X-ray stellar feedback in low-metallicity starbursts: Insights from the template starburst galaxy ESO 338-IG04 and its halo

Abstract

The X-ray output of low-metallicity starburst galaxies is a key component of stellar feedback, tracing the processes responsible for gas ionization and chemical enrichment. The integrated X-ray luminosity ($L_X$) from high-mass X-ray binaries in star-forming galaxies scales with both the star formation rate (SFR) and host-galaxy metallicity $Z$. Due to the inverse correlation between $L_X/\mathrm{SFR}$ and $Z$, the contribution of X-ray binaries to the ionizing photon budget is enhanced in metal-poor systems and may ionize He II in the surrounding interstellar medium, powering nebular He II $\lambda4686$ emission. The blue compact dwarf galaxy ESO 338-IG04 (ESO 338-4) provides a nearby laboratory for studying stellar feedback in a low-metallicity starburst, combining vigorous recent star formation, low metallicity ($12+\log(\mathrm{O/H})\approx7.9$), and a rich population of massive stellar clusters. We characterize the X-ray emission of ESO 338-4 and its halo using new deep Chandra and XMM-Newton observations. We analyze X-ray spectra, light curves, and images to constrain the nature of its X-ray sources. We identify five ultra-luminous X-ray sources (ULXs) and diffuse hot gas surrounding the galaxy. Two ULXs are spatially associated with stellar clusters. The total X-ray luminosity exceeds $10^{41}\,\mathrm{erg\,s^{-1}}$. The brightest source, ULX1, shows variability on day timescales and lacks a stellar-cluster counterpart. Photoionization modeling shows that X-ray sources significantly impact the ionizing photon budget; models with ULX1 as the ionizing source predict nebular He II $\lambda4686$ luminosities of $\sim10^{39}\,\mathrm{erg\,s^{-1}}$.