risaliti et al. 2006, Medling et al. 2011, Feruglio et al. 2013). HST and Spitzer images (Bush et al. 2008) show how the thermal emission follows the optical dust obscuration very closely. ...
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Two main physical processes characterize the activity in the nuclear region of active galaxies: an intense star formation (starburst, SB) and an Active Galactic Nucleus (AGN). While the existence of a starburst-AGN connection is undisputed, still it is not clear which process dominates the energetic output in both local and high redshift Universe. Moreover there is no consensus on whether AGN fueling is synchronous with star formation or follows it during a post-starburst phase. Here I first review how to disentangle the relative SB-AGN contribution, then I focus on the physical and geometrical properties of the circumnuclear environment.
Using archived data from the Chandra X-ray telescope, we have extracted the diffuse X-ray emission from 49 equal-mass interacting/merging galaxy pairs in a merger sequence, from widely separated pairs to merger remnants. After removal of contributions from unresolved point sources, we compared the diffuse thermal X-ray luminosity from hot gas (L(X)(gas)) with the global star formation rate (SFR). After correction for absorption within the target galaxy, we do not see strong trend of L(X)(gas)/SFR with SFR or merger stage for galaxies with SFR > 1 M(sun) yr^-1. For these galaxies, the median L(X)(gas)/SFR is 5.5 X 10^39 ((erg s^-1)/M(sun) yr^-1)), similar to that of normal spiral galaxies. These results suggest that stellar feedback in star forming galaxies reaches an approximately steady state condition, in which a relatively constant fraction of about 2% of the total energy output from supernovae and stellar winds is converted into X-ray flux. Three late-stage merger remnants with low SFRs and high K band luminosities (L(K)) have enhanced L(X)(gas)/SFR; their UV/IR/optical colors suggest that they are post-starburst galaxies, perhaps in the process of becoming ellipticals. Systems with L(K) < 10^10 L(sun) have lower L(X)(gas)/SFR ratios than the other galaxies in our sample, perhaps due to lower gravitational fields or lower metallicities. We see no relation between L(X)(gas)/SFR and Seyfert activity in this sample, suggesting that feedback from active galactic nuclei is not a major contributor to the hot gas in our sample galaxies.
Nuclear starbursts and AGN activity are the main heating processes in luminous infrared galaxies (LIRGs) and their relationship is fundamental to understand galaxy evolution. In this paper, we study the star-formation and AGN activity of a sample of 11 local LIRGs imaged with subarcsecond angular resolution at radio (8.4GHz) and near-infrared ($2.2\mu$m) wavelengths. This allows us to characterize the central kpc of these galaxies with a spatial resolution of $\simeq100$pc. In general, we find a good spatial correlation between the radio and the near-IR emission, although radio emission tends to be more concentrated in the nuclear regions. Additionally, we use an MCMC code to model their multi-wavelength spectral energy distribution (SED) using template libraries of starburst, AGN and spheroidal/cirrus models, determining the luminosity contribution of each component, and finding that all sources in our sample are starburst-dominated, except for NGC6926 with an AGN contribution of $\simeq64$\%. Our sources show high star formation rates ($40$ to $167M_\odot\mathrm{yr}^{-1}$), supernova rates (0.4 to $2.0\mathrm{SN}\mathrm{yr}^{-1}$), and similar starburst ages (13 to $29\mathrm{Myr}$), except for the young starburst (9Myr) in NGC6926. A comparison of our derived star-forming parameters with estimates obtained from different IR and radio tracers shows an overall consistency among the different star formation tracers. AGN tracers based on mid-IR, high-ionization line ratios also show an overall agreement with our SED model fit estimates for the AGN. Finally, we use our wide-band VLA observations to determine pixel-by-pixel radio spectral indices for all galaxies in our sample, finding a typical median value ($\alpha\simeq-0.8$) for synchrotron-powered LIRGs.
