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Ultraluminous X-ray sources (ULXs) are thought to be powerful X-ray binaries(XRBs) and may contribute significantly to the redshift-dependent X-rayemission from star forming galaxies. We have assembled a uniform sample of 259ULXs over the redshift range z=0.002-0.51 to constrain their physical natureand their contribution to the Cosmic X-Ray Background (CXB). The sample isconstructed by crossmatching galaxies from the Sloan Digital Sky Survey withthe Chandra Source Catalog and selecting off-nuclear X-ray sources afterapplying astrometric corrections. The fraction of contaminants is ~30% andshows no evolution with redshift. The host galaxy star formation rates (SFRs)are systematically elevated relative to the parent sample when matched in hoststellar mass. The specific SFRs suggest a slight preference for high-mass XRBs,and the X-ray luminosity scaling relations with host galaxy stellar mass andSFR indicate that the highest redshift sources represent relatively luminousXRB populations that dominate their host galaxy X-ray emission. The fraction ofgalaxies hosting at least one ULX of a given luminosity increases with redshiftover the full range of our sample, as expected if ULXs are preferentially foundin galaxies with high SFRs and low metallicities. At z~0.5, the ULX X-ray fluxis consistent with the X-ray emission from star-forming galaxies. Moreover,ULXs may account for up to ~40% of the integrated flux from XRBs in the normalgalaxy population out to z~0.5, suggesting they may contribute significantly tothe overall ionizing radiation from galaxies.

The Redshift Evolution of Ultraluminous X-Ray Sources out to z~0.5:  Comparison with X-Ray Binary Populations and Contribution to the Cosmic X-Ray  Background