Diffuse x-ray emission from star-forming galaxies

Abstract

We study the diffuse X-ray luminosity (LX) of star-forming galaxies using two-dimensional axisymmetric hydrodynamical simulations and analytical considerations of supernovae-(SNe-)driven galactic outflows. We find that the mass loading of the outflows, a crucial parameter for determining the X-ray luminosity, is constrained by the availability of gas in the central star-forming region, and a competition between cooling and expansion. We show that the allowed range of the mass loading factor can explain the observed scaling of LX with star formation rate (SFR) as ${L}_{{\rm{X}}}\quad \propto $ SFR2 for SFR $\gtrsim \quad 1$ ${M}_{\odot }$ yr−1, and a flatter relation at low SFRs. We also show that the emission from the hot circumgalactic medium (CGM) in the halo of massive galaxies can explain the large scatter in the ${L}_{{\rm{X}}}\mbox{--}\mathrm{SFR}$ relation for low SFRs (lesssimfew ${M}_{\odot }$ yr−1). Our results suggest that galaxies with small SFRs and large diffuse X-ray luminosities are excellent candidates for the detection of the elusive CGM.