Andy D. Goulding

Defining the intrinsic AGN infrared spectral energy distribution and measuring its contribution to the infrared output of composite galaxies

ABSTRACT We use infrared spectroscopy and photometry to empirically define the intrinsic, thermal in-frared spectral energy distribution (i.e., 6-100µm SED) of typical active galactic nuclei (i.e.,2-10 keV luminosity,L 2−10keV ∼10 42 −10 44 ergs s −1 AGNs). On average, the infrared SED oftypical AGNs is best describedas a brokenpower-law at.40 µm that falls steeply at &40 µm(i.e., at far-infrared wavelengths). Despite this fall-off at long wavelengths, at least 3 of the 11AGNs in our sample have observed SEDs that are AGN-dominatedeven at 60 µm, demon-strating the importance of accounting for possible AGN contribution even at far-infraredwavelengths. Our results also suggest that the average intrinsic AGN 6-100µm SED getsbluer with increasing X-ray luminosity, a trend seen both within our sample and also when wecompare against the intrinsic SEDs of more luminous quasars (i.e., L 2−10keV &10 44 ergs s −1 ).We compare our intrinsic AGN SEDs with predictions from dusty torus models and find theyare more closely matched by clumpy, rather than continuous, torus models. Next, we use ourintrinsic AGN SEDs to define a set of correction factors to con vert either monochromatic in-frared or X-ray luminosities into total intrinsic AGN infrared (i.e., 8-1000µm) luminosities.Finally, we outline a procedure that uses our newly defined in trinsic AGN infrared SEDs, inconjunction with a selection of host-galaxy templates, to fit the infrared photometry of com-posite galaxies and measure the AGN contribution to their total infrared output. We verify theaccuracy of our SED fitting procedure by comparing our result s to two independent measuresof AGN contribution: (1) 12 µm luminosities obtained from high-spatial resolution observa-tions of nearby galaxies and (2) the equivalent width of the 11.25 µm PAH feature. Our SEDfitting procedureopens up the possibility of measuringthe i ntrinsic AGN luminosities of largenumbers of galaxies with well-sampled infrared data (e.g.,IRAS, ISO, Spitzer and Herschel).Key words: Galaxies, Seyfert, Active, Quasars, Infrared, X-rays

A Chandra Perspective on Galaxy-wide X-ray Binary Emission and its Correlation with Star Formation Rate and Stellar Mass: New Results from Luminous Infrared Galaxies

We present newChandra observations that complete a sample of seventeen (17) luminous infrared galaxies (LIRGs) with D< 60 Mpc and low Galactic column densities of NH 5 × 10 20 cm −2 . The LIRGs in our sample have total infrared (8–1000 μm) luminosities in the range of LIR ≈ (1–8) × 10 11 L� . The high-resolution imaging and X-ray spectral information from our Chandra observations allow us to measure separately X-ray contributions from active galactic nuclei and normal galaxy processes (e.g., X-ray binaries and hot gas). We utilized total infrared plus UV luminosities to estimate star formation rates (SFRs) and K-band luminosities and optical colors to estimate stellar masses (M� ) for the sample. Under the assumption that the galaxy-wide 2–10 keV luminosity (L gal ) traces the combined emission from high-mass X-ray binaries (HMXBs) and low-mass X-ray binaries, and that the power output from these components is linearly correlated with SFR and M� , respectively, we constrain the relation L gal = αM� + βSFR. To achieve this, we construct a Chandra-based data set composed of our new LIRG sample combined with additional samples of less actively star-forming normal galaxies and more powerful LIRGs and ultraluminous infrared galaxies (ULIRGs) from the literature. Using these data, we measure best-fit values of α = (9.05 ± 0.37) × 10 28 erg s −1 M −1 ...

