Using Chandra X-ray observations in the All-Wavelength Extended Groth Strip International Survey (AEGIS) we identify 241 X-ray-selected active galactic nuclei (AGNs; L 2 Y 10 > 10 42 ergs s À1 ) and study the properties of their host galaxies in the range 0:4 < z < 1:4. By making use of infrared photometry from the Palomar Observatory and BRI imaging from the Canada-France-Hawaii Telescope, we estimate AGN host galaxy stellar masses and show that both stellar mass and photometric redshift estimates (where necessary) are robust to the possible contamination from AGNs in our X-ray-selected sample. Accounting for the photometric and X-ray sensitivity limits of the survey, we construct the stellar mass function of X-ray-selected AGN host galaxies and find that their abundance decreases by a factor of $2 since z $ 1 but remains roughly flat as a function of stellar mass. We compare the abundance of AGN hosts to the rate of star formation quenching observed in the total galaxy population. If the timescale for X-ray-detectable AGN activity is roughly 0.5Y1 Gyr, as suggested by black hole demographics and recent simulations, then we deduce that the inferred AGN ''trigger'' rate matches the star formation quenching rate, suggesting a link between these phenomena. However, given the large range of nuclear accretion rates we infer for the most massive and red hosts, X-ray-selected AGNs may not be directly responsible for quenching star formation.

We explore the role of active galactic nuclei (AGN) in establishing and/or maintaining the bimodal colour distribution of galaxies by quenching their star formation and hence, causing their transition from the blue to the red cloud. Important tests for this scenario include (i) the X-ray properties of galaxies in the transition zone between the two clouds and (ii) the incidence of AGN in post-starbursts, i.e. systems observed shortly after (<1 Gyr) the termination of their star formation. We perform these tests by combining deep Chandra observations with multiwavelength data from the All-wavelength Extended Groth strip International Survey (AEGIS). Stacking the X-ray photons at the positions of galaxies (0.4 < z < 0.9) not individually detected at X-ray wavelengths suggests a population of obscured AGN among sources in the transition zone and in the red cloud. Their mean X-ray and mid-infrared (IR) properties are consistent with moderately obscured low-luminosity AGN, Compton thick sources or a mix of both. Morphologies show that major mergers are unlikely to drive the evolution of this population but minor interactions may play a role. The incidence of obscured AGN in the red cloud (both direct detections and stacking results) suggests that black hole (BH) accretion outlives the termination of the star formation. This is also supported by our finding that poststarburst galaxies at z ≈ 0.8 and AGN are associated, in agreement with recent results at low z. A large fraction of post-starbursts and red cloud galaxies show evidence for at least moderate levels of AGN obscuration. This implies that if AGN outflows cause the colour transformation of galaxies, then some nuclear gas and dust clouds either remain unaffected or relax to the central galaxy regions after quenching their star formation.

We use morphological information of X‐ray selected active galactic nuclei (AGN) hosts to set limits on the fraction of the accretion density of the Universe at z≈ 1 that is not likely to be associated with major mergers. Deep X‐ray observations are combined with high‐resolution optical data from the Hubble Space Telescope in the All‐wavelength Extended Groth strip International Survey, Great Observatories Origins Deep Survey (GOODS) North and GOODS South fields to explore the morphological breakdown of X‐ray sources in the redshift interval 0.5 < z < 1.3. The sample is split into discs, early‐type bulge‐dominated galaxies, peculiar systems and point sources in which the nuclear source outshines the host galaxy. The X‐ray luminosity function and luminosity density of AGN at z≈ 1 are then calculated as a function of morphological type. We find that disc‐dominated hosts contribute 30 ± 9 per cent to the total AGN space density and 23 ± 6 per cent to the luminosity density at z≈ 1. We argue that AGN in disc galaxies are most likely fuelled not by major merger events but by minor interactions or internal instabilities. We find evidence that these mechanisms may be more efficient in producing luminous AGN compared to predictions for the stochastic fuelling of massive black holes in disc galaxies.

We visually and quantitatively determine the host galaxy morphologies of 94 intermediate-redshift (0.2 ≤ ) active galactic nuclei (AGNs), selected using Chandra X-ray data and Spitzer mid-infrared data in the z ! 1.2 Extended Groth Strip. Using recently developed morphology measures, the second-order moment of the brightest 20% of a galaxy's flux ( ) and the Gini coefficient, we find that X-ray-selected AGNs mostly reside in E/S0/ M 20 Sa galaxies ( ), while IR-selected AGNs show no clear preference for host morphology. X-ray-selected ϩ11 53 % Ϫ10AGN hosts are members of close pairs more often than the field population by a factor of , but most 3.3 ‫ע‬ 1.4 of these pair members appear to be undisturbed early-type galaxies and do not tend to show direct evidence of gravitational perturbations or interactions. Thus, the activation mechanism for AGN activity remains unknown, even for pair members.

We assess the effects of simulated active galactic nuclei (AGN) on the colour and morphology measurements of their host galaxies. To test the morphology measurements, we select a sample of galaxies not known to host AGN and add a series of point sources scaled to represent specified fractions of the observed V‐band light detected from the resulting systems; we then compare morphology measurements of the simulated systems to measurements of the original galaxies. AGN contributions >rsim 20 per cent bias most of the morphology measurements tested, though the extent of the apparent bias depends on the morphological characteristics of the original galaxies. We test colour measurements by adding to non‐AGN galaxy spectra a quasar spectrum scaled to contribute specified fractions of the rest‐frame B‐band light detected from the resulting systems. A quasar fraction of 5 per cent can move the NUV −r colour of an elliptical galaxy from the ultraviolet–optical red sequence to the green valley, and 20 per cent can move it into the blue cloud. Combining the colour and morphology results, we find that a galaxy/AGN system with an AGN contribution >rsim 20 per cent may appear bluer and more bulge dominated than the underlying galaxy. We conclude that (1) bulge‐dominated, E/S0/Sa, and early‐type morphology classifications are accurate for red AGN host galaxies and may be accurate for blue host galaxies, unless the AGN manifests itself as a well‐defined point source and (2) although highly unobscured AGN, such as the quasar used for our experiments, can significantly bias the measured colours of AGN host galaxies, it is possible to identify such systems by examining optical images of the hosts for the presence of a point source and/or measuring the level of nuclear obscuration.