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 2-10 keV background isnow resolved into discrete sources. Here we present deep optical, NIR, submm,and 20 cm (radio) images, as well as high-quality optical spectra, of acomplete sample of 20 hard X-ray sources in a deep Chandra observation of theSSA13 field. The thirteen I<23.5 galaxies have redshifts in the range 0.1 to2.6. Two are quasars, five show AGN signatures, and six are z<1.5 luminousbulge-dominated galaxies whose spectra show no obvious optical AGN signatures.The seven spectroscopically unidentified sources have colors that areconsistent with evolved early galaxies at z=1.5-3. Only one hard X-ray sourceis significantly detected in an ultradeep submm map; its millimetric redshiftis in the range z=1.2-2.4. None of the remaining 19 sources is detected in thesubmm. These results probably reflect the fact that the 850-micron flux limitsobtainable with SCUBA are quite close to the expected fluxes from obscured AGN.The hard X-ray sources have an average L(FIR)/L(2-10 keV)~60, similar to thatof local obscured AGN. The same ratio for a sample of submm selected sources isin excess of 1100, suggesting that their FIR light is primarily produced bystar formation. Our data show that luminous hard X-ray sources are common inbulge-dominated optically luminous galaxies. We use our measured bolometriccorrections with the 2-10 keV EBL to infer the growth of supermassive blackholes. Even with a high radiative efficiency of accretion (e=0.1), the blackhole mass density required to account for the observed light is comparable tothe local black hole mass density. (Abridged)Comment: 25 pages + 1 extra table. Accepted by The Astronomical Journal for the February 2001 issue. Minor changes to match accepted versio