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

TL;DR

This study analyzes the evolution of ultraluminous X-ray sources (ULXs) up to redshift 0.5, revealing their increasing prevalence in star-forming galaxies and their significant contribution to the cosmic X-ray background.

Contribution

It provides a uniform sample of 259 ULXs across redshifts up to 0.5 and quantifies their contribution to the cosmic X-ray background, linking ULX properties with galaxy star formation and metallicity.

Findings

ULX fraction increases with redshift.

ULXs may account for up to 40% of XRB flux at z~0.5.

ULXs are associated with high SFR and low metallicity galaxies.

Abstract

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