We analyze a population of intermediate-redshift ( z \approx 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 _ { \mbox { \scriptsize { X } } } \geq 10 ^ { 39 } erg s ^ { -1 } 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 _ { \mbox { \scriptsize { X } } } \lower 2.15 pt \hbox { $ \buildrel > \over { \sim } $ } 10 ^ { 39.5 } erg 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 _ { \mbox { \scriptsize { X } } } \buildrel > \over { \sim } 10 ^ { 39 } erg s ^ { -1 } is suggestively elevated ( \approx 80 % confidence level ) with respect to that observed for IXOs in the local universe ; we calculate this elevation to be a factor of \approx 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 .