X-ray emission from star cluster winds in starburst galaxies

TL;DR

This study models the soft X-ray emission from star cluster winds in starburst galaxies, finding that stellar winds alone cannot explain the excess emission observed in Green Pea galaxies, suggesting other sources are involved.

Contribution

It provides a detailed modeling of X-ray emission from star cluster winds at different metallicities, highlighting the limitations of stellar winds in explaining observed X-ray excesses.

Findings

Stellar winds contribute minimally to soft X-ray emission at low metallicity.

Models reproduce the SFR-X-ray luminosity correlation at higher metallicities.

Other mechanisms likely cause the X-ray excess in Green Pea galaxies.

Abstract

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.