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(f_X/f_R)<~-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present spectroscopic identifications for 42 of the OBXF sources and optical spectra for 25, including five previously unpublished redshifts.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(f<sub>X</sub>/f<sub>R</sub>)<~-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 Γ~2.0 power law. The 0.5-2 keV logN-logS for the OBXF galaxies is much steeper (α~-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 (L<sub>X</sub>>~10<sup>39</sup> ergs s<sup>-1</sup>) 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.

Based on observations obtained at the W. M. Keck Observatory, which is operated jointly by the California Institute of Technology and the University of California. Based on observations obtained by the Hobby-Eberly Telescope, which is a joint project of the University of Texas at Austin, Pennsylvania State University, Stanford University, Ludwig-Maximillians-Universität München, and Georg-August-Universität Göttingen. Also based on observations obtained at the Mayall 4 m telescope, Kitt Peak National Observatory, National Optical Astronomy Observatory, which is operated by the Association of Universities for Research in Astronomy, Inc., under cooperative agreement with the National Science Foundation.