Elevated Hot Gas and High-Mass X-ray Binary Emission in Low Metallicity Galaxies: Implications for Nebular Ionization and Intergalactic Medium Heating in the Early Universe

Abstract

High-energy emission associated with star formation has been proposed as a significant source of interstellar medium (ISM) ionization in low-metallicity starbursts and an important contributor to the heating of the intergalactic medium (IGM) in the high-redshift ($z>8$) Universe. Using Chandra observations of a sample of 30 galaxies at $D\approx$~200--450 Mpc that have high specific star-formation rates of 3--9 Gyr${}^{-1}$ and metallicities near $Z\approx 0.3Z_{\odot }$, we provide new measurements of the average 0.5--8 keV spectral shape and normalization per unit star-formation rate (SFR). We model the sample-combined X-ray spectrum as a combination of hot gas and high-mass X-ray binary (HMXB) populations and constrain their relative contributions. We derive scaling relations of $\log L_{0.5-8keV}^{\mathrm{HMXB}}$/SFR $=40.19\pm 0.06$ and $\log L_{0.5-2keV}^{\mathrm{gas}}$/SFR $=39.58_{-0.28}^{+0.17}$; significantly elevated compared to local relations. The HMXB scaling is also somewhat higher than $L_{0.5-8keV}^{\mathrm{HMXB}}$-SFR-$Z$ relations presented in the literature, potentially due to our galaxies having relatively low HMXB obscuration and young and X-ray luminous stellar populations. The elevation of the hot gas scaling relation is at the level expected for diminished attenuation due to a reduction of metals; however, we cannot conclude that an $L_{0.5-2keV}^{\mathrm{gas}}$-SFR-$Z$ relation is driven solely by changes in ISM metal content. Finally, we present SFR-scaled spectral models (both emergent and intrinsic) that span the X-ray--to--IR band, providing new benchmarks for studies of the impact of ISM ionization and IGM heating in the early Universe.

Accepted for publication in ApJ (25 pages, 9 figures, and 4 tables)

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