We report on the first ten identifications of sources serendipitously detected by the NuSTAR to provide the first sensitive census of the cosmic X-ray background (CXB) source population at >10 keV. We find that these NuSTAR-detected sources are ~100x
fainter than those previously detected at >10 keV and have a broad range in redshift and luminosity (z=0.020-2.923 and L_10-40 keV~4x10^{41}-5x10^{45} erg/s); the median redshift and luminosity are z~0.7 and L_10-40 keV~3x10^{44} erg/s, respectively. We characterize these sources on the basis of broad-band ~0.5-32 keV spectroscopy, optical spectroscopy, and broad-band ultraviolet-to-mid-infrared SED analyzes. We find that the dominant source population is quasars with L_10-40 keV>10^{44} erg/s, of which ~50% are obscured with N_H>10^{22} cm^{-2}. However, none of the ten NuSTAR sources are Compton thick (N_H>10^{24} cm^{-2}) and we place a 90% confidence upper limit on the fraction of Compton-thick quasars (L_10-40 keV>10^{44} erg/s) selected at >10 keV of ~33% over the redshift range z=0.5-1.1. We jointly fitted the rest-frame ~10-40 keV data for all of the non-beamed sources with L_10-40 keV>10^{43} erg/s to constrain the average strength of reflection; we find R<1.4 for Gamma=1.8, broadly consistent with that found for local AGNs observed at >10 keV. We also constrain the host galaxy masses and find a median stellar mass of ~10^{11} M_sun, a factor ~5 times higher than the median stellar mass of nearby high-energy selected AGNs, which may be at least partially driven by the order of magnitude higher X-ray luminosities of the NuSTAR sources. Within the low source-statistic limitations of our study, our results suggest that the overall properties of the NuSTAR sources are broadly similar to those of nearby high-energy selected AGNs but scaled up in luminosity and mass.
We use the 4Ms CDF-S survey to place direct X-ray constraints on the ubiquity of z~2 heavily obscured AGNs in K<22 BzK galaxies. Forty seven of the 222 BzK galaxies in the central region of the CDF-S are detected at X-ray energies, 11 of which have h
ard X-ray spectral slopes (Gamma<1) indicating the presence of heavily obscured AGN activity. The other 36 X-ray detected BzK galaxies appear to be relatively unobscured AGNs and starburst galaxies; we use X-ray variability analyses over a rest-frame baseline of ~3 years to further confirm the presence of AGN activity in many of these systems. The majority (7 out of 11) of the heavily obscured AGNs have excess IR emission over that expected from star formation (termed IR-excess galaxies). However, we find that X-ray detected heavily obscured AGNs only comprise ~25% of the IR-excess galaxy population, which is otherwise composed of relatively unobscured AGNs and starburst galaxies. We find that the typical X-ray spectrum of the heavily obscured AGNs is better characterized by a pure reflection model than an absorbed power-law model, suggesting extreme Compton-thick absorption in some systems. We verify this result by producing a composite rest-frame 2-20 keV spectrum, which has a similar shape as a reflection-dominated X-ray spectrum and reveals an emission feature at rest-frame energy ~6.4 keV, likely to be due to Fe K. These heavily obscured AGNs are likely to be the distant analogs of the reflection-dominated AGNs recently identified at z~0 with >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.
It is well established that a dominant phase in the growth of massive galaxies occurred at high redshift and was heavily obscured by gas and dust. Many studies have explored the stellar growth of massive galaxies but few have combined these constrain
ts with the growth of the supermassive black hole (SMBH; i.e., identified as AGN activity). In this brief contribution we highlight our work aimed at identifying AGNs in z~2 luminous dust-obscured galaxies. Using both sensitive X-ray and infrared (IR)-submillimeter (submm) observations, we show that AGN activity is common in z~2 dust-obscured systems. With a variety of techniques we have found that the majority of the AGN activity is heavily obscured, and construct diagnostics based on X-ray-IR data to identify some of the most heavily obscured AGNs in the Universe (i.e., AGNs obscured by Compton-thick material; N_H>1.5x10^24 cm^-2). On the basis of these techniques we show that SMBH growth was typically heavily obscured (N_H>10^23 cm^-2) at z~2, and find that the growth of the SMBH and spheroid was closely connected, even in the most rapidly evolving systems.