Towards a complete census of AGN in nearby Galaxies: a large population of optically unidentified AGN

Using Spitzer-infrared spectrograph (IRS) spectroscopy, we investigate the ubiquity of active galactic nuclei (AGN) in a complete (≈94 per cent), volume-limited sample of the most bolometrically luminous galaxies [L IR , 8-1000 μm ≥(0.3-20) × 10 10 L ⊙ ] to D < 15 Mpc. Our analyses are based on the detection of the high-excitation emission line [Ne v]λ14.32 μm (97.1 eV) to unambiguously identify AGN activity. We find that 17 of the 64 infrared (IR) bright galaxies in our sample host AGN activity (≈27+8/-6 per cent), ≥50 per cent of which are not identified as AGN using optical spectroscopy. The large AGN fraction indicates a tighter connection between AGN activity and IR luminosity for galaxies in the local Universe than previously found, potentially indicating a close association between AGN activity and star formation. The optically unidentified AGN span a wide range of galaxy type (S0-Ir) and are typically starburst-dominated systems hosting modest-luminosity AGN activity (L [Ne V] ≈ 10 37 -10 39 erg s -1 ). The non-identification of optical AGN signatures in the majority of these galaxies appears to be due to extinction towards the AGN, rather than intrinsically low-luminosity AGN activity; however, we note that the AGN optical signatures are diluted in some galaxies due to strong star formation activity. Examination of optical images shows that the optically unidentified AGN with evidence for extinction are hosted in either highly inclined galaxies or galaxies with dust lanes, indicating that obscuration of the AGN is not necessarily due to an obscuring torus. We therefore conclude that optical spectroscopic surveys miss approximately half of the AGN population simply due to extinction through the host galaxy.

DEEP SILICATE ABSORPTION FEATURES IN COMPTON-THICK ACTIVE GALACTIC NUCLEI PREDOMINANTLY ARISE DUE TO DUST IN THE HOST GALAXY

We explore the origin of mid-infrared (mid-IR) dust extinction in all 20 nearby (z 1.5 × 1024 cm–2) active galactic nuclei (AGNs) with hard energy (E > 10 keV) X-ray spectral measurements. We accurately measure the silicate absorption features at λ ~ 9.7 μm in archival low-resolution (R ~ 57-127) Spitzer Infrared Spectrograph spectroscopy, and show that only a minority (≈45%) of nearby Compton-thick AGNs have strong Si-absorption features (S 9.7 = ln (f int/f obs) 0.5) which would indicate significant dust attenuation. The majority (≈60%) are star formation dominated (AGN:SB < 0.5) at mid-IR wavelengths and lack the spectral signatures of AGN activity at optical wavelengths, most likely because the AGN emission lines are optically extinguished. Those Compton-thick AGNs hosted in low-inclination-angle galaxies exhibit a narrow range in Si-absorption (S 9.7 ~ 0-0.3), which is consistent with that predicted by clumpy-torus models. However, on the basis of the IR spectra and additional lines of evidence, we conclude that the dominant contribution to the observed mid-IR dust extinction is dust located in the host galaxy (i.e., due to disturbed morphologies, dust lanes, galaxy inclination angles) and not necessarily a compact obscuring torus surrounding the central engine.

Narrow-line region gas kinematics of 24 264 optically selected AGN: the radio connection

Using a sample of 24 264 optically selected active galactic nuclei (AGNs) from the SDSS DR7 data base, we characterize how the profile of the [O III] λ5007 emission line relates to bolometric luminosity (LAGN), Eddington ratio, radio loudness, radio luminosity (L1.4 GHz) and optical class (i.e. broad/narrow-line Seyfert 1, type 2) to determine what drives the kinematics of this kpc-scale line emitting gas. First, we use spectral stacking to characterize how the average [O III] λ5007 profile changes as a function of these five variables. After accounting for the known correlation between LAGN and L1.4 GHz, we report that L1.4 GHz has the strongest influence on the [O III] λ5007 profile, with AGNs of moderate radio luminosity (L1.4 GHz = 1023–1025 W Hz−1) having the broadest [O III] λ5007 profiles. Conversely, we find only a modest change in the [O III] λ5007 profile with increasing radio loudness and find no significant difference between the [O III] λ5007 profiles of broad- and narrow-line Seyfert 1s. When binned according to Eddington ratio, only the AGNs in our highest bin (i.e. >0.3) show any signs of having broadened [O III] λ5007 profiles, although the small numbers of such extreme AGNs in our sample mean we cannot rule out that other processes (e.g. radio jets) are responsible for this broadening. The [O III] λ5007 profiles of type 1 and type 2 AGNs show the same trends in terms of line width, but type 1 AGNs display a much stronger ‘blue wing’, which we interpret as evidence of outflowing ionized gas. We perform multicomponent fitting to the Hβ, [O III] λλ4959, 5007, [N II] λλ6548, 6584 and Hα lines for all the AGNs in our sample to calculate the proportions of AGNs with broad [O III] λ5007 profiles. The individual fits confirm the results from our stacked spectra; AGNs with L1.4 GHz > 1023 W Hz−1 are roughly five times more likely to have extremely broad [O III] λ5007 lines (full width at half-maximum, FWHMAvg > 1000 km s−1) compared to lower L1.4 GHz AGNs, and the width of the [O III] λ5007 line peaks in moderate-radio-luminosity AGNs (L1.4 GHz ∼ 1024 W Hz−1). Our results are consistent with the most disturbed gas kinematics being induced by compact radio cores (rather than powerful radio jets), although broadened [O III] λ5007 lines are also present, but much rarer, in low-L1.4 GHz systems. Our catalogue of multicomponent fits is freely available as an online resource for statistical studies of the kinematics and luminosities of the narrow- and broad-line AGN regions and the identification of potential targets for follow-up observations at http://sites.google.com/site/sdssalpaka.

