The Chandra Deep Field North Survey. XV. Optically Bright, X-Ray-Faint Sources

We have analyzed optically bright, X-ray?faint [OBXF; i.e., log (fX/fR)-2] sources identified in an 178.9 arcmin2 area having high exposure (greater than 1500 ks) within the Chandra Deep Field North 2 Ms survey. We find 43 OBXF sources in this area, making up ?15% of the X-ray sources above a 0.5?2 keV flux of ?2.3 ? 10-17 ergs cm-2 s-1. We present spectroscopic identifications for 42 of the OBXF sources and optical spectra for 25, including five previously unpublished redshifts. Deep optical imaging data (either Hubble Space Telescope [HST] or ground-based) are presented for all the OBXF sources; we measure the optical morphologies of the 20 galaxies having HST imaging data. The OBXF population consists mainly of normal and starburst galaxies detected out to cosmologically significant distances (i.e., to a median redshift of z = 0.297 and a full redshift range z = 0.06?0.845). This is notable since these distances equate to look-back times of up to ?8 Gyr; we are thus provided with a window on the X-ray emission from galaxies at redshifts much closer to the cosmic star formation peak than was possible prior to the Chandra X-Ray Observatory. The X-ray luminosity distribution of OBXF sources extends to higher luminosity than does that of normal galaxies, indicating that a significant fraction are likely dominated by low-luminosity active galactic nuclei or vigorous star formation. The lowest redshift galaxies (z ? 0.06?0.2) have very low X-ray?to?optical flux ratios [i.e., log (fX/fR) -3], which are consistent with those of normal galaxies in the local universe. By combining the detected X-ray counts, we find the average OBXF X-ray spectrum to be consistent with a ? ? 2.0 power law. The 0.5?2 keV log N?log S for the OBXF galaxies is much steeper (? ? -1.7) than for the general X-ray source population. Indeed, the number of OBXF sources has doubled between the 1 and 2 Ms surveys, rising sharply in numbers at faint fluxes. The extragalactic OBXF sources are found to contribute ?1%?2% of the soft extragalactic X-ray background. We report on the discovery of five candidate off-nuclear ultraluminous X-ray sources (LX 1039 ergs s-1) with z ? 0.1?0.2 within the OBXF population. These sources are ultraluminous in that they are typically more X-ray luminous than, e.g., Eddington-limited accretion onto stellar-mass black holes; these sources are found to dominate the X-ray emission of their host galaxies.