Diffuse X-ray emission from star-forming galaxies

We study the diffuse X-ray luminosity (L_X) 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 L_X with star formation rate (SFR) as $L_X\propto$ SFR² for SFR $\gtrsim 1$ $M_{\odot }$ 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 $L_X\text{--}SFR$ relation for low SFRs (≲few $M_{\odot }$ 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.