The formerly X-ray reflection-dominated Seyfert~2 galaxy NGC6300

In this paper, a BeppoSAX observation of the bright Seyfert 2 galaxy NGC6300 is presented. The rapidly variable emission from the active nucleus is seen through a Compton-thin (NH ~ 3x10^23 atomoms/cm/cm) absorber. A Compton-reflection component with an unusually high reflection fraction (R ~ 4.2), and the comparison with a reflection-dominated spectrum measured by RXTE two and half years earlier suggest that NGC6300 belongs to the class of "transient" AGN, undergoing long and repeated periods of low-activity. The spectral transition provides support to the idea that Compton-thick and Compton-thin X-ray absorbers in Seyfert 2 galaxies are decoupled, the former being most likely associated with the "torus", whereas the latter is probably located at much larger distances.Comment: 5 Latex pages, 5 figures, To appear in Monthly Notices of the Royal Astronomical Society (Letters

Enhanced Star Formation in Narrow Line Seyfert 1 AGN revealed by Spitzer

We present new low resolution Spitzer mid-infrared spectroscopy of a sample of 20 ROSAT selected local Narrow Line Seyfert 1 galaxies (NLS1s). We detect strong AGN continuum in all and clear PAH emission in 70% of the sources. The 6.2 micron PAH luminosity spans three orders of magnitudes, from ~10^(39) erg/s to ~10^(42) erg/s providing strong evidence for intense ongoing star formation in the circumnuclear regions of these sources. Using the IRS/Spitzer archive we gather a large number of additional NLS1s and their broad line counterparts (BLS1s) and constructed NLS1 and BLS1 sub-samples to compare them in various ways. The comparison shows a clear separation according to FWHM(H_beta) such that objects with narrower broad H_beta lines are the strongest PAH emitters. We test this division in various ways trying to remove biases due to luminosity and aperture size. Specifically, we find that star formation activity around NLS1 AGN is larger than around BLS1 of the same AGN luminosity. The above result seems to hold over the entire range of distance and luminosity. Moreover the star formation rate is higher in low black hole mass and high L/L_Edd systems indicating that black hole growth and star formation are occurring simultaneously.Comment: 30 pages, 11 figures, 4 tables. Now accepted in MNRA

Testing current synthesis models of the X-ray background

We present synthesis models of the X-ray background where the available X-ray observational constraints are used to derive information on the AGN population properties. We show the need for luminous X-ray absorbed AGNs, the QSO2s, in reproducing the 2-10 keV source counts at relatively bright fluxes. We compare a model where the evolution of absorbed AGNs is faster than that of unabsorbed ones, with a standard model where absorbed and unabsorbed AGNs evolve at the same rate. It is found that an increase by a factor of ~2 from z=0 to z~1.3 in the ratio between absorbed and unabsorbed AGNs would provide a significant improvement in the data description. Finally, we make predictions on the AGNs to be observed in deep X-ray surveys which contain information on the AGN space density at high redshift.Comment: 11 pages with 8 figures, A&A accepte

Chandra unveils a binary Active Galactic Nucleus in Mrk463

We analyse Chandra, XMM-Newton and HST data of the double-nucleus Ultraluminous Infrared Galaxy (ULIRG), Mrk463. The Chandra detection of two luminous ($\mathrm{L}_\mathrm{2-10 keV}=1.5\times10^{43}$ and $3.8\times10^{42}$ erg cm$^{-2}$ s$^{-1}$), unresolved nuclei in Mrk~463 indicates that this galaxy hosts a binary AGN, with a projected separation of $\simeq3.8$ kpc ($3.83\pm0.01$ arcsec). While the East nucleus was already known to be a Seyfert 2 (and this is further confirmed by our Chandra detection of a neutral iron line), this is the first unambiguous evidence in favour of the AGN nature of the West nucleus. Mrk463 is therefore the clearest case so far for a binary AGN, after NGC6240.Comment: 7 pages, 7 figures, accepted for publication in MNRA

