E. S. Laird

The evolution of the hard X-ray luminosity function of AGN

We present new observational determinations of the evolution of the 2–10 keV X-ray luminosity function (XLF) of active galactic nuclei (AGN). We utilize data from a number of surveys including both the 2 Ms Chandra Deep Fields and the AEGIS-X 200 ks 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 50 per cent of black hole growth takes place at z > 1 , with around half in L_X < 10^(44) erg s^(−1) AGN.

CANDELS: Constraining the AGN-Merger Connection with Host Morphologies at z 2

Using Hubble Space Telescope/WFC3 imaging taken as part of the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey, we examine the role that major galaxy mergers play in triggering active galactic nucleus (AGN) activity at z ~ 2. Our sample consists of 72 moderate-luminosity (L X ~ 1042-44 erg s–1) AGNs at 1.5 < z < 2.5 that are selected using the 4 Ms 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 mass-matched control sample of 216 non-active galaxies at the same redshift. We find that most of the AGNs 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 AGNs 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 the stochastic accretion of gas plays a greater role in fueling AGN activity at z ~ 2 than previously thought.

AEGIS-X: the Chandra Deep Survey of the Extended Groth Strip

We present the AEGIS-X survey, a series of deep Chandra ACIS-I observations of the Extended Groth Strip. The survey comprises pointings at eight separate positions, each with nominal exposure of 200 ks, covering a total area of approximately 0.67 deg2 in a strip of length 2 degrees. We describe in detail an updated version of our data reduction and point-source-detection algorithms used to analyze these data. A total of 1325 band-merged sources have been found to a Poisson probability limit of 4 × 10–6, with limiting fluxes of 5.3 × 10–17 erg cm2 s–1 in the soft (0.5-2 keV) band and 3.8 × 10–16 erg cm–2 s–1 in the hard (2-10 keV) band. We present simulations verifying the validity of our source-detection procedure and showing a very small, <1.5%, contamination rate from spurious sources. Optical/NIR counterparts have been identified from the DEEP2, CFHTLS, and Spitzer/Infrared Array Camera (IRAC) surveys of the same region. Using a likelihood ratio method, we find optical counterparts for 76% of our sources, complete to R AB = 24.1, and, of the 66% of the sources that have IRAC coverage, 94% have a counterpart to a limit of 0.9 μJy at 3.6 μm (m AB = 23.8). After accounting for (small) positional offsets in the eight Chandra fields, the astrometric accuracy of Chandra positions is found to be 08 rms; however, this number depends both on the off-axis angle and the number of detected counts for a given source. All data products described in this paper are made available via a public Web site.

Host galaxy morphologies of X-ray selected AGN: assessing the significance of different black hole fuelling mechanisms to the accretion density of the Universe at z∼ 1.

We use morphological information of X-ray selected 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 AEGIS, GOODS North and GOODS So uth fields to explore the morphological breakdown of X-ray sources in the redshift interval 0:5 10 44 ergs 1 ) compared to predictions for the stochastic fueling of

A new method for determining the sensitivity of X-ray imaging observations and the X-ray number counts

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 widearea Chandra observations. The sample has a total of 6295 unique sources over an area of 11.8 deg 2 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.

The role of AGN in the colour transformation of galaxies at redshifts z≈ 1

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 post-starburst 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.

A deep Chandra survey of the Groth Strip – I. The X-ray data

We present the results of a 200-ks Chandra observation of part of the Groth Strip region, using the ACIS-I instrument. We present a relatively simple method for the detection of point sources and calculation of limiting sensitivities, which we argue is at least as sensitive and more self-consistent than previous methods presented in the literature. A total of 158 distinct X-ray sources are included in our point-source catalogue in the ACIS-I area. The number counts show a relative dearth of X-ray sources in this region. For example, at a flux limit of 10 -15 erg cm -2 s -1 , around 20 per cent more soft-band sources are detected in the HDF-N and almost 50 per cent more in the ELAIS-N1 field, which we have analysed by the same method for comparison. We find, however, that these differences are consistent with Poisson variations at <2σ significance, and therefore there is no evidence for cosmic variance based on these number counts alone. We determine the average spectra of the objects and find a marked difference between the soft-band-selected sources, which have r = 1.9 typical of unobscured active galactic nuclei (AGN), and the hard-band-selected sources, which have r = 1.0. Reassuringly, the sample as a whole has a mean spectrum of Γ = 1.4 ± 0.1, the same as the X-ray background. None the less, our results imply that the fraction of sources with significant obscuration is only ∼25 per cent, much less than predicted by standard AGN population synthesis models. This is confirmed by direct spectral fitting, with only a handful of objects showing evidence for absorption. After accounting for absorption, all objects are consistent with a mean intrinsic spectrum of r = 1.76 ± 0.08, very similar to local Seyfert galaxies. The survey area is distinguished by having outstanding multiwaveband coverage. Comparison with these observations and detailed discussion of the X-ray source properties will be presented in future papers.

