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

High-energy emission associated with star formation has been proposed as a significant source of interstellarmedium (ISM) ionization in low-metallicity starbursts and an important contributor to the heating of theintergalactic medium (IGM) in the high-redshift (z  8) universe. Using Chandra observations of a sample of 30galaxies at D ≈ 200–450 Mpc that have high specific star formation rates of 3–9 Gyr−1 and metallicities nearZ ≈ 0.3Ze, we provide new measurements of the average 0.5–8 keV spectral shape and normalization per unit starformation rate (SFR). We model the sample-combined X-ray spectrum as a combination of hot gas and high-massX-ray binary (HMXB) populations and constrain their relative contributions. We derive scaling relations ofLlog 0.5 8 keVHMXB– /SFR = 40.19 ± 0.06 and Llog 0.5 2 keVgas– /SFR 39.58 ;0.280.17= -+ significantly elevated compared to localrelations. The HMXB scaling is also somewhat higher than L0.5 8 keVHMXB– –SFR-Z relations presented in the literature,potentially due to our galaxies having relatively low HMXB obscuration and young and X-ray luminous stellarpopulations. The elevation of the hot gas scaling relation is at the level expected for diminished attenuation due to areduction of metals; however, we cannot conclude that an L0.5 2 keVgas– –SFR-Z relation is driven solely by changes inISM metal content. Finally, we present SFR-scaled spectral models (both emergent and intrinsic) that span theX-ray-to-IR band, providing new benchmarks for studies of the impact of ISM ionization and IGM heating in theearly universe.