R. C. Hickox

What Drives the Growth of Black Holes

Massive black holes (BHs) are at once exotic and yet ubiquitous, residing in the centers of massive galaxies in the local Universe. Recent years have seen remarkable advances in our understanding of how these BHs form and grow over cosmic time, during which they are revealed as active galactic nuclei (AGN). However, despite decades of research, we still lack a coherent picture of the physical drivers of BH growth, the connection between the growth of BHs and their host galaxies, the role of large-scale environment on the fueling of BHs, and the impact of BH-driven outflows on the growth of galaxies. In this paper we review our progress in addressing these key issues, motivated by the science presented at the “What Drives the Growth of Black Holes?” workshop held at Durham on 26 th ‐29 th July 2010, and discuss how these questions may be tackled with current and future facilities.

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

Absolute Measurement of the Unresolved Cosmic X-Ray Background in the 0.5-8 keV Band with Chandra

We present the absolute measurement of the unresolved 0.5-8 keV cosmic X-ray background (CXB) in the Chandra Deep Fields (CDFs) North and South, the longest observations with Chandra (2 and 1 Ms, respectively). We measure the unresolved CXB intensity by extracting spectra of the sky and removing all point and extended sources detected in the CDF. To model and subtract the instrumental background, we use observations obtained with ACIS in stowed position, not exposed to the sky. The unresolved signal in the 0.5-1 keV band is dominated by diffuse Galactic and local thermal-like emission. We find unresolved intensites in the 0.5-1 keV band of (4.1 ? 0.3) ? 10-12 ergs cm-2 s-1 deg-2 for CDF-N and (5.0 ? 0.4) ? 10-12 for CDF-S. In the 1-8 keV band, the unresolved spectrum is adequately described by a power law with a photon index ? = 1.5 and normalization 2.6 ? 0.3 photons s-1 keV-1 sr-1 at 1 keV. We find unresolved CXB intensities of (1.04 ? 0.14) ? 10-12 ergs cm-2 s-1 deg-2 for the 1-2 keV band and (3.4 ? 1.7) ? 10-12 ergs cm-2 s-1 deg-2 for the 2-8 keV band. Our detected unresolved intensities in these bands significantly exceed the expected flux from sources below the CDF detection limits, if one extrapolates the log N/ log S curve to zero flux. Thus, these background intensities imply either a genuine diffuse component or a steepening of the log N/ log S curve at low fluxes, most significantly for energies <2 keV. Adding the unresolved intensity to the total contribution from sources detected in these fields and wider field surveys, we obtain a total intensity of the extragalactic CXB of (4.6 ? 0.3) ? 10-12 ergs cm-2 s-1 deg-2 for 1-2 keV and (1.7 ? 0.2) ? 10-11 ergs cm-2 s-1 deg-2 for 2-8 keV. These totals correspond to a CXB power-law normalization (for ? = 1.4) of 10.9 photons cm-2 s-1 keV-1 sr-1 at 1 keV. This corresponds to resolved fractions of 77% ? 3% and 80% ? 8% for 1-2 and 2-8 keV, respectively.

Low-Resolution Spectral Templates for Active Galactic Nuclei and Galaxies from 0.03 to 30 μm