A statistical relation between the X-ray spectral index and Eddington ratio of active galactic nuclei in deep surveys

We present an investigation into how well the properties of the accretion flow on to a supermassive black hole may be coupled to those of the overlying hot corona. To do so, we specifically measure the characteristic spectral index, Γ, of a power-law energy distribution, over an energy range of 2–10 keV, for X-ray selected, broad-lined radio-quiet active galactic nuclei (AGN) up to z ∼ 2 in Cosmic Evolution Survey (COSMOS) and Extended Chandra Deep Field South (E-CDF-S). We test the previously reported dependence between Γ and black hole mass, full width at half-maximum (FWHM) and Eddington ratio using a sample of AGN covering a broad range in these parameters based on both the Mg II and Hα emission lines with the later afforded by recent near-infrared spectroscopic observations using Subaru/Fibre Multi Object Spectrograph. We calculate the Eddington ratios, λ_(Edd), for sources where a bolometric luminosity (L_(Bol)) has been presented in the literature, based on spectral energy distribution fitting, or, for sources where these data do not exist, we calculate L_(Bol) using a bolometric correction to the X-ray luminosity, derived from a relationship between the bolometric correction and L_X/L_(3000). From a sample of 69 X-ray bright sources (>250 counts), where Γ can be measured with greatest precision, with an estimate of L_(Bol), we find a statistically significant correlation between Γ and λ_(Edd), which is highly significant with a chance probability of 6.59× 10^(−8). A statistically significant correlation between Γ and the FWHM of the optical lines is confirmed, but at lower significance than with λ_(Edd) indicating that λ_(Edd) is the key parameter driving conditions in the corona. Linear regression analysis reveals that Γ = (0.32 ± 0.05) log_(10)λ_(Edd) + (2.27 ± 0.06) and Γ = (−0.69 ± 0.11) log_(10)(FWHM/km s^(−1)) + (4.44 ± 0.42). Our results on Γ–λ_(Edd) are in very good agreement with previous results. While the Γ–λ_(Edd) relationship means that X-ray spectroscopy may be used to estimate black hole accretion rate, considerable dispersion in the correlation does not make this viable for single sources, however could be valuable for large X-ray spectral samples, such as those to be produced by eROSITA.

AEGIS: DEMOGRAPHICS OF X-RAY AND OPTICALLY SELECTED ACTIVE GALACTIC NUCLEI

We develop a new diagnostic method to classify galaxies into active galactic nucleus (AGN) hosts, star-forming galaxies, and absorption-dominated galaxies by combining the [O III]/Hβ ratio with rest-frame U – B color. This can be used to robustly select AGNs in galaxy samples at intermediate redshifts (z 1044 erg s-1 in our sample are not detected in our 200 ks Chandra images, most likely due to moderate or heavy absorption by gas near the AGN. The 2-7 keV detection rate of Seyfert 2s at z ~ 0.6 suggests that their column density distribution and Compton-thick fraction are similar to that of local Seyferts. Multiple sample selection techniques are needed to obtain as complete a sample as possible.

Infrared Spectrograph Spectroscopy and Multi-Wavelength Study of Luminous Star-Forming Galaxies at z ≃ 1.9

We analyze a sample of galaxies chosen to have F_(24μm) > 0.5 mJy and satisfy a certain IRAC color criterion. Infrared Spectrograph (IRS) spectra yield redshifts, spectral types, and polycyclic aromatic hydrocarbons (PAH) luminosities, to which we add broadband photometry from optical through IRAC wavelengths, MIPS from 24-160 μm, 1.1 mm, and radio at 1.4 GHz. Stellar population modeling and IRS spectra together demonstrate that the double criteria used to select this sample have efficiently isolated massive star-forming galaxies at z ~ 1.9. This is the first starburst (SB)-dominated ultraluminous infrared galaxies (ULIRG) sample at high redshift with total infrared luminosity measured directly from FIR and millimeter photometry, and as such gives us the first accurate view of broadband spectral energy distributions for SB galaxies at extremely high luminosity and at all wavelengths. Similar broadband data are assembled for three other galaxy samples—local SB galaxies, local active galactic nucleus (AGN)/ULIRGs, and a second 24 μm-luminous z ~ 2 sample dominated by AGN. L_(PAH)/L_(IR) for the new z ~ 2 SB sample is the highest ever seen, some three times higher than in local SBs, whereas in AGNs this ratio is depressed below the SB trend, often severely. Several pieces of evidence imply that AGNs exist in this SB-dominated sample, except two of which even host very strong AGN, while they still have very strong PAH emission. The Advanced Camera for Surveys images show that most objects have very extended morphologies in the rest-frame ultraviolet band, thus extended distribution of PAH molecules. Such an extended distribution prevents further destruction PAH molecules by central AGNs. We conclude that objects in this sample are ULIRGs powered mainly by SB; and the total infrared luminosity density contributed by this type of objects is 0.9-2.6 × 10^7 L_☉ Mpc^(–3).