The Properties and Redshift Evolution of Intermediate-Luminosity Off-Nuclear X-ray Sources in the Chandra Deep Fields

Abstract

We analyze a population of intermediate-redshift (z ~ 0.05-0.3), off-nuclear X-ray sources located within optically-bright galaxies in the Great Observatories Origins Deep Survey (GOODS) and Galaxy Evolution from Morphology and SEDs (GEMS) fields. A total of 24 off-nuclear source candidates are classified using deep Chandra exposures from the Chandra Deep Field-North, Chandra Deep Field-South, and Extended Chandra Deep Field-South; 15 of these are newly identified. These sources have average X-ray spectral shapes and optical environments similar to those observed for off-nuclear intermediate-luminosity (L_X >~ 10^{39} erg/s in the 0.5-2.0 keV band) X-ray objects (IXOs; sometimes referred to as ultraluminous X-ray sources [ULXs]) in the local universe. This sample improves the available source statistics for intermediate-redshift, off-nuclear sources with L_X >~ 10^{39.5} erg/s, and it places significant new constraints on the redshift evolution of the off-nuclear source frequency in field galaxies. The fraction of intermediate-redshift field galaxies containing an off-nuclear source with L_X >~ 10^{39} erg/s is suggestively elevated (~80% confidence level) with respect to that observed for IXOs in the local universe; we calculate this elevation to be a factor of 1.9^{+1.4}_{-1.3}. A rise in this fraction is plausibly expected as a consequence of the observed increase in global star-formation density with redshift, and our results are consistent with the expected magnitude of the rise in this fraction.