The Nature of the Hard X-Ray Background Sources: Optical, Near-Infrared, Submillimeter, and Radio Properties

By A. Barger, L. Cowie, R. Mushotzky, E. Richards. 2000

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With recent Chandra observations, at least 60% of the X-ray background in the 2–10 keV energy range is now resolved into discrete sources. Here we present deep optical, near-infrared, submillimeter, and 20 cm (radio) images, as well as high-quality optical spectra, of a complete sample of 20 sources selected to lie above a 2–10 keV flux of 3.8 × 10-15 ergs cm-2 s-1 in a deep Chandra observation of the Hawaii Deep Survey Field SSA13. The 13 galaxies with I < 23.5 have redshifts in the range 0.1 to 2.6. Two are quasars, five show active galactic nucleus (AGN) signatures, and six are z < 1.5 luminous bulge-dominated galaxies whose spectra show no obvious optical AGN signatures. The seven spectroscopically unidentified I > 23.5 sources have colors that are consistent with evolved early galaxies at z = 1.5–3. Only one hard X-ray source is significantly detected in an ultradeep submillimeter map; from the submillimeter to radio flux ratio we estimate a millimetric redshift in the range 1.2–2.4. None of the remaining 19 hard X-ray sources is individually detected in the submillimeter. These results probably reflect the fact that the 850 μm flux limits obtainable with SCUBA are quite close to the expected fluxes from obscured AGNs. The ensemble of hard X-ray sources contribute about 10% of the extragalactic background light at submillimeter wavelengths. From the submillimeter and radio data we obtain bolometric far-infrared luminosities. The hard X-ray sources have an average ratio of bolometric far-infrared to 2–10 keV luminosity of about 60, similar to that of local obscured AGNs. The same ratio for a sample of submillimeter-selected sources is in excess of 1100; this suggests that their far-infrared light is primarily produced by star formation. Our data show that luminous hard X-ray sources are common in bulge-dominated optically luminous galaxies, with about 10% of the population showing activity at any given time. We use our measured bolometric corrections with the 2–10 keV extragalactic background light to infer the growth of supermassive black holes. Even with a high radiative efficiency of accretion (ϵ = 0.1), the black hole mass density required to account for the observed light is comparable to the local black hole mass density.

