Connecting the metallicity dependence and redshift evolution of high-mass X-ray binaries

ABSTRACT The integrated X-ray luminosity (LX) of high-mass X-ray binaries (HMXBs) in a galaxy is correlated with its star formation rate (SFR), and the normalization of this correlation increases with redshift. Population synthesis models suggest that the redshift evolution of LX/SFR is driven by the metallicity (Z) dependence of HMXBs, and the first direct evidence of this connection was recently presented using galaxies at z ∼ 2. To confirm this result with more robust measurements and better constrain the LX–SFR–Z relation, we have studied the Z dependence of LX/SFR at lower redshifts. Using samples of star-forming galaxies at z = 0.1–0.9 with optical spectra from the hCOSMOS and zCOSMOS surveys, we stacked Chandra data from the COSMOS Legacy survey to measure the average LX/SFR as a function of Z in three redshift ranges: z = 0.1–0.25, 0.25–0.4, and 0.5–0.9. We find no significant variation of the LX–SFR–Z relation with redshift. Our results provide further evidence that the Z dependence of HMXBs is responsible for the redshift evolution of LX/SFR. Combining all available z > 0 measurements together, we derive a best-fitting LX–SFR–Z relation and assess how different population synthesis models describe the data. These results provide the strongest constraints to date on the LX–SFR–Z relation in the range of 8.0