Sub-galactic scaling relations between X-ray luminosity, star formation rate, and stellar mass

Abstract

ABSTRACT X-ray luminosity (LX) originating from high-mass X-ray binaries (HMXBs) is tightly correlated with the host galaxy’s star formation rate (SFR). We explore this connection at sub-galactic scales spanning ∼7 dex in SFR and ∼8 dex in specific SFR (sSFR). There is good agreement with established relations down to SFR ≃ 10−3 M$_{\odot }\, \rm {yr^{-1}}$, below which an excess of X-ray luminosity emerges. This excess likely arises from low-mass X-ray binaries. The intrinsic scatter of the LX–SFR relation is constant, not correlated with SFR. Different star formation indicators scale with LX in different ways, and we attribute the differences to the effect of star formation history. The SFR derived from H α shows the tightest correlation with X-ray luminosity because H α emission probes stellar populations with ages similar to HMXB formation time-scales, but the H α-based SFR is reliable only for $\rm sSFR{\gt }10^{-12}$ M$_{\odot }\, \rm {yr^{-1}}$/M⊙.