Leading models of galaxy formation require large-scale energetic outflows to regulate the growth of distant galaxies and their central black holes. However, current observational support for this hypothesis at high redshift is mostly limited to rare
z>2 radio galaxies. Here we present Gemini-North NIFS Intregral Field Unit (IFU) observations of the [O III]5007 emission from a z~2 ultraluminous infrared galaxy (ULIRG; L_IR>10^12 L_sol) with an optically identified Active Galactic Nucleus (AGN). The spatial extent (~4-8 kpc) of the high velocity and broad [O III] emission are consistent with that found in z>2 radio galaxies, indicating the presence of a large-scale energetic outflow in a galaxy population potentially orders of magnitude more common than distant radio galaxies. The low radio luminosity of this system indicates that radio-bright jets are unlikely to be responsible for driving the outflow. However, the estimated energy input required to produce the large-scale outflow signatures (of order ~10^59 ergs over ~30 Myrs) could be delivered by a wind radiatively driven by the AGN and/or supernovae winds from intense star formation. The energy injection required to drive the outflow is comparable to the estimated binding energy of the galaxy spheroid, suggesting that it can have a significant impact on the evolution of the galaxy. We argue that the outflow observed in this system is likely to be comparatively typical of the high-redshift ULIRG population and discuss the implications of these observations for galaxy formation models.
Leading models of galaxy formation require large-scale energetic outflows to regulate the growth of distant galaxies and their central black holes. However, current observational support for this hypothesis at high redshift is mostly limited to rare
z>2 radio galaxies. Here we present Gemini-North NIFS Intregral Field Unit (IFU) observations of the [OIII] emission from a z~2 ultraluminous infrared galaxy (L_IR>10^12 solar luminosities) with an optically identified Active Galactic Nucleus (AGN). The spatial extent (~4-8 kpc) of the high velocity and broad [OIII] emission are consistent with that found in z>2 radio galaxies, indicating the presence of a large-scale energetic outflow in a galaxy population potentially orders of magnitude more common than distant radio galaxies. The low radio luminosity of this system indicates that radio-bright jets are unlikely to be responsible for driving the outflow. However, the estimated energy input required to produce the large-scale outflow signatures (of order ~10^59 ergs over ~30 Myrs) could be delivered by a wind radiatively driven by the AGN and/or supernovae winds from intense star formation. The energy injection required to drive the outflow is comparable to the estimated binding energy of the galaxy spheroid, suggesting that it can have a significant impact on the evolution of the galaxy. We argue that the outflow observed in this system is likely to be comparatively typical of the high-redshift ULIRG population and discuss the implications of these observations for galaxy formation models.
Submillimeter-emitting galaxies (SMGs) are z~2 bolometrically luminous systems hosting energetic starburst and AGN activity. SMGs may represent a rapid growth phase that every massive galaxy undergoes before lying on the well-established black-hole-s
pheroid mass relationship in the local Universe. Here we briefly discuss our recent results from Alexander et al. (2008) where we estimated the masses of the black holes in SMGs using the black-hole virial mass estimator, finding M_BH~6x10^7 M_solar for typical SMGs. We show that the black-hole-spheroid mass ratio for SMGs at z~2 was suggestively below that found for massive galaxies in the local Universe and more than an order of magnitude below the black-hole-spheroid mass ratio estimated for z~2 quasars and radio galaxies. We demonstrate that SMGs and their progeny cannot lie on the elevated z~2 black-hole-spheroid mass relationship of quasars-radio galaxies without overproducing the space density of the most massive black holes (M_BH~10^9 M_solar), unless the galaxy spheroid of SMGs is an order of magnitude lower than that typically assumed (M_SPH~10^10 M_solar). We also show that the relative black-hole-spheroid growth rates of typical SMGs appear to be insufficient to significantly increase the black-hole-spheroid mass ratio, without requiring long duty cycles (~10^9 years), and argue that a more AGN-dominated phase (e.g., an optically bright quasar) is required to significantly move SMGs (and their progeny) up the black-hole-spheroid mass plane.
