On the X-ray Spectral Energy Distributions of Star-Forming Galaxies: the 0.3-30 keV Spectrum of the Low-Metallicity Starburst Galaxy VV 114

TL;DR

This study presents detailed X-ray spectral energy distributions of the low-metallicity starburst galaxy VV 114, highlighting the dominance of X-ray binaries and ULXs in its 0.3-30 keV emission, with implications for early universe heating.

Contribution

First comprehensive spectral modeling of VV 114 across 0.3-30 keV, combining multiple X-ray observatories to constrain low-metallicity galaxy X-ray SEDs.

Findings

X-ray emission dominated by XRBs and ULXs above 1.5 keV

Elevated X-ray luminosity per star formation rate compared to higher-metallicity galaxies

Implications for heating of the intergalactic medium in the early universe

Abstract

Binary population synthesis combined with cosmological models suggest that X-ray emission from star-forming galaxies, consisting primarily of emission from X-ray binaries (XRBs) and the hot interstellar medium (ISM), could be an important, and perhaps dominant, source of heating of the intergalactic medium prior to the epoch of reionization. However, such models rely on empirical constraints for the X-ray spectral energy distributions (SEDs) of star-forming galaxies, which are currently lacking for low-metallicity galaxies. Using a combination of Chandra, XMM-Newton, and NuSTAR observations, we present new constraints on the 0.3-30 keV SED of the low-metallicity starburst galaxy VV 114, which is known to host several ultra-luminous X-ray sources (ULXs) with luminosities above 10$^{40}$ erg s$^{-1}$. We use an archival Chandra observation of VV 114 to constrain the contributions to the X-ray SED from the major X-ray emitting components of the galaxy, and newly acquired, nearly simultaneous XMM-Newton and NuSTAR observations to extend the spectral model derived from Chandra to cover the 0.3-30 keV range. Using our best-fit galaxy-wide spectral model, we derive the 0.3-30 keV SED of VV 114, which we find is dominated by emission from the XRB population, and in particular ULXs, at energies > 1.5 keV, and which we find to have an elevated galaxy-integrated X-ray luminosity per unit star formation rate relative to higher-metallicity star-forming galaxies. We discuss our results in terms of the effect of metallicity on XRB populations and the hot ISM, and the importance of X-ray emission from star-forming galaxies in the high redshift Universe.