The location and kinematics of the emission line regions in Active Galactic Nuclei

This thesis contains the results from a study of the optical emission lines of various samples of active galactic nuclei (AGN). We have used three separate techniques to determine the precise kinematics and the relative location of the emission line regions of AGN: a detailed study of a small sample (10) of AGN, a study of the general emission line properties of two larger samples, and the interpretation of the results from observations using numeric photoionisation models. Although we present results on both the broad line and narrow line regions of AGN the main drive behind this work is the study of the poorly defined intermediate line region using emission from highly ionised species (high ionisation emission lines). Our results derived from observations indicate that a significant proportion of the emission from these species is produced within the central void of the putative dusty torus and that emission from more highly ionised species increases at radii closer to the central engine of the AGN. Furthermore, we find strong evidence that the intermediate region traced by these high ionisation emission lines is generally outflowing from the central engine but there is little evidence of any significant acceleration or deceleration of this material between the inner and outer regions traced by the higher and lower ionisation lines, respectively. Our results derived from numerical photoionisation codes support a model in which this outflowing material responsible for the high ionisation line emission is launched from the inner edge of the the dusty torus

Black hole variability and the star formation-active galactic nucleus connection : do all star-forming galaxies host an active galactic nucleus?

We investigate the effect of active galactic nucleus (AGN) variability on the observed connection between star formation and black hole accretion in extragalactic surveys. Recent studies have reported relatively weak correlations between observed AGN luminosities and the properties of AGN hosts, which has been interpreted to imply that there is no direct connection between AGN activity and star formation. However, AGNs may be expected to vary significantly on a wide range of timescales (from hours to Myr) that are far shorter than the typical timescale for star formation (gsim100 Myr). This variability can have important consequences for observed correlations. We present a simple model in which all star-forming galaxies host an AGN when averaged over ~100 Myr timescales, with long-term average AGN accretion rates that are perfectly correlated with the star formation rate (SFR). We show that reasonable prescriptions for AGN variability reproduce the observed weak correlations between SFR and L AGN in typical AGN host galaxies, as well as the general trends in the observed AGN luminosity functions, merger fractions, and measurements of the average AGN luminosity as a function of SFR. These results imply that there may be a tight connection between AGN activity and SFR over galaxy evolution timescales, and that the apparent similarities in rest-frame colors, merger rates, and clustering of AGNs compared to "inactive" galaxies may be due primarily to AGN variability. The results provide motivation for future deep, wide extragalactic surveys that can measure the distribution of AGN accretion rates as a function of SFR

Inferring a difference in the star-forming properties of lower versus higher X-ray luminosity AGNs

We explore the distribution of RMS ≡ SFR/SFRMS (where SFRMS is the star formation rate of ‘main-sequence’ star-forming galaxies) for AGN hosts at z = 1. We split our sample into two bins of X-ray luminosity divided at LX = 2 × 1043 erg s−1 to investigate whether the RMS distribution changes as a function of AGN power. Our main results suggest that, when the RMS distribution of AGN hosts is modelled as a log-normal distribution (i.e. the same shape as that of MS galaxies), galaxies hosting more powerful X-ray AGNs (i.e. LX > 2 × 1043 erg s−1) display a narrower RMS distribution that is shifted to higher values compared to their lower LX counterparts. In addition, we find that more powerful X-ray AGNs have SFRs that are more consistent with that of MS galaxies compared to lower LX AGNs. Despite this, the mean SFRs (as opposed to RMS) measured from these distributions are consistent with the previously observed flat relationship between SFR and LX. Our results suggest that the typical star-forming properties of AGN hosts change with LX, and that more powerful AGNs typically reside in more MS-like star-forming galaxies compared to lower LX AGNs

A wide search for obscured active galactic nuclei using XMM-Newton and WISE

Heavily obscured and Compton-thick active galactic nuclei (AGNs) are missing even in the deepest X-ray surveys, and indirect methods are required to detect them. Here we use a combination of the XMM–Newton serendipitous X-ray survey with the optical Sloan Digital Sky Survey (SDSS), and the infrared WISE all-sky survey in order to check the efficiency of the low X-ray-to-infrared luminosity selection method in finding heavily obscured AGNs. We select the sources which are detected in the hard X-ray band (2–8 keV), and also have a redshift determination (photometric or spectroscopic) in the SDSS catalogue. We match this sample with the WISE catalogue, and fit the spectral energy distributions of the 2844 sources which have three, or more, photometric data points in the infrared. We then select the heavily obscured AGN candidates by comparing their 12 μm luminosity to the observed 2–10 keV X-ray luminosity and the intrinsic relation between the X-ray and the mid-infrared luminosities. With this approach, we find 20 candidate heavily obscured AGNs and we then examine their X-ray and optical spectra. Of the 20 initial candidates, we find nine (64 per cent; out of the 14, for which X-ray spectra could be fitted) based on the X-ray spectra, and seven (78 per cent; out of the nine detected spectroscopically in the SDSS) based on the [O III] line fluxes. Combining all criteria, we determine the final number of heavily obscured AGNs to be 12–19, and the number of Compton-thick AGNs to be 2–5, showing that the method is reliable in finding obscured AGNs, but not Compton thick. However, those numbers are smaller than what would be expected from X-ray background population synthesis models, which demonstrates how the optical–infrared selection and the scatter of the Lx-LMIR relation limit the efficiency of the method. Finally, we test popular obscured AGN selection methods based on mid-infrared colours, and find that the probability of an AGN to be selected by its mid-infrared colours increases with the X-ray luminosity. The (observed) X-ray luminosities of heavily obscured AGNs are relatively low (L2−10keV<1044ergs−1), even though most of them are located in the ‘quasi stellar object (QSO) locus’. However, a selection scheme based on a relatively low X-ray luminosity and mid-infrared colours characteristic of QSOs would not select ∼25 per cent of the heavily obscured AGNs of our sample

