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 ≈ 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 0.5 – 8 keV HMXB /SFR = 40.19 ± 0.06 and log L 0.5 – 2 keV 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 – 8 keV 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 – 2 keV 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.

The ionizing and heating power of ultraluminous X-ray sources under the geometrical beaming model

“…We find that in only a small fraction (∼1%) of galaxies the ULXs compete the underlying stellar populations in He ii emission, in agreement with recent studies (e.g., Senchyna et al 2020), while they are significant in the case of Ne v (e.g., Simmonds et al 2021). On the other hand, observational studies may also put constraints on the contribution of the hot gas component in star-forming galaxies, which is found to be comparable to the stellar component in ionizing the He ii in low-metallicity galaxies (e.g., Lehmer et al 2022). From our modeling, the Ne v-ionizing power of ULXs points at the possibility of constraints for the EUV emission from ULXs with the use of emission lines associated with highionization potentials.…”

“…One of the most important difficulties in understanding the contribution of X-ray sources, which are typically observed at energy bands 0.2 − 10 keV, is that their spectra are poorly constrained at the EUV regime (He ii: ∼54 eV, Ne v: ∼97.1 eV; see Lehmer et al 2022). However, ULXs are strong candidates for four reasons: (i) ULXs exhibit softer spectra than typical XRBs and can potentially produce high rate of He-ionizing photons (e.g., Simmonds et al 2021), (ii) they have been shown to dominate the X-ray emission from normal galaxies due to the shallow slope of the high-mass XRB (HMXB) luminosity function (XLF; e.g., Lehmer et al 2019), (iii) they are abundant in low-metallicity environments (e.g., Mapelli et al 2010), and (iv) there is evidence of geometrical beaming (e.g., King 2009), which indicates a larger underlying population of ULXs, and hence a stronger contribution in the ionization of the ISM than what is expected by the observed population (e.g., Rickards Vaught et al 2021).…”

First Insights into the ISM at $z>8$ with JWST: Possible Physical Implications of a High [O III]$\mathbf{λ4363}$/[O III]$\mathbf{λ5007}$

“…High-energy photons from X-ray sources (including high-mass Xray binaries or HMXBs) can potentially provide additional heating in the ISM (e.g. Lebouteiller et al 2017;Lehmer et al 2022), above what one would expect from photoionization due to stars alone. Observational data has shown that the X-ray luminosity per unit star formation rate increases significantly at low metallicities (e.g.…”

The ionizing and heating power of ultraluminous X-ray sources under the