We present a broad-band (~0.3-70 keV) spectral and temporal analysis of
NuSTAR observations of the luminous infrared galaxy NGC 6240, combined with
archival Chandra, XMM-Newton and BeppoSAX data. NGC 6240 is a galaxy in a
relatively early merger state with two distinct nuclei separated by ~1."5.
Previous Chandra observations have resolved the two nuclei, showing that they
are both active and obscured by Compton-thick material. Although they cannot be
resolved by NuSTAR, thanks to the unprecedented quality of the NuSTAR data at
energies >10 keV, we clearly detect, for the first time, both the primary and
the reflection continuum components. The NuSTAR hard X-ray spectrum is
dominated by the primary continuum piercing through an absorbing column density
which is mildly optically thick to Compton scattering (tau ~ 1.2, N_H ~ 1.5 x
10^(24) cm^-2). We detect moderate hard X-ray (> 10 keV) flux variability up to
20% on short (15-20 ksec) timescales. The amplitude of the variability is
maximum at ~30 keV and is likely to originate from the primary continuum of the
southern nucleus. Nevertheless, the mean hard X-ray flux on longer timescales
(years) is relatively constant. Moreover, the two nuclei remain Compton-thick,
although we find evidence of variability of the material along the line of
sight with column densities N_H <~ 2 x 10^(23) cm-2 over long (~3-15 years)
timescales. The observed X-ray emission in the NuSTAR energy range is fully
consistent with the sum of the best-fit models of the spatially resolved
Chandra spectra of the two nuclei.
We present an updated mid-infrared (MIR) versus X-ray correlation for the
local active galactic nuclei (AGN) population based on the high angular
resolution 12 and 18um continuum fluxes from the AGN subarcsecond MIR atlas and
2-10 keV and 14-195 keV data collected from the literature. We isolate a sample
of 152 objects with reliable AGN nature and multi-epoch X-ray data and minimal
MIR contribution from star formation. Although the sample is not homogeneous or
complete, we show that our results are unlikely to be affected by biases. The
MIR--X-ray correlation is nearly linear and within a factor of two independent
of the AGN type and the wavebands used. The observed scatter is <0.4 dex. A
possible flattening of the correlation slope at the highest luminosities probed
(~ 10^45 erg/s) is indicated but not significant. Unobscured objects have, on
average, an MIR--X-ray ratio that is only <= 0.15 dex higher than that of
obscured objects. Objects with intermediate X-ray column densities (22 < log
N_H < 23) actually show the highest MIR--X-ray ratio on average. Radio-loud
objects show a higher mean MIR--X-ray ratio at low luminosities, while the
ratio is lower than average at high luminosities. This may be explained by
synchrotron emission from the jet contributing to the MIR at low-luminosities
and additional X-ray emission at high luminosities. True Seyfert 2 candidates
and double AGN do not show any deviation from the general behaviour. Finally,
we show that the MIR--X-ray correlation can be used to verify the AGN nature of
uncertain objects. Specifically, we give equations that allow to determine the
intrinsic 2-10 keV luminosities and column densities for objects with complex
X-ray properties to within 0.34 dex. These techniques are applied to the
uncertain objects of the remaining AGN MIR atlas, demonstrating the usefulness
of the MIR--X-ray correlation as an empirical tool.
We present the first subarcsecond-resolution mid-infrared (MIR) atlas of
local active galactic nuclei (AGN) containing 253 objects with a median
redshift of z = 0.016. It comprises all available MIR imaging observations
performed to date with ground-based 8-meter class telescopes and includes in
total 895 independent photometric measurements, of which more than 60% are
previously unpublished.We detect extended nuclear emission in at least 21% of
the objects, while another 19% appear clearly point-like, and the remaining
objects cannot be constrained. Subarcsecond resolution allows us to isolate the
emission of the AGN on scales of a few tens of parsecs for the bulk of the
sample and obtain nuclear photometry in multiple filters for the objects. The
photometry is used to construct median spectral energy distributions (SEDs) for
the different optical AGN types and estimate the individual MIR 12 and 18 um
continuum luminosities, which range over more than six orders of magnitude. We
also analyse the arcsecond-scale MIR emission as probed by Spitzer and compare
it to the subarcsecond-scale emission. The continuum emission is on average 20%
lower on subarcsecond scales with a large scatter that depends mostly on the
object distance or luminosity. The silicate feature strength is similar on both
scales and generally appears in emission (absorption) in type I (II) AGN.