The evolving AGN duty cycle in galaxies since z ∼ 3 as encoded in the X-ray luminosity function

We present a new modeling of the X-ray luminosity function (XLF) of active galactic nuclei (AGNs) out to z ~ 3, dissecting the contributions of main-sequence (MS) and starburst (SB) galaxies. For each galaxy population, we convolved the observed galaxy stellar mass (M sstarf) function with a grid of M sstarf-independent Eddington ratio (λ EDD) distributions, normalized via empirical black hole accretion rate (BHAR) to star formation rate (SFR) relations. Our simple approach yields an excellent agreement with the observed XLF since z ~ 3. We find that the redshift evolution of the observed XLF can only be reproduced through an intrinsic flattening of the λ EDD distribution and with a positive shift of the break λ*, consistent with an antihierarchical behavior. The AGN accretion history is predominantly made by massive (1010 44.36 + 1.28 × (1 + z). We infer that the probability of finding highly accreting (λ EDD > 10%) AGNs significantly increases with redshift, from 0.4% (3.0%) at z = 0.5%–6.5% (15.3%) at z = 3 for MS (SB) galaxies, implying a longer AGN duty cycle in the early universe. Our results strongly favor a M sstarf-dependent ratio between BHAR and SFR, as BHAR/SFR ∝ ${M}_{\star }^{0.73[+0.22,-0.29]}$, supporting a nonlinear BH buildup relative to the host. Finally, this framework opens potential questions on super-Eddington BH accretion and different λ EDD prescriptions for understanding the cosmic BH mass assembly

Mid-infrared luminous quasars in the GOODS–Herschel fields: a large population of heavily obscured, Compton-thick quasars at z ≈ 2

We present the infrared (IR) and X-ray properties of a sample of 33 mid-IR luminous quasars (νL6 μm ≥ 6 × 1044 erg s−1) at redshift z ≈ 1–3, identified through detailed spectral energy distribution analyses of distant star-forming galaxies, using the deepest IR data from Spitzer and Herschel in the GOODS–Herschel fields. The aim is to constrain the fraction of obscured, and Compton-thick (CT, NH > 1.5 × 1024 cm−2) quasars at the peak era of nuclear and star formation activities. Despite being very bright in the mid-IR band, ≈30 per cent of these quasars are not detected in the extremely deep 2 and 4 Ms Chandra X-ray data available in these fields. X-ray spectral analysis of the detected sources reveals that the majority (≈67 per cent) are obscured by column densities NH > 1022 cm−2; this fraction reaches ≈80 per cent when including the X-ray-undetected sources (9 out of 33), which are likely to be the most heavily obscured, CT quasars. We constrain the fraction of CT quasars in our sample to be ≈24–48 per cent, and their space density to be Φ = (6.7 ± 2.2) × 10−6 Mpc−3. From the investigation of the quasar host galaxies in terms of star formation rates (SFRs) and morphological distortions, as a sign of galaxy mergers/interactions, we do not find any direct relation between SFRs and quasar luminosity or X-ray obscuration. On the other hand, there is tentative evidence that the most heavily obscured quasars have, on average, more disturbed morphologies than the unobscured/moderately obscured quasar hosts, which preferentially live in undisturbed systems. However, the fraction of quasars with disturbed morphology amongst the whole sample is ≈40 per cent, suggesting that galaxy mergers are not the main fuelling mechanism of quasars at z ≈ 2

ALMA and Herschel reveal that AGN and main-sequence galaxies have different star formation rate distributions