The Link between the Hidden Broad Line Region and the Accretion Rate in Seyfert 2 Galaxies

In the past few years, more and more pieces of evidence have been presented for a revision of the widely accepted unified model of active galactic nuclei. A model based solely on orientation cannot explain all the observed phenomenology. In the following, we will present evidence that accretion rate is also a key parameter for the presence of hidden broad line regions (HBLRs) in Seyfert 2 galaxies. Our sample consists of 21 sources with polarized hidden broad lines and 18 sources without hidden broad lines. We use stellar velocity dispersions from several studies on the Ca II and Mg b triplets in Seyfert 2 galaxies to estimate the mass of the central black holes via the M BH-σ relation. The ratio between the bolometric luminosity, derived from the intrinsic (i.e., unabsorbed) X-ray luminosity, and the Eddington luminosity is a measure of the rate at which matter accretes onto the central supermassive black hole. A separation between Compton-thin HBLR and non-HBLR sources is clear, both in accretion rate (log L bol/L Edd = –1.9) and in luminosity (log L bol = 43.90). When properly luminosity-corrected Compton-thick sources are included, the separation between HBLR and non-HBLR is less sharp but no HBLR source falls below the Eddington ratio threshold. We speculate that non-HBLR Compton-thick sources with accretion rate higher than the threshold do possess a BLR, but something, probably related to their heavy absorption, is preventing us from observing it even in polarized light. Our results for Compton-thin sources support theoretical expectations. In a model presented by Nicastro, the presence of broad emission lines is intrinsically connected with disk instabilities occurring in proximity of a transition radius, which is a function of the accretion rate, becoming smaller than the innermost stable orbit for very low accretion rates and therefore luminosities.

X-RAY SPECTRAL CONSTRAINTS FOR z ≈ 2 MASSIVE GALAXIES: THE IDENTIFICATION OF REFLECTION-DOMINATED ACTIVE GALACTIC NUCLEI

We use the 4Ms CDF-S survey to place direct X-ray constraints on the ubiquity of z~2 heavily obscured 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_X>1E43 erg/s) 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.

Searching for Compton-thick active galactic nuclei at z∼ 0.1

Using a suite of X-ray, mid-infrared (mid-IR) and optical active galactic nuclei (AGN) luminosity indicators, we search for Compton-thick AGNs with intrinsic LX > 10 42 erg s −1 at z ∼ 0.03–0.2, a region of parameter space which is currently poorly constrained by deep narrow-field and high-energy (E > 10 keV) all-sky X-ray surveys. We have used the widest XMM–Newton survey (the serendipitous source catalogue) to select a representative subsample (14; ≈10 per cent) of the 147 X-ray undetected candidate Compton-thick AGNs in the Sloan Digital Sky Survey (SDSS) with f X/f [O III] 43 per cent) of our sample appears to be obscured by Compton-thick material with NH > 1.5 × 10 24 cm −2 . Under the reasonable assumption that our 14 AGNs are representative of the overall X-ray undetected AGN population in the SDSS–XMM parent sample, we find that >20 per cent of the optical type-2 AGN population are likely to be obscured by Compton-thick material. This implies a space density of log(� ) > −4. 9M pc −3 for Comptonthick AGNs with LX > 10 42 erg s −1 at z ∼ 0.1, which we suggest may be consistent with that predicted by X-ray background synthesis models. Furthermore, using the 6-µm continuum luminosity to infer the intrinsic AGN luminosity and the stellar velocity dispersion to estimate MBH, we find that the most conservatively identified Compton-thick AGNs in this sample may harbour some of the most rapidly growing black holes (median MBH ≈ 3 × 10 7 M� )i n the nearby Universe, with a median Eddington ratio of η ≈ 0.2.

