X-Ray Emission from Star-cluster Winds in Starburst Galaxies

Inspired by the excess soft X-ray emission recently detected in Green Pea galaxies, we model the soft X-ray emission (0.5-2.0 keV) of hot gas from star-cluster winds. By combining individual star clusters, we estimate the soft X-ray emission expected from the typically unresolved diffuse hot gas in starburst galaxies, devoid of competing emission from, e.g., active galactic nuclei (AGNs) or other unresolved point sources. We use stellar models of subsolar metallicities (0.02 Z (circle dot) and 0.4 Z (circle dot)) and take into account supernova explosions for massive stars. For lower metallicities, we find that stellar winds do not contribute significantly (less than or similar to 3% of the mechanical energy) to the observed soft X-ray emission of normal star-forming galaxies. For higher metallicities and possibly also for larger proportions of massive star clusters in the simulated starburst galaxies, we reproduce well the observed correlation between star formation rate and X-ray luminosity previously reported in the literature. However, we find that no combination of model assumptions is capable of reproducing the substantial soft X-ray emission observed from Green Pea galaxies, indicating that other emission mechanisms (i.e., unusually large quantities of high-/low-mass X-ray binaries, ultraluminous X-ray sources, a modified initial mass function, intermediate-mass black holes, or AGNs) are more likely to be responsible for the X-ray excess.