  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.
