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Register a new userObservational Limits on Type 1 AGN Accretion Rate in COSMOS
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We present black hole masses and accretion rates for 182 Type 1 AGN in COSMOS. We estimate masses using the scaling relations for the broad Hb, MgII, and CIV emission lines in the redshift ranges 0.16<z<0.88, 1<z<2.4, and 2.7<z<4.9. We estimate the accretion rate using an Eddington ratio L_I/L_Edd estimated from optical and X-ray data. We find that very few Type 1 AGN accrete below L_I/L_Edd ~ 0.01, despite simulations of synthetic spectra which show that the survey is sensitive to such Type 1 AGN. At lower accretion rates the BLR may become obscured, diluted or nonexistent. We find evidence that Type 1 AGN at higher accretion rates have higher optical luminosities, as more of their emission comes from the cool (optical) accretion disk with respect to shorter wavelengths. We measure a larger range in accretion rate than previous works, suggesting that COSMOS is more efficient at finding low accretion rate Type 1 AGN. However the measured range in accretion rate is still comparable to the intrinsic scatter from the scaling relations, suggesting that Type 1 AGN accrete at a narrow range of Eddington ratio, with L_I/L_Edd ~ 0.1.
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The Cosmic Evolution Survey (COSMOS) enables the study of the Spectral Energy Distributions (SEDs) of Active Galactic Nuclei (AGN) because of the deep coverage and rich sampling of frequencies from X-ray to radio. Here we present a SED catalog of 413 X-ray (xmm) selected type 1 (emission line FWHM$>2000$ km s$^{-1}$) AGN with Magellan, SDSS or VLT spectrum. The SEDs are corrected for the Galactic extinction, for broad emission line contributions, constrained variability, and for host galaxy contribution. We present the mean SED and the dispersion SEDs after the above corrections in the rest frame 1.4 GHz to 40 keV, and show examples of the variety of SEDs encountered. In the near-infrared to optical (rest frame $sim 8mu m$-- 4000AA), the photometry is complete for the whole sample and the mean SED is derived from detections only. Reddening and host galaxy contamination could account for a large fraction of the observed SED variety. The SEDs are all available on-line.
This paper estimates the specific accretion-rate distribution of AGN using a sample of 4821 X-ray sources from both deep and shallow surveys. The specific accretion-rate distribution is defined as the probability of a galaxy with a given stellar mass and redshift hosting an active nucleus with a certain specific accretion rate. We find that the probability of a galaxy hosting an AGN increases with decreasing specific accretion rate. There is evidence that this trend reverses at low specific accretion rates, $lambda<10^{-4}-10^{-3}$ (in Eddington units). There is also a break close to the Eddington limit, above which the probability of an accretion event decreases steeply. The specific accretion-rate distribution evolves such that the fraction of AGN among galaxies drops toward lower redshifts. This decrease in the AGN duty cycle is responsible for the strong evolution of the accretion density of the Universe from redshift $zapprox1-1.5$ to the present day. Our analysis also suggests that this evolution is accompanied by a decoupling of accretion events onto black holes from the formation of stars in galaxies. There is also evidence that at earlier times the relative probability of high vs low specific accretion-rate events among galaxies increases. We argue that this differential redshift evolution of the AGN duty cycle with respect to $lambda$ produces the AGN downsizing trend, whereby luminous sources peak at earlier epochs compared to less luminous ones. Finally, we also find a stellar-mass dependence of the specific accretion-rate distribution, with more massive galaxies avoiding high specific accretion-rate events.
We present results of a statistical study of the cosmic evolution of the mass dependent major-merger rate since z=1. A stellar mass limited sample of close major-merger pairs (the CPAIR sample) was selected from the archive of the COSMOS survey. Pair fractions at different redshifts derived using the CPAIR sample and a local K-band selected pair sample show no significant variations with stellar mass. The pair fraction exhibits moderately strong cosmic evolution, with the best-fitting evolutionary index m=2.2+-0.2. The best-fitting function for the merger rate implies that galaxies with stellar mass between 1E+10 -- 3E+11 M_sun have undergone 0.5 -- 1.5 major-mergers since z=1. Our results show that, for massive galaxies at z<1, major mergers involving star forming galaxies (i.e. wet and mixed mergers) can account for the formation of both ellipticals and red quiescent galaxies (RQGs). On the other hand, major mergers cannot be responsible for the formation of most low mass ellipticals and RQGs. Our quantitative estimates indicate that major mergers have significant impact on the stellar mass assembly of the most massive galaxies, but for less massive galaxies the stellar mass assembly is dominated by the star formation. Comparison with the mass dependent (U)LIRG rates suggests that the frequency of major-merger events is comparable to or higher than that of (U)LIRGs.
We explore the evolution with redshift of the rest-frame colours and space densities of AGN hosts (relative to normal galaxies) to shed light on the dominant mechanism that triggers accretion onto supermassive black holes as a function of cosmic time. Data from serendipitous wide-area XMM surveys of the SDSS footprint (XMM/SDSS, Needles in the Haystack survey) are combined with Chandra deep observations in the AEGIS, GOODS-North and GOODS-South to compile uniformly selected samples of moderate luminosity X-ray AGN [L_X(2-10keV) = 1e41-1e44erg/s] at redshifts 0.1, 0.3 and 0.8. It is found that the fraction of AGN hosted by red versus blue galaxies does not change with redshift. Also, the X-ray luminosity density associated with either red or blue AGN hosts remains nearly constant since z=0.8. X-ray AGN represent a roughly fixed fraction of the space density of galaxies of given optical luminosity at all redshifts probed by our samples. In contrast the fraction of X-ray AGN among galaxies of a given stellar mass decreases with decreasing redshift. These findings suggest that the same process or combination of processes for fueling supermassive black holes are in operation in the last 5 Gyrs of cosmic time. The data are consistent with a picture in which the drop of the accretion power during that period (1dex since z=0.8) is related to the decline of the space density of available AGN hosts, as a result of the evolution of the specific star-formation rate of the overall galaxy population. Scenarios which attribute the evolution of moderate luminosity AGN since z approx 1 to changes in the suppermassive black hole accretion mode are not favored by our results.
The mid-infrared to ultraviolet (0.1 -- 10 $mu m$) spectral energy distribution (SED) shapes of 407 X-ray-selected radio-quiet type 1 AGN in the wide-field ``Cosmic Evolution Survey (COSMOS) have been studied for signs of evolution. For a sub-sample of 200 radio-quiet quasars with black hole mass estimates and host galaxy corrections, we studied their mean SEDs as a function of a broad range of redshift, bolometric luminosity, black hole mass and Eddington ratio, and compared them with the Elvis et al. (1994, E94) type 1 AGN mean SED. We found that the mean SEDs in each bin are closely similar to each other, showing no statistical significant evidence of dependence on any of the analyzed parameters. We also measured the SED dispersion as a function of these four parameters, and found no significant dependencies. The dispersion of the XMM-COSMOS SEDs is generally larger than E94 SED dispersion in the ultraviolet, which might be due to the broader ``window function for COSMOS quasars, and their X-ray based selection.
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