We present a set of low-resolution empirical spectral energy distribution (SED) templates for active galactic nuclei (AGNs) and galaxies in the wavelength range from 0.03 ?m to 30 ?m based on the multi-wavelength photometric observations of the NOAO Deep-Wide Field Survey Bo?tes field and the spectroscopic observations of the AGN and Galaxy Evolution Survey. Our training sample is comprised of 14,448 galaxies in the redshift range 0 z 1 and 5347 likely AGNs in the range 0 z 5.58. The galaxy templates correspond to the SED templates presented in 2008 by Assef et al. extended into the UV and mid-IR by the addition of FUV and NUV GALEX and MIPS 24 ?m data for the field. We use our templates to determine photometric redshifts for galaxies and AGNs. While they are relatively accurate for galaxies (? z /(1 + z) = 0.04, with 5% outlier rejection), their accuracies for AGNs are a strong function of the luminosity ratio between the AGN and galaxy components. Somewhat surprisingly, the relative luminosities of the AGN and its host are well determined even when the photometric redshift is significantly in error. We also use our templates to study the mid-IR AGN selection criteria developed by Stern et al. in 2005 and Lacy et al. in 2004. We find that the Stern et al. criterion suffers from significant incompleteness when there is a strong host galaxy component and at z 4.5, when the broad H? emission line is redshifted into the [3.6] band, but that it is little contaminated by low- and intermediate-redshift galaxies. The Lacy et al. criterion is not affected by incompleteness at z 4.5 and is somewhat less affected by strong galaxy host components, but is heavily contaminated by low-redshift star-forming galaxies. Finally, we use our templates to predict the color-color distribution of sources in the upcoming Wide-Field Infrared Survey Explorer (WISE) mission and define a color criterion to select AGNs analogous to those developed for IRAC photometry. We estimate that in between 640,000 and 1,700,000 AGNs will be identified by these criteria, but without additional information, WISE-selected quasars will have serious completeness problems for z 3.4.We present a set of low resolution empirical SED templates for AGNs and galaxies in the wavelength range from 0.03 to 30 microns. These templates form a non-negative basis of the color space of such objects and have been derived from a combination 14448 galaxies and 5347 likely AGNs in the NDWFS Bootes field. We briefly describe how the templates are derived and discuss some applications of them. In particular, we discuss biases in commonly used AGN mid-IR color selection criteria and the expected distribution of sources in the current WISE satellite mission.

The LABOCA Survey of the Extended Chandra Deep Field South : Clustering of submillimetre galaxies

We present a measurement of the spatial clustering of submillimetre galaxies (SMGs) at z= 1-3. Using data from the 870μm Large APEX Bolometer Camera (LABOCA) submillimetre survey of the Extended Chandra Deep Field-South, we employ a novel technique to measure the cross-correlation between SMGs and galaxies, accounting for the full probability distributions for photometric redshifts of the galaxies. From the observed projected two-point cross-correlation function we derive the linear bias and characteristic dark matter halo masses for the SMGs. We detect clustering in the cross-correlation between SMGs and galaxies at the >4σ level. Accounting for the clustering of galaxies from their autocorrelation function, we estimate an autocorrelation length for SMGs of r o = 7.7 -2.3 +1.8 h -1 Mpc assuming a power-law slope γ= 1.8, and derive a corresponding dark matter halo mass of log(M halo[h -1M ⊙]) = 12.8 -0.5 +0.3. Based on the evolution of dark matter haloes derived from simulations, we show that that the z= 0 descendants of SMGs are typically massive (~2-3L *) elliptical galaxies residing in moderate- to high-mass groups (log(M halo[h -1M ⊙]) = 13.3 -0.5 +0.3). From the observed clustering we estimate an SMG lifetime of ~100Myr, consistent with lifetimes derived from gas consumption times and star formation time-scales, although with considerable uncertainties. The clustering of SMGs at z~ 2 is consistent with measurements for optically selected quasi-stellar objects (QSOs), supporting evolutionary scenarios in which powerful starbursts and QSOs occur in the same systems. Given that SMGs reside in haloes of characteristic mass ~6 × 10 12h -1M ⊙, we demonstrate that the redshift distribution of SMGs can be described remarkably well by the combination of two effects: the cosmological growth of structure and the evolution of the molecular gas fraction in galaxies. We conclude that the powerful starbursts in SMGs likely represent a short-lived but universal phase in massive galaxy evolution, associated with the transition between cold gas-rich, star-forming galaxies and passively evolving systems.

