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present spectroscopic identifications for 42 of the OBXF sources and optical spectra for 25, including five previously unpublished redshifts.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_X/f_R) ≲ -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 log N-log S 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_X ≳ 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.