Local Supermassive Black Holes, Relics of Active Galactic Nuclei and the X-ray Background

We quantify the importance of mass accretion during AGN phases in the growth of supermassive black holes (BH) by comparing the mass function of black holes in the local universe with that expected from AGN relics, which are black holes grown entirely with mass accretion during AGN phases. The local BH mass function (BHMF) is estimated by applying the well-known correlations between BH mass, bulge luminosity and stellar velocity dispersion to galaxy luminosity and velocity functions. The density of BH's in the local universe is 4.6 (-1.4; +1.9) (h/0.7)^2 10^5 Msun Mpc^-3. The relic BHMF is derived from the continuity equation with the only assumption that AGN activity is due to accretion onto massive BH's and that merging is not important. We find that the relic BHMF at z=0 is generated mainly at z<3. Moreover, the BH growth is anti-hierarchical in the sense that smaller BH's (MBH< 10^7 Msun) grow at lower redshifts (z<1) with respect to more massive one's (z~1-3). Unlike previous work, we find that the BHMF of AGN relics is perfectly consistent with the local BHMF indicating the local BH's were mainly grown during AGN activity. This agreement is obtained while satisfying, at the same time, the constraints imposed from the X-ray background. The comparison with the local BHMF also suggests that the merging process is not important in shaping the relic BHMF, at least at low redshifts (z<3). Our analysis thus suggests the following scenario: local black holes grew during AGN phases in which accreting matter was converted into radiation with efficiencies epsilon = 0.04-0.16 and emitted at a fraction lambda = 0.1-1.7 of the Eddington luminosity. The average total lifetime of these active phases ranges from ~4.5 10^8 yr for MBH 10^9 Msun. (abridged)Comment: 19 pages, 18 figures, MNRAS in press, minor changes following referee's comment

The host galaxy/AGN connection. Brightness profiles of early-type galaxies hosting Seyfert nuclei

[ABRIDGED] We recently presented evidence of a connection between the brightness profiles of nearby early-type galaxies and the properties of the AGN they host. The radio loudness of the AGN appears to be univocally related to the host's brightness profile: radio-loud nuclei are only hosted by ``core'' galaxies while radio-quiet AGN are only found in ``power-law'' galaxies. We extend our analysis here to a sample of 42 nearby (V < 7000 km/s) Seyfert galaxies hosted by early-type galaxies. We used the available HST images to study their brightness profiles. Having excluded complex and highly nucleated galaxies, in the remaining 16 objects the brightness profiles can be successfully modeled with a Nuker law with a steep nuclear cusp characteristic of ``power-law'' galaxies (with logarithmic slope 0.51 - 1.07). This result is what is expected for these radio-quiet AGN based on our previous findings, thus extending the validity of the connection between brightness profile and radio loudness to AGN of a far higher luminosity. We explored the robustness of this result against a different choice of the analytic form for the brightness profiles, using a Sersic law. In no object could we find evidence of a central light deficit with respect to a pure Sersic model, the defining feature of ``core'' galaxies in this modeling framework. We conclude that, regardless of the modeling strategy, the dichotomy of AGN radio loudness can be univocally related to the host's brightness profile. Our general results can be re-phrased as ``radio-loud nuclei are hosted by core galaxies, while radio-quiet AGN are found in non-core galaxies''.Comment: Accepted for publication in A&

Detecting quasars at very high redshift with next generation X-ray telescopes

The next generation of X-ray telescopes have the potential to detect faint quasars at very high redshift and probe the early growth of massive black holes (BHs). We present modelling of the evolution of the optical and X-ray AGN luminosity function at 2 < z < 6 based on a CDM merger-driven model for the triggering of nuclear activity combined with a variety of fading laws. We extrapolate the merger-driven models to z > 6 for a range of BH growth scenarios. We predict significant numbers of sources at z ~ 6 with fluxes just an order of magnitude below the current detection limits and thus detectable with XEUS and Constellation-X, relatively independently of the fading law chosen. The predicted number of sources at even higher redshift depends sensitively on the early growth history of BHs. For passive evolution models in which BHs grow constantly at their Eddington limit, detectable BHs may be rare beyond z ~ 10 even with Generation-X. However, in the more probable scenario that BH growth at z > 6 can be described by passive evolution with a small duty cycle, or by our merger driven accretion model, then we predict that XEUS and Generation-X will detect significant numbers of black holes out to z ~ 10 and perhaps beyond.Comment: 18 pages, 11 Figures. Version accepted to MNRAS; extra data plotted, XEUS and Con-X sensitivities corrected and predictions amended accordingl