THE HIDDEN “AGN MAIN SEQUENCE”: EVIDENCE FOR A UNIVERSAL BLACK HOLE ACCRETION TO STAR FORMATION RATE RATIO SINCE z ∼ 2 PRODUCING AN M BH-M * RELATION

Using X-ray stacking analyses we estimate the average amounts of supermassive black hole (SMBH) growth taking place in star-forming galaxies at z ~ 1 and z ~ 2 as a function of galaxy stellar mass (M *). We find that the average SMBH growth rate follows remarkably similar trends with M * and redshift as the average star formation rates (SFRs) of their host galaxies (i.e., ∝ M * 0.86 ± 0.39 for the z ~ 1 sample and ∝ M * 1.05 ± 0.36 for the z ~ 2 sample). It follows that the ratio of SMBH growth rate to SFR is (1) flat with respect to M *, (2) not evolving with redshift, and (3) close to the ratio required to maintain/establish an SMBH to stellar mass ratio of ≈10–3 as also inferred from todays M BH-M Bulge relationship. We interpret this as evidence that SMBHs have, on average, grown in step with their host galaxies since at least z ~ 2, irrespective of host galaxy mass and active galactic nucleus triggering mechanism. As such, we suggest that the same secular processes that drive the bulk of star formation are also responsible for the majority of SMBH growth. From this, we speculate that it is the availability of gas reservoirs that regulate both cosmological SMBH growth and star formation.

Energetic galaxy-wide outflows in high-redshift ultraluminous infrared galaxies hosting AGN activity

We present integral field spectroscopy observations, covering the [O iii] λλ4959, 5007 emission-line doublet of eight high-redshift (z = 1.4–3.4) ultraluminous infrared galaxies (ULIRGs) that host active galactic nucleus (AGN) activity, including known submillimetre luminous galaxies. The targets have moderate radio luminosities that are typical of high-redshift ULIRGs (L1.4 GHz = 1024–1025 W Hz−1) and therefore are not radio-loud AGNs. We decouple kinematic components due to the galaxy dynamics and mergers from those due to outflows. We find evidence in the four most luminous systems ( erg s−1) for the signatures of large-scale energetic outflows: extremely broad [O iii] emission (full width at half-maximum ≈ 700–1400 km s−1) across ≈4–15 kpc, with high velocity offsets from the systemic redshifts (up to ≈850 km s−1). The four less luminous systems have lower quality data displaying weaker evidence for spatially extended outflows. We estimate that these outflows are potentially depositing energy into their host galaxies at considerable rates (–1045 erg s−1); however, due to the lack of constraints on the density of the outflowing material and the structure of the outflow, these estimates should be taken as illustrative only. Based on the measured maximum velocities (vmax ≈ 400–1400 km s−1) the outflows observed are likely to unbind some fraction of the gas from their host galaxies, but are unlikely to completely remove gas from the galaxy haloes. By using a combination of energetic arguments and a comparison to ULIRGs without clear evidence for AGN activity, we show that the AGN activity could be the dominant power source for driving all of the observed outflows, although star formation may also play a significant role in some of the sources.

Origin of the Soft Excess in X-Ray Pulsars

The spectra of many X-ray pulsars show, in addition to a power law, a low-energy component that has often been modeled as a blackbody with kTBB ~ 0.1 keV. However, the physical origin of this soft excess has remained a mystery. We examine a sample of well-studied bright X-ray pulsars, which have been observed using ROSAT, ASCA, Ginga, RXTE, BeppoSAX, Chandra, and XMM-Newton. In particular, we consider the Magellanic Cloud pulsars SMC X-1, LMC X-4, XTE J0111.2-7317, and RX J0059.2-7138 and the Galactic sources Her X-1, 4U 1626-67, Cen X-3, and Vela X-1. We show that the soft excess is a very common if not ubiquitous feature intrinsic to X-ray pulsars. We evaluate several possible mechanisms for the soft emission, using theoretical arguments, as well as observational clues, such as spectral shapes, eclipses, pulsations of the soft component, and superorbital modulation of the source flux. We find that reprocessing of hard X-rays from the neutron star by the inner region of the accretion disk is the only process that can explain the soft excess in all the pulsars with LX 1038 ergs s-1. Other mechanisms, such as emission from diffuse gas in the system, are important in less luminous objects.

