We present new observational determinations of the evolution of the 2-10keV X-ray luminosity function (XLF) of AGN. We utilise data from a number of surveys including both the 2Ms Chandra Deep Fields and the AEGIS-X 200ks survey, enabling accurate measurements of the evolution of the faint end of the XLF. We combine direct, hard X-ray selection and spectroscopic follow-up or photometric redshift estimates at z<1.2 with a rest-frame UV colour pre-selection approach at higher redshifts to avoid biases associated with catastrophic failure of the photometric redshifts. Only robust optical counterparts to X-ray sources are considered using a likelihood ratio matching technique. A Bayesian methodology is developed that considers redshift probability distributions, incorporates selection functions for our high redshift samples, and allows robust comparison of different evolutionary models. We find that the XLF retains the same shape at all redshifts, but undergoes strong luminosity evolution out to z~1, and an overall negative density evolution with increasing redshift, which thus dominates the evolution at earlier times. We do not find evidence that a Luminosity-Dependent Density Evolution, and the associated flattening of the faint-end slope, is required to describe the evolution of the XLF. We find significantly higher space densities of low-luminosity, high-redshift AGN than in prior studies, and a smaller shift in the peak of the number density to lower redshifts with decreasing luminosity. The total luminosity density of AGN peaks at z=1.2+/-0.1, but there is a mild decline to higher redshifts. We find >50% of black hole growth takes place at z>1, with around half in Lx<10^44 erg/s AGN.Comment: 24 pages, 13 figures, accepted for publication in MNRA

Using HST /WFC3 imaging taken as part of the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS), we examine the role that major galaxy mergers play in triggering active galactic nuclei (AGN) activity at z ∼ 2. Our sample consists of 72 moderate-luminosity (L X ∼ 10 42−44 erg s −1 ) AGN at 1.5 < z < 2.5 that are selected using the 4 Msec Chandra observations in the Chandra Deep Field South, the deepest X-ray observations to date. Employing visual classifications, we have analyzed the rest-frame optical morphologies of the AGN host galaxies and compared them to a massmatched control sample of 216 non-active galaxies at the same redshift. We find that most of the AGN reside in disk galaxies (51.4 +5.8 −5.9 %), while a smaller percentage are found in spheroids (27.8 +5.8 −4.6 %). Roughly 16.7 +5.3 −3.5 % of the AGN hosts have highly disturbed morphologies and appear to be involved in a major merger or interaction, while most of the hosts (55.6 +5.6 −5.9 %) appear relatively relaxed and undisturbed. These fractions are statistically consistent with the fraction of control galaxies that show similar morphological disturbances. These results suggest that the hosts of moderate-luminosity AGN are no more likely to be involved in an ongoing merger or interaction relative to non-active galaxies of similar mass at z ∼ 2. The high disk fraction observed among the AGN hosts also appears to be at odds with predictions that merger-driven accretion should be the dominant AGN fueling mode at z ∼ 2, even at moderate X-ray luminosities. Although we cannot rule out that minor mergers are responsible for triggering these systems, the presence of a large population of relatively undisturbed disk-like hosts suggests that secular processes play a greater role in fueling AGN activity at z ∼ 2 than previously thought.

We discuss the relationship between rest-frame color and optical luminosity for X-ray sources in the range 0.6 < z < 1.4 selected from the Chandra survey of the Extended Groth Strip (EGS). These objects are almost exclusively active galactic nuclei (AGN). While there are a few luminous QSOs, most are relatively weak or obscured AGN whose optical colors should be dominated by host galaxy light. The vast majority of AGN hosts at z ∼ 1 are luminous and red, with very few objects fainter than M B = −20.5 or bluer than U − B = 0.6. This places the AGN in a distinct region of color-magnitude space, on the "red sequence" or at the top of the "blue cloud", with many in between these two modes in galaxy color. A key stage in the evolution of massive galaxies is when star formation is quenched, resulting in a migration from the blue cloud to the red sequence. Our results are consistent with scenarios in which AGN either cause or maintain this quenching. The large numbers of red sequence AGN imply that strong, ongoing star formation is not a necessary ingredient for AGN activity, as black hole accretion appears often to persist after star formation has been terminated.

