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., AGN or other
unresolved point sources. We use stellar models of sub-solar metallicities
(0.02 $Z_{\odot}$ and 0.4 $Z_{\odot}$), and take into account supernova
explosions for massive stars. For lower metallicities, we find that stellar
winds do not contribute significantly ($\lesssim 3$ % 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, Ultra-Luminous X-ray sources, a
modified initial mass function, Intermediate-Mass Black Holes, or AGN) are more
likely to be responsible for the X-ray excess.