Subfields: Astronomy and Astrophysics, Instrumentation

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A long-wavelength view on galaxy evolution from deep surveys by the Infrared Space Observatory (2001, background): “The Chandra X-ray observatory has performed several deep investigations of the high-z SCUBA population (Fabian et al. 2000; Hornschemeier et al. 2000; and Barger et al. 2001).”
The Chandra Deep Field-South: Optical Spectroscopy. I. (2003, background): “…fields are still in progress, however, a mixture of obscured and unobscured AGNs with an increasing fraction of obscuration at lower flux levels seems to be the dominant population in these samples (Fiore et al. 2000; Barger et al. 2001a; Tozzi et al. 2001; Rosati et al. 2002; Stern et al. 2002).”
The HELLAS2XMM survey IV. Optical identifications and the evolution of the accretion luminosity in the Universe (2003, methodology): “We combine the results on the optical identification of the HELLAS2XMM 1dF sources with those from the CDFN, Chandra SSA13 (Mushotzky et al. 2000; Barger et al. 2001), and XMM-Newton Lockman Hole fields to evaluate the evolution of the hard X-ray selected AGN number and luminosity density up toz =…”
Swift BAT Survey of AGNs (2007, background): “…recent Chandra and XMM data (e.g., Alexander et al. 2003; Giacconi et al. 2002; Yang et al. 2004;Mainieri et al. 2002, 2005; Szokoly et al. 2004; Zheng et al. 2004; Barger et al. 2001, 2003) there has been great progress in understanding the origin of the X-ray background and the evolution of AGNs.”
The Cosmic Evolution of Hard X-Ray-selected Active Galactic Nuclei (2004, methodology): “Submillimeter Data The submillimeter properties of the X-ray sample in the CDF-N are discussed in Barger et al. (2001c) and Alexander et al. (2003a), using earlier, more restricted SCUBA observations, whereas analyses on other fields are discussed in Almaini et al. (2003) and Waskett et al. (2003).”
THE COSMIC INFRARED BACKGROUND: Measurements and Implications ⁄ (2001, methodology): “Model parameters are constrained so that the AGN produce the hard CXB.”
A Redshift Survey of the Submillimeter Galaxy Population (2004, background): “20+424344.5 by Barger et al. (2001a, 2001b, 2003).9 The RAB mags of these sources are amongst the brightest in our sample, with an average R = 22.3 ± 2.2.”
The Dust Opacity of Star‐forming Galaxies (2001, background): “…based on the X-ray luminosity of the sources favor an AGN fraction ∼20% (Severgnini et al. 2000; Hornschemeier et al. 2000; Fabian et al. 2000; Barger et al. 2001); this value will generally depend on the source luminosity range, as it does in the local Universe (Genzel et al. 1998; Lutz et…”
The Chandra Deep Field North Survey. XIV. X-Ray-detected Obscured AGNs and Starburst Galaxies in the Bright Submillimeter Source Population (2002, background): “…X-ray surveys with SCUBA surveys has yielded little overlap between the X-ray and submm detected source populations (Fabian et al. 2000; Hornschemeier et al. 2000, 2001; Severgnini et al. 2000; Barger et al. 2001a; Almaini et al. 2002; however, see also Bautz et al. 2000 and Ivison et al. 2002).”
Spectral Energy Distributions of Hard X-Ray Selected Active Galactic Nuclei in the XMM-Newton Medium Deep Survey (2007, unknown): “…can be applied to sources that are too faint at optical wavelengths for optical spectroscopic observations, or with emission lines that are diluted by the host-galaxy light or affected by obscuration (Moran et al. 2002; Barger et al. 2001; Rigby et al. 2006), and to sources in the redshift desert.”
Cosmic X-ray surveys of distant active galaxies (2015, unknown): “E; e.g., Alexander et al 2001, 2002; Barger et al 2001b, 2002; Cowie et al 2001; Severgnini et al 2003; Gandhi et al 2004).”
The ROSAT Deep Survey - VI. X-ray sources and Optical identifications of the Ultra Deep Survey (2001, background): “…revealed a large fraction of very red sources in the 2-10 keV energy band, which are probably heavily obscured AGNs (Hasinger et al. 2001, see also Barger et al. 2001 for similar finding in the Chandra observation of the Hawaii Deep Survey Field SSA13), but no spectroscopic identifications for…”
Optical and Infrared Properties of the 2 Ms Chandra Deep Field North X-Ray Sources (2003, background): “Thus, an impressive amount of the 2–8 keV extragalactic background light arises at recent times, as was first shown by Barger et al. (2001a).”
Cosmological Evolution of the Hard X-Ray Active Galactic Nucleus Luminosity Function and the Origin of the Hard X-Ray Background (2003, methodology): “The use of the ASCA value for self consistency of our analysis is justified, because the contribution of HEAO1 sources to the 2–10 keV CXB is less than 3% and the absolute flux calibration between ASCA and Chandra is accurate within 10% (see e.g., Barger et al. 2001).”
Evidence for Extended, Obscured Starbursts in Submillimeter Galaxies (2004, methodology): “…been used to search for the signatures of AGNs, in both local ULIRGs and those at high redshifts (Sanders & Mirabel 1996; Fabian et al. 2000; Ivison et al. 2000; Barger et al. 2001; Frayer et al. 2003; Chapman et al. 2003a; Alexander et al. 2002, 2004; A. M. Swinbank et al. 2004, in preparation).”
Chandra Deep Field South: The 1 Ms Catalog (2002, background): “This population of sources has been already partially identified as nearby, bright normal galaxies by Tozzi et al. (2001), Hornschemeier et al. (2001), and Barger et al. (2001).”