However, the PAH emission appears weaker or absent on subarcsecond scales,
indicating that most of the star formation has been resolved out. The
differences of the MIR SEDs on both scales are particularly large for
AGN/starburst composites and close-by (and weak) AGN. Because of its size and
characteristics, the AGN MIR atlas is well-suited not only for detailed
investigation of individual sources but also for sample studies of AGN
unification.
We construct a 2D-dynamical model in order to study the motion in a galaxy with a double nucleus. Numerical calculations show that the majority of high energy stars, near the double nuclear region, are on chaotic orbits. On the contrary, low energy stars are on chaotic orbits only near massive nuclei while, for less massive nuclei, the motion is regular. Using a semi-numerical approach we explain the chaotic scattering of orbits near each nucleus. The high values of the velocities near the dense nuclei are also explained. Computation of the LCEs indicates a very high degree of chaos. The results derived from our double nucleus dynamical model are compared with models with single ones. Our outcomes are in satisfactory agreement with observational data for the double nucleus system NGC6240.
We present a 5–8 μm analysis of the Spitzer spectra of 71 ultraluminous infrared galaxies (ULIRGs) with redshift z < 0.15, devoted to the study of the role of active galactic nuclei (AGN) and starbursts (SBs) as the power source of the
extreme infrared emission. Around ∼5 μm, an AGN is much brighter (by a factor of ≈30) than an SB of equal bolometric luminosity.
This allows us to detect the presence of even faint accretion-driven cores inside ULIRGs: signatures of AGN activity are found
in ∼70 per cent of our sample (50/71 sources). Through a simple analytical model, we are also able to obtain a quantitative estimate of the AGN/SB contribution to the overall energy output of each source. Although the main fraction of ULIRG luminosity
is confirmed to arise from star formation events, the AGN contribution is non-negligible (∼23 per cent) and is shown to increase
with luminosity. The existence of a rather heterogeneous pattern in the composition and geometrical structure of the dust
among ULIRGs is newly supported by the comparison between individual absorption features and continuum extinction.
We report on a recent ~150 ks long Chandra observation of the ultraluminous infrared galaxy merger NGC 6240, which allows a detailed investigation of the diffuse galactic halo. Extended soft X-ray emission is detected at the 3σ confidence level over a diamond-shaped region with projected physical size of ~110 × 80 kpc, and a single-component thermal model provides a reasonably good fit to the observed X-ray spectrum. The hot gas has a temperature of ~7.5 million K, an estimated density of 2.5 × 10–3 cm–3, and a total mass of ~1010M
☉, resulting in an intrinsic 0.4-2.5 keV luminosity of 4 × 1041 erg s–1. The average temperature of 0.65 keV is quite high to be obviously related to either the binding energy in the dark-matter gravitational potential of the system or the energy dissipation and shocks following the galactic collision, yet the spatially resolved spectral analysis reveals limited variations across the halo. The relative abundance of the main α-elements with respect to iron is several times the solar value, and nearly constant as well, implying a uniform enrichment by type II supernovae out to the largest scales. Taken as a whole, the observational evidence is not compatible with a superwind originated by a recent, nuclear starburst, but rather hints at widespread, enhanced star formation proceeding at a steady rate over the entire dynamical timescale (~200 Myr). The preferred scenario is that of a starburst-processed gas component gently expanding into, and mixing with, a pre-existing halo medium of lower metallicity (Z ~ 0.1 solar) and temperature (kT ~ 0.25 keV). This picture cannot be probed more extensively with the present data, and the ultimate fate of the diffuse, hot gas remains uncertain. Under some favorable conditions, at least a fraction of it might be retained after the merger completion, and evolve into the hot halo of a young elliptical galaxy.
