Properties of Primitive Galaxies

TL;DR

This study analyzes nine nearby low-metallicity, star-forming galaxies using Hubble's UV spectra, revealing evidence of X-ray irradiation from stellar black holes, which impacts their nebular emission and distinguishes them from more metal-rich galaxies.

Contribution

It provides new observational evidence linking low metallicity to increased X-ray irradiation from stellar black holes in primitive galaxies.

Findings

Primitive galaxies show strong X-ray irradiation evidence.

Lower metallicity correlates with higher X-ray activity.

Embedded stellar black holes influence nebular spectra.

Abstract

We report on a study of 9 nearby star-forming, very low-metallicity galaxies observed by Hubble's COS far-UV spectrograph that can serve as templates of high-z galaxies to be observed by JWST. We find that the nebular spectra of these primitive galaxies show evidence of irradiation by X-ray emitters. Following Thuan et al. (2004), we identify the sources of X-ray emission as massive X-ray binaries containing a massive accreting stellar black hole. We further find that the lower the metallicity, the higher the probability of strong X-irradiation. Following Heger et al. (2003), we suggest that these accreting black holes are produced by direct collapse of stars having initial masses greater than $\sim50\, M_\odot$. Our models of young star clusters with an embedded stellar black hole produce effects on the surrounding gaseous medium that are consistent with the observed spectra. We conclude that primitive galaxies are qualitatively different from more metal-rich galaxies in showing evidence of hard radiation that can best be explained by the presence of one or more embedded stellar black holes.