The properties and redshift evolution of intermediate-luminosity off-nuclear X-ray sources in the chandra deep fields

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 and Galaxy Evolution from Morphology and SEDs fields. A total of 24 off-nuclear source candidates are classified using deep Chandra X-Ray Observatory 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 ergs s ^-1 in the 0.5-2.0 keV band) X-ray objects (IXOs; sometimes referred to as ultraluminous X-ray sources) in the local universe. This sample improves the available source statistics for intermediate-redshift off-nuclear sources with L _x ≳ 10 ^39.5 ergs s ^-1, 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 ergs s ^-1 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 _-1.3-1.4. 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.