The physics of galactic winds driven by active galactic nuclei

Active galactic nuclei (AGN) drive fast winds in the interstellar medium of their host galaxies. It is commonly assumed that the high ambient densities and intense radiation fields in galactic nuclei imply short cooling times, thus making the outflows momentum-conserving. We show that cooling of high-velocity, shocked winds in AGN is in fact inefficient in a wide range of circumstances, including conditions relevant to ultra-luminous infrared galaxies (ULIRGs), resulting in energy-conserving outflows. We further show that fast energy-conserving outflows can tolerate a large amount of mixing with cooler gas before radiative losses become important. For winds with initial velocity v_in>~10,000 km s^-1, as observed in ultra-violet and X-ray absorption, the shocked wind develops a two-temperature structure. While most of the thermal pressure support is provided by the protons, the cooling processes operate directly only on the electrons. This significantly slows down inverse Compton cooling, while free free cooling is negligible. Slower winds with v_in~1,000 km s^-1, such as may be driven by radiation pressure on dust, can also experience energy-conserving phases but under more restrictive conditions. During the energy-conserving phase, the momentum flux of an outflow is boosted by a factor ~v_in/2v_s by work done by the hot post-shock gas, where v_s is the velocity of the swept-up material. Energy-conserving outflows driven by fast AGN winds (v_in~0.1c) may therefore explain the momentum fluxes Pdot>>L_AGN/c of galaxy-scale outflows recently measured in luminous quasars and ULIRGs. Shocked wind bubbles expanding normal to galactic disks may also explain the large-scale bipolar structures observed in some systems, including around the Galactic Center, and can produce significant radio, X-ray, and gamma-ray emission. [Abridged]Comment: 20 pages, 8 figures. MNRAS, in pres

A physical model for the origin of the diffuse cosmic infrared background

We present a physical model for origin of the cosmic diffuse infrared background (CDIRB). By utilizing the observed stellar mass function and its evolution as input to a semi-empirical model of galaxy formation, we isolate the physics driving diffuse IR emission. The model includes contributions from three primary sources of IR emission: steady-state star formation owing to isolated disk galaxies, interaction-driven bursts of star formation owing to close encounters and mergers, and obscured active galactic nuclei (AGN). We find that most of the CDIRB is produced by equal contributions from objects at z=0.5-1 and z>1, as suggested by recent observations. Of those sources, the vast majority of the emission originates in systems with low to moderate IR luminosities (L_{IR}<10^{12} $L_sun); the most luminous objects contribute significant flux only at high-redshifts (z>2). All star formation in ongoing mergers accounts for <10% of the total at all wavelengths and redshifts, while emission directly attributable to the interaction-driven burst itself accounts for <5%. We furthermore find that obscured AGN contribute <1-2% of the CDIRB at all wavelengths and redshifts, with a strong upper limit of less than 4% of the total emission. Finally, since electron-positron pair production interactions with the CDIRB represent the primary source of opacity to very high energy (VHE: E_\gamma > 1 TeV) \gamma-rays, the model provides predictions for the optical depth of the Universe to the most energetic photons. We find that these predictions agree with observations of high-energy cutoffs at TeV energies in nearby blazars, and suggest that while the Universe is extremely optically thick at >10 TeV, the next generation of VHE \gamma-ray telescopes can reasonably expect detections from out to 50-150 Mpc.Comment: 14 pages, 13 figures, submitted to MNRA