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

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Abstract
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 ( = 0.1), the black hole mass density required to account for the observed light is comparable to the local black hole mass density.