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We present a new method for determining the sensitivity of X‐ray imaging observations, which correctly accounts for the observational biases that affect the probability of detecting a source of a given X‐ray flux, without the need to perform a large number of time‐consuming simulations. We use this new technique to estimate the X‐ray source counts in different spectral bands (0.5–2, 0.5–10, 2–10 and 5–10 keV) by combining deep pencil‐beam and shallow wide‐area Chandra observations. The sample has a total of 6295 unique sources over an area of 11.8 deg2 and is the largest used to date to determine the X‐ray number counts. We determine, for the first time, the break flux in the 5–10 keV band, in the case of a double power‐law source count distribution. We also find an upturn in the 0.5–2 keV counts at fluxes below about 6 × 10−17 erg s−1 cm−2. We show that this can be explained by the emergence of normal star‐forming galaxies which dominate the X‐ray population at faint fluxes. The fraction of the diffuse X‐ray background resolved into point sources at different spectral bands is also estimated. It is argued that a single population of Compton thick active galactic nuclei (AGN) cannot be responsible for the entire unresolved X‐ray background in the energy range 2–10 keV.

In this the first of a series of Letters, we present a panchromatic data set in the Extended Groth Strip region of the sky. Our survey, the All-Wavelength Extended Groth Strip International Survey (AEGIS), aims to study the physical properties and evolutionary processes of galaxies at . It includes the following deep, wide-field imaging data sets: z ∼ 1 Chandra/ACIS X-ray, GALEX ultraviolet, CFHT/MegaCam Legacy Survey optical, CFHT/CFH12K optical, Hubble Space Telescope/ACS optical and NICMOS near-infrared, Palomar/WIRC near-infrared, Spitzer/IRAC mid-infrared, Spitzer/MIPS far-infrared, and VLA radio continuum. In addition, this region of the sky has been targeted for extensive spectroscopy using the Deep Imaging Multi-Object Spectrograph (DEIMOS) on the Keck II 10 m telescope. Our survey is compared to other large multiwavelength surveys in terms of depth and sky coverage.

We measure the clustering of nonquasar X-ray active galactic nucleus (AGN) at z = 0.7-1.4 in the AEGIS field. Using the cross-correlation of 113 Chandra-selected AGN, with a median log L X = 42.8 erg s −1 , with ∼5000 DEEP2 galaxies, we find that the X-ray AGNs are fitted by a power law with a clustering scale length of r 0 = 5.95 ± 0.90 h −1 Mpc and slope γ = 1.66 ± 0.22. X-ray AGNs have a similar clustering amplitude as red, quiescent and "green" transition galaxies at z ∼ 1 and are significantly more clustered than blue, starforming galaxies. The X-ray AGN clustering strength is primarily determined by the host galaxy color; AGNs in red host galaxies are significantly more clustered than AGNs in blue host galaxies, with a relative bias that is similar to that of red to blue DEEP2 galaxies. We detect no dependence of clustering on optical brightness, X-ray luminosity, or hardness ratio within the ranges probed here. We find evidence for galaxies hosting X-ray AGN to be more clustered than a sample of galaxies with matching joint optical color and magnitude distributions. This implies that galaxies hosting X-ray AGN are more likely to reside in groups and more massive dark matter halos than galaxies of the same color and luminosity without an X-ray AGN. In comparison to optically selected quasars in the DEEP2 fields, we find that X-ray AGNs at z ∼ 1 are more clustered than optically selected quasars (with a 2.6σ significance) and therefore may reside in more massive dark matter halos. Our results are consistent with galaxies undergoing a quasar phase while in the blue cloud before settling on the red sequence with a lower-luminosity X-ray AGN, if they are similar objects at different evolutionary stages.

Mid-infrared observations of Active Galactic Nuclei (AGN) are important for understanding of the physical conditions around the central accretion engines. Chandra and XMM-Newton X-ray observations of a 300 arcmin 2 region in the Extended Groth Strip are used to select a sample of ∼ 150 AGN. The Spitzer instruments IRAC and MIPS detect 68-80% of these sources, which show a wide range of mid-infrared properties. About 40% of the sources have red powerlaw spectral energy distributions (f ν ∝ ν α , α < 0) in the 3.6 − 8 µm IRAC bands. In these sources the central engine dominates the emission at both Xray and IR wavelengths. Another 40% of the sources have blue mid-IR spectral energy distributions (α > 0) with their infrared emission dominated by the host galaxy; the remaining 20% are not well-fit by a power law. Published IRAC color criteria for AGN select most of the red sources, but only some of the blue sources. As with all other known methods, selecting AGN with mid-IR colors will not produce a sample that is simultaneously complete and reliable. The IRAC SED type does not directly correspond to X-ray spectral type (hard/soft). The mid-IR properties of X-ray-detected Lyman-break, radio, submillimeter, and optically-faint sources vary widely and, for the most part, are not distinct from -2those of the general X-ray/infrared source population. X-ray sources emit 6-11% of the integrated mid-IR light, making them significant contributors to the cosmic infrared background.