The nature of the hard X-ray background sources: Optical, near-infrared, submillimeter, and radio properties

With recent Chandra observations, at least 60% of the X-ray background in t he 2-10 keV energy range is now resolved into discrete sources. Here we pre sent 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 x 10(-15) ergs cm(-2) s(-1) i n 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 quas ars, five show active galactic nucleus (AGN) signatures, and six are z < 1. 5 luminous bulge-dominated galaxies whose spectra show no obvious optical A GN signatures. The seven spectroscopically unidentified I > 23.5 sources ha ve colors that are consistent with evolved early galaxies at z = 1.5-3. Onl y one hard X-ray source is significantly detected in an ultradeep submillim eter map; from the submillimeter to radio flux ratio we estimate a millimet ric redshift in the range 1.2-2.4. None of the remaining 19 hard X-ray sour ces is individually detected in the submillimeter. These results probably r eflect the fact that the 850 km flux limits obtainable with SCUBA are quite close to the expected fluxes from obscured AGNs. The ensemble of hard X-ra y sources contribute about 10% of the extragalactic background light at sub millimeter wavelengths. From the submillimeter and radio data we obtain bol ometric far-infrared luminosities. The hard X-ray sources have an average r atio 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 submillimet er-selected sources is in excess of 1100; this suggests that their far-infr ared light is primarily produced by star formation. Our data show that lumi nous hard X-ray sources are common in bulge-dominated optically luminous ga laxies, with about 10% of the population showing activity at any given time . We use our measured bolometric corrections with the 2-10 keV extragalacti c background light to infer the growth of supermassive black holes. Even wi th a high radiative efficiency of accretion (epsilon = 0.1), the black hole mass density required to account for the observed light is comparable to t he local black hole mass density.