We constrain the total accreted mass density in supermassive black holes at z>6, inferred via the upper limit derived from the integrated X-ray emission from a sample of photometrically selected galaxy candidates. Studying galaxies obtained from the
deepest Hubble Space Telescope images combined with the Chandra 4 Msec observations of the Chandra Deep Field South, we achieve the most restrictive constraints on total black hole growth in the early Universe. We estimate an accreted mass density <1000Mo Mpc^-3 at z~6, significantly lower than the previous predictions from some existing models of early black hole growth and earlier prior observations. These results place interesting constraints on early black growth and mass assembly by accretion and imply one or more of the following: (1) only a fraction of the luminous galaxies at this epoch contain active black holes; (2) most black hole growth at early epochs happens in dusty and/or less massive - as yet undetected - host galaxies; (3) there is a significant fraction of low-z interlopers in the galaxy sample; (4) early black hole growth is radiatively inefficient, heavily obscured and/or is due to black hole mergers as opposed to accretion or (5) the bulk of the black hole growth occurs at late times. All of these possibilities have important implications for our understanding of high redshift seed formation models.
Using multiwavelength surveys of active galactic nuclei across a wide range of bolometric luminosities (10^{43}<L_{bol}(erg/s<5x10^{46}) and redshifts (0<z<3), we find a strong, redshift-independent correlation between the AGN luminosity and the frac
tion of host galaxies undergoing a major merger. That is, only the most luminous AGN phases are connected to major mergers, while less luminous AGN appear to be driven by secular processes. Combining this trend with AGN luminosity functions to assess the overall cosmic growth of black holes, we find that ~50% by mass is associated with major mergers, while only 10% of AGN by number, the most luminous, are connected to these violent events. Our results suggest that to reach the highest AGN luminosities -where the most massive black holes accreted the bulk of their mass - a major merger appears to be required. The luminosity dependence of the fraction of AGN triggered by major mergers can successfully explain why the observed scatter in the M-sigma relation for elliptical galaxies is significantly lower than in spirals. The lack of a significant redshift dependence of the L_{bol}-f_{merger} relation suggests that downsizing, i.e., the general decline in AGN and star formation activity with decreasing redshift, is driven by a decline in the frequency of major mergers combined with a decrease in the availability of gas at lower redshifts.
We constrain the number density and evolution of Compton-thick Active Galactic Nuclei (AGN), and their contribution to the extragalactic X-ray background. In the local Universe we use the wide area surveys from the Swift and INTEGRAL satellites, whil
e for high redshifts we explore candidate selections based on mid-IR parameters. We present the properties of a sample of 211 heavily-obscured AGN candidates in the Extended Chandra Deep Field-South (ECDF-S) selecting objects with f24/fR>1000 and R-K>4.5. The X-ray to mid-IR ratios for these sources are significantly larger than that of star-forming galaxies and ~2 orders of magnitude smaller than for the general AGN population, suggesting column densities of NH>5x10^24 cm^-2. The space density of CT AGN at z~2 derived from these observations is ~10^-5 Mpc^{-3}, finding a strong evolution in the number of LX>10^44 erg/s sources from z=1.5 to 2.5.
We study the properties of a sample of 211 heavily-obscured Active Galactic Nucleus (AGN) candidates in the Extended Chandra Deep Field-South selecting objects with f_24/f_R>1000 and R-K>4.5. Of these, 18 were detected in X-rays and found to be obscu
red AGN with neutral hydrogen column densities of ~10^23 cm^-2. In the X-ray undetected sample, the following evidence suggests a large fraction of heavily-obscured (Compton Thick) AGN: (i) The stacked X-ray signal of the sample is strong, with an observed ratio of soft to hard X-ray counts consistent with a population of ~90% heavily obscured AGN combined with 10% star-forming galaxies. (ii) The X-ray to mid-IR ratios for these sources are significantly larger than that of star-forming galaxies and ~2 orders of magnitude smaller than for the general AGN population, suggesting column densities of N_H>5x10^24 cm^-2. (iii) The Spitzer near- and mid-IR colors of these sources are consistent with those of the X-ray-detected sample if the effects of dust self-absorption are considered. Spectral fitting to the rest-frame UV/optical light (dominated by the host galaxy) returns stellar masses of ~10^11 M_sun and <E(B-V)> =0.5, and reveals evidence for a significant young stellar population, indicating that these sources are experiencing considerable star-formation. This sample of heavily-obscured AGN candidates implies a space density at z~2 of ~10^-5 Mpc^-3, finding a strong evolution in the number of L_X>10^44 erg/s sources from z=1.5 to 2.5, possibly consistent with a short-lived heavily-obscured phase before an unobscured quasar is visible.
We constrain the number density and evolution of Compton-thick Active Galactic Nuclei (AGN). In the local Universe we use the wide area surveys from the Swift and INTEGRAL satellites, while for high redshifts we explore candidate selections based on
a combination of X-ray and mid-IR parameters. We find a significantly lower space density of Compton-thick AGN in the local Universe than expected from published AGN population synthesis models to explain the X-ray background. This can be explained by the numerous degeneracies in the parameters of those models; we use the high-energy surveys described here to remove those degeneracies. We show that only direct observations of CT AGN can currently constrain the number of heavily-obscured supermassive black holes. At high redshift, the inclusion of IR-selected Compton-thick AGN candidates leads to a much higher space density, implying (a) a different (steeper) evolution for these sources compared to less-obscured AGN, (b) that the IR selection includes a large number of interlopers, and/or (c) that there is a large number of reflection-dominated AGN missed in the INTEGRAL and Swift observations. The contribution of CT AGN to the X-ray background is small, ~9%, with a comparable contribution to the total cosmic accretion, unless reflection-dominated CT AGN significantly outnumber transmission-dominated CT AGN, in which case their contribution can be much higher. Using estimates derived here for the accretion luminosity over cosmic time we estimate the local mass density in supermassive black holes and find a good agreement with available constraints for an accretion efficiency of ~10%. Transmission-dominated CT AGN contribute only ~8% to total black hole growth.
We present the first results of our optical spectroscopy program aimed to provide redshifts and identifications for the X-ray sources in the Extended Chandra Deep Field South. A total of 339 sources were targeted using the IMACS spectrograph at the M
agellan telescopes and the VIMOS spectrograph at the VLT. We measured redshifts for 186 X-ray sources, including archival data and a literature search. We find that the AGN host galaxies have on average redder rest-frame optical colors than non-active galaxies, and that they live mostly in the green valley. The dependence of the fraction of AGN that are obscured on both luminosity and redshift is confirmed at high significance and the observed AGN space density is compared with the expectations from existing luminosity functions. These AGN show a significant difference in the mid-IR to X-ray flux ratio for obscured and unobscured AGN, which can be explained by the effects of dust self-absorption on the former. This difference is larger for lower luminosity sources, which is consistent with the dust opening angle depending on AGN luminosity.