Many models that seek to explain the origin of the unresolved X-ray background predict that Compton-thick Active Galactic Nuclei (AGNs) are ubiquitious at high redshift. However, few distant Compton-thick AGNs have been reliably identified to date. H
ere we present Spitzer-IRS spectroscopy and 3.6-70um photometry of a z=2.2 optically identified AGN (HDF-oMD49) that is formally undetected in the 2Ms Chandra Deep Field-North (CDF-N) survey. The Spitzer-IRS spectrum and spectral energy distribution of this object is AGN dominated, and a comparison of the energetics at X-ray wavelengths to those derived from mid-infrared (mid-IR) and optical spectroscopy shows that the AGN is intrinsically luminous (L_X~3x10^44 erg/s) but heavily absorbed by Compton-thick material (N_H>>10^24 cm^{-2}); i.e., this object is a Compton-thick quasar. Adopting the same approach that we applied to HDF-oMD49, we found a further six objects at z~2-2.5 in the literature that are also X-ray weak/undetected but have evidence for AGN activity from optical and/or mid-IR spectroscopy, and show that all of these sources are also Compton-thick quasars with L_X>10^44 erg/s. On the basis of the definition of Daddi etal. (2007), these Compton-thick quasars would be classified as mid-IR excess galaxies, and our study provides the first spectroscopic confirmation of Compton-thick AGN activity in a subsample of these z~2 mid-IR bright galaxies. Using the four objects that lie in the CDF-N field, we estimate the space-density of Compton-thick quasars [Phi~(0.7-2.5)x10^-5 Mpc^-3 for L_X>10^44 erg/s objects at z~2-2.5] and show that Compton-thick accretion is as ubiquitious as unobscured accretion in the distant Universe.
We place direct observational constraints on the black-hole masses of the cosmologically important z~2 submillimeter-emitting galaxy (SMG; f850>4mJy) population, and use measured host-galaxy masses to explore their evolutionary status. We employ the
well-established virial black-hole mass estimator to weigh the black holes of a sample of z~2 SMGs with broad Halpha or Hbeta emission. The average black-hole mass and Eddington ratio (eta) of the lower-luminosity broad-line SMGs (L_X~10^44 erg/s} are log(M_BH/M_sol)~8.0 and eta~0.2, respectively. These lower-luminosity broad-line SMGs lie in the same location of the L_X-L_FIR plane as more typical SMGs hosting X-ray obscured AGN and may be intrinsically similar systems, but orientated so that the rest-frame optical nucleus is visible. Under this hypothesis, we conclude that SMGs host black holes with log(M_BH/M_odot)~7.8; we find supporting evidence from observations of local ULIRGs. Combining these black-hole mass constraints with measured host-galaxy masses, we find that the black holes in SMGs are >3 times smaller than those found in comparably massive normal galaxies in the local Universe, albeit with considerable uncertainty, and >10 times smaller than those predicted for z~2 luminous quasars and radio galaxies. These results imply that the growth of the black hole lags that of the host galaxy in SMGs, in stark contrast with that previously suggested for radio galaxies and luminous quasars at z~2. On the basis of current host-galaxy mass constraints, we show that SMGs and their descendants cannot lie significantly above the locally defined M_BH-M_GAL relationship. We argue that the black holes in the z~0 descendents of SMGs will have log(M_BH/M_odot)~8.6, indicating that they only need to grow by a factor of ~6 by the present day (ABRIDGED).
