The MOSDEF Survey: The Metallicity Dependence of X-Ray Binary Populations at z ∼ 2

Abstract

Population synthesis models predict that high-mass X-ray binary (HMXB) populations produced in low-metallicity environments should be more X-ray luminous, a trend supported by studies of nearby galaxies. This trend may be responsible for the observed increase of the X-ray luminosity ( L X ) per star formation rate (SFR) with redshift due to the decrease of metallicity ( Z ) at fixed stellar mass as a function of redshift. To test this hypothesis, we use a sample of 79 z  ∼ 2 star-forming galaxies with oxygen abundance measurements from the MOSDEF survey, which obtained rest-frame optical spectra for ∼1500 galaxies in the CANDELS fields at 1.37 <  z  < 3.80. Using Chandra data from the Chandra AEGIS-X Deep, Chandra Deep Field North, and Chandra Deep Field South surveys, we stack the X-ray data at the galaxy locations in bins of redshift and Z because the galaxies are too faint to be individually detected. In agreement with previous studies, the average L X /SFR of our z  ∼ 2 galaxy sample is enhanced by ≈0.4–0.8 dex relative to local HMXB L X –SFR scaling relations. Splitting our sample by Z , we find that L X /SFR and Z are anticorrelated with 97% confidence. This observed Z dependence for HMXB-dominated galaxies is consistent with both the local L X –SFR– Z relation and a subset of population synthesis models. Although the statistical significance of the observed trends is weak owing to the low X-ray statistics, these results constitute the first direct evidence connecting the redshift evolution of L X /SFR and the Z dependence of HMXBs.