Characterizing the far‐infrared properties of distant X‐ray detected AGNs: evidence for evolution in the infrared–X‐ray luminosity ratio

We investigate the far-infrared (FIR) properties of X-ray sources detected in the Chandra Deep Field-South (CDF-S) survey using the ultradeep 70 and 24 μm Spitzer observations taken in this field. Since only 30 (i.e. ≈ 10 per cent) of the 266 X-ray sources in the region of the 70 μm observations are detected at 70 μm, we rely on stacking analyses of the 70 μm data to characterize the average 70 μm properties of the X-ray sources as a function of redshift, X-ray luminosity and X-ray absorption. Using Spitzer-IRS data of the Swift-Burst Alert Telescope (BAT) sample of z ≈ 0 active galactic nuclei (AGNs), we show that the 70/24 μm flux ratio can distinguish between AGN-dominated and starburst-dominated systems out to z ≈ 1.5 . Among the X-ray sources detected at 70 μm, we note a large scatter in the observed 70/24 μm flux ratios, spanning almost a factor of 10 at similar redshifts, irrespective of object classification, suggesting a range of AGN:starburst ratios. From stacking analyses we find that the average observed 70/24 μm flux ratios of AGNs out to an average redshift of 1.5 are similar to z ≈ 0 AGNs with similar X-ray luminosities (L_X = 10^(42-44) erg s^(−1)) and absorbing column densities (N_H ≤ 10^(23) cm^(−2)) . Furthermore, both high-redshift and z ≈ 0 AGNs follow the same tendency towards warmer 70/24 μm colours with increasing X-ray luminosity (LX). From analyses of the Swift-BAT sample of z ≈ 0 AGNs, we note that the 70 μm flux can be used to determine the IR (8–1000 μm) luminosities of high-redshift AGNs. We use this information to show that L_X = 10^(42-43) erg s^(−1) AGNs at high redshifts (z = 1–2) have IR to X-ray luminosity ratios (L_(IR)/L_X) that are, on average, 4.7^(+10.2)_(−2.0) and 12.7+7.1−2.6 times higher than AGNs with similar X-ray luminosities at z = 0.5–1 and ≈0, respectively. By comparison, we find that the L_(IR)/L_X ratios of L_X= 10^(43-44) erg s^(−1) AGNs remain largely unchanged across this same redshift interval. We explore the consequences that these results may have on the identification of distant, potentially Compton-thick AGNs using L_(IR)/L_X ratios. In addition, we discuss possible scenarios for the observed increase in the L_(IR)/L_X ratio with redshift, including changes in the dust covering factor of AGNs and/or the star formation rates of their host galaxies. Finally, we show how deep observations to be undertaken by the Herschel Space Observatory will enable us to discriminate between these proposed scenarios and also identify Compton-thick AGNs at high redshifts.

TRACING THE EVOLUTION OF ACTIVE GALACTIC NUCLEI HOST GALAXIES OVER THE LAST 9 Gyr OF COSMIC TIME

We present the results of a combined galaxy population analysis for the host galaxies of active galactic nuclei (AGN) identified at 0 < z < 1.4 within the Sloan Digital Sky Survey, Bootes, and DEEP2 surveys. We identified AGN in a uniform and unbiased manner at X-ray, infrared, and radio wavelengths. Supermassive black holes undergoing radiatively efficient accretion (detected as X-ray and/or infrared AGN) appear to be hosted in a separate and distinct galaxy population than AGN undergoing powerful mechanically dominated accretion (radio AGN). Consistent with some previous studies, radiatively efficient AGN appear to be preferentially hosted in modest star-forming galaxies, with little dependence on AGN or galaxy luminosity. AGN exhibiting radio-emitting jets due to mechanically dominated accretion are almost exclusively observed in massive, passive galaxies. Crucially, we now provide strong evidence that the observed host-galaxy trends are independent of redshift. In particular, these different accretion-mode AGN have remained as separate galaxy populations throughout the last 9 Gyr. Furthermore, it appears that galaxies hosting AGN have evolved along the same path as galaxies that are not hosting AGN with little evidence for distinctly separate evolution.