Using deep Herschel and ALMA observations, we investigate the star formation rate (SFR) distributions of X-ray AGN host galaxies at 0.5<z<1.5 and 1.5<z<4, comparing them to that of normal, star-forming (i.e., "main-sequence", or MS) galaxies. We find 34-55 per cent of AGNs have SFRs at least a factor of two below that of the average MS galaxy, compared to ~15 per cent of all MS galaxies, suggesting significantly different SFR distributions. Indeed, when both are modelled as log-normal distributions, the mass and redshift-normalised SFR distributions of AGNs are roughly twice as broad, and peak ~0.4 dex lower, than that of MS galaxies. However, like MS galaxies, the normalised SFR distribution of AGNs appears not to evolve with redshift. Despite AGNs and MS galaxies having different SFR distributions, the linear-mean SFR of AGNs derived from our distributions is remarkably consistent with that of MS galaxies, and thus with previous results derived from stacked Herschel data. This apparent contradiction is due to the linear-mean SFR being biased by bright outliers, and thus does not necessarily represent a true characterisation of the typical SFR of AGNs

ZFOURGE catalogue of AGN candidates: an enhancement of 160-μm-derived star formation rates in active galaxies to z = 3.2

We investigate active galactic nuclei (AGN) candidates within the FourStar Galaxy Evolution Survey (ZFOURGE) to determine the impact they have on star formation in their host galaxies. We first identify a population of radio, X-ray, and infrared-selected AGN by cross-matching the deep Ks-band imaging of ZFOURGE with overlapping multiwavelength data. From this, we construct a mass-complete (log(M∗/M⊙M∗/M⊙) ≥9.75), AGN luminosity limited sample of 235 AGN hosts over z = 0.2–3.2. We compare the rest-frame U − V versus V − J (UVJ) colours and specific star formation rates (sSFRs) of the AGN hosts to a mass-matched control sample of inactive (non-AGN) galaxies. UVJ diagnostics reveal AGN tend to be hosted in a lower fraction of quiescent galaxies and a higher fraction of dusty galaxies than the control sample. Using 160 μm Herschel PACS data, we find the mean specific star formation rate of AGN hosts to be elevated by 0.34 ± 0.07 dex with respect to the control sample across all redshifts. This offset is primarily driven by infrared-selected AGN, where the mean sSFR is found to be elevated by as much as a factor of ∼5. The remaining population, comprised predominantly of X-ray AGN hosts, is found mostly consistent with inactive galaxies, exhibiting only a marginal elevation. We discuss scenarios that may explain these findings and postulate that AGN are less likely to be a dominant mechanism for moderating galaxy growth via quenching than has previously been suggested

The effect of active galactic nuclei on the cold interstellar medium in distant star-forming galaxies

In the framework of a systematic study with the ALMA interferometer of IR-selected main-sequence and starburst galaxies at z ∼ 1 − 1.7 at typical ∼1″ resolution, we report on the effects of mid-IR- and X-ray-detected active galactic nuclei (AGN) on the reservoirs and excitation of molecular gas in a sample of 55 objects. We find widespread detectable nuclear activity in ∼30% of the sample. The presence of dusty tori influences the IR spectral energy distribution of galaxies, as highlighted by the strong correlation among the AGN contribution to the total IR luminosity budget (fAGN = LIR,  AGN/LIR), its hard X-ray emission, and the Rayleigh-Jeans to mid-IR (S1.2 mm/S24 μm) observed color, with evident consequences on the ensuing empirical star formation rate estimates. Nevertheless, we find only marginal effects of the presence and strength of AGN on the carbon monoxide CO (J = 2, 4, 5, 7) or neutral carbon ([C I](3P1  −  3P0), [C I](3P2  −  3P1)) line luminosities and on the derived molecular gas excitation as gauged by line ratios and the full spectral line energy distributions. The [C I] and CO emission up to J = 5, 7 thus primarily traces the properties of the host in typical IR luminous galaxies. However, our analysis highlights the existence of a large variety of line luminosities and ratios despite the homogeneous selection. In particular, we find a sparse group of AGN-dominated sources with the highest LIR,  AGN/LIR,  SFR ratios, ≳3, that are more luminous in CO (5−4) than what is predicted by the L′CO(5-4)−LIR, SFR relation, which might be the result of the nuclear activity. For the general population, our findings translate into AGN having minimal effects on quantities such as gas and dust fractions and star formation efficiencies. If anything, we find hints of a marginal tendency of AGN hosts to be compact at far-IR wavelengths and to display 1.8 times larger dust optical depths. In general, this is consistent with a marginal impact of the nuclear activity on the gas reservoirs and star formation in average star-forming AGN hosts with LIR > 5 × 1011 L⊙, typically underrepresented in surveys of quasars and submillimeter galaxies

X-Ray Spectral Constraints for z ≈ 2 Massive Galaxies: The Identification of Reflection-dominated Active Galactic Nuclei

We use the 4 Ms Chandra Deep Field-South (CDF-S) survey to place direct constraints on the ubiquity of z 2 heavily obscured active galactic nuclei (AGNs) in K 10 keV observatories. On the basis of these analyses, we estimate the space density for typical (intrinsic X-ray luminosities of L 2-10 keV 1043 erg s–1) heavily obscured and Compton-thick AGNs at z 2. Our space-density constraints are conservative lower limits but they are already consistent with the range of predictions from X-ray background models