A Large Population of Mid-Infrared-selected, Obscured Active Galaxies in the Boötes Field

We identify a population of 640 obscured and 839 unobscured AGNs at redshifts 0.7 < z 3 using multiwavelength observations of the 9 deg2 NOAO Deep Wide-Field Survey (NDWFS) region in Bootes. We select AGNs on the basis of Spitzer IRAC colors obtained by the IRAC Shallow Survey. Redshifts are obtained from optical spectroscopy or photometric redshift estimators. We classify the IR-selected AGNs as IRAGN 1 (unobscured) and IRAGN 2 (obscured) using a simple criterion based on the observed optical to mid-IR color, with a selection boundary of R − [ 4.5] = 6.1, where R and [4.5] are the Vega magnitudes in the R and IRAC 4.5 μm bands, respectively. We verify this selection using X-ray stacking analyses with data from the Chandra XBootes survey, as well as optical photometry from NDWFS and spectroscopy from MMT/AGES. We show that (1) these sources are indeed AGNs, and (2) the optical/IR color selection separates obscured sources (with average NH ~ 3 × 1022 cm −2 obtained from X-ray hardness ratios, and optical colors and morphologies typical of galaxies) and unobscured sources (with no X-ray absorption, and quasar colors and morphologies), with a reliability of 80%. The observed numbers of IRAGNs are comparable to predictions from previous X-ray, optical, and IR luminosity functions, for the given redshifts and IRAC flux limits. We observe a bimodal distribution in R − [ 4.5] color, suggesting that luminous IR-selected AGNs have either low or significant dust extinction, which may have implications for models of AGN obscuration.

GOODS-Herschel: the far-infrared view of star formation in active galactic nucleus host galaxies since z ≈ 3

We present a study of the infrared properties of X-ray selected, moderate-luminosity (i.e. L_X= 10^(42)–10^(44) erg s^(−1)) active galactic nuclei (AGNs) up to z ≈ 3, in order to explore the links between star formation in galaxies and accretion on to their central black holes. We use 100 and 160 μ m fluxes from GOODS-Herschel – the deepest survey yet undertaken by the Herschel telescope – and show that in the vast majority of cases (i.e. >94 per cent) these fluxes are dominated by emission from the host galaxy. As such, these far-infrared bands provide an uncontaminated view of star formation in the AGN host galaxies. We find no evidence of any correlation between the X-ray and infrared luminosities of moderate AGNs at any redshift, suggesting that global star formation is decoupled from nuclear (i.e. AGN) activity in these galaxies. On the other hand, we confirm that the star formation rates of AGN hosts increase strongly with redshift, by a factor of 43^(+27)_(−18) from z < 0.1 to z = 2–3 for AGNs with the same range of X-ray luminosities. This increase is entirely consistent with the factor of 25–50 increase in the specific star formation rates (SSFRs) of normal, star-forming (i.e. main-sequence) galaxies over the same redshift range. Indeed, the average SSFRs of AGN hosts are only marginally (i.e. ≈20 per cent) lower than those of main-sequence galaxies at all surveyed redshifts, with this small deficit being due to a fraction of AGNs residing in quiescent (i.e. low SSFR) galaxies. We estimate that 79 ± 10 per cent of moderate-luminosity AGNs are hosted in main-sequence galaxies, 15 ± 7 per cent in quiescent galaxies and <10 per cent in strongly starbursting galaxies. We derive the fractions of all main-sequence galaxies at z < 2 that are experiencing a period of moderate nuclear activity, noting that it is strongly dependent on galaxy stellar mass (M_(stars)), rising from just a few per cent at M_(stars) ∼ 10^(10) M_⊙ to ≳20 per cent at M_(stars)≥ 10^(11) M_⊙. Our results indicate that it is galaxy stellar mass that is most important in dictating whether a galaxy hosts a moderate-luminosity AGN. We argue that the majority of moderate nuclear activity is fuelled by internal mechanisms rather than violent mergers, which suggests that high-redshift disc instabilities could be an important AGN feeding mechanism.