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

• Published in: The Astrophysical journal 2022-05, Vol.930 (2), p.135
• Main Authors: Lehmer, Bret D., Eufrasio, Rafael T., Basu-Zych, Antara, Garofali, Kristen, Gilbertson, Woodrow, Mesinger, Andrei, Yukita, Mihoko
• Format: Article
• Language: English
• Subjects: Astrophysics; Benchmarks; Binary stars; Compact objects; Emission; Galaxies; Heating; High mass x-ray binary stars; Intergalactic media; Interstellar matter; Interstellar medium; Ionization; Metal content; Metallicity; Occultation; Red shift; Reduction (metal working); Scaling; Space Sciences (General); Star & galaxy formation; Star formation; Star formation rate; Starburst galaxies; Stars & galaxies; Stellar populations; Universe; X ray binaries; X ray spectra; X ray stars; X-ray astronomy; X-ray binary stars; X-rays
• ISSN: 0004-637X; 1538-4357
• DOI: 10.3847/1538-4357/ac63a7

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 ≈ 200–450 Mpc that have high specific star formation rates of 3–9 Gyr^(−1) and metallicities near Z ≈ 0.3Z_⊙, 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^(HMXB)_(0.5-8keV) / SFR = 40.19 ± 0.06 and log L^(gas)_(0.5-2keV) / SFR = 39.58 ^(+0.17)_(-0.28), significantly elevated compared to local relations. The HMXB scaling is also somewhat higher than L^(HMXB)_(0.5-8keV) -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^(gas)_(0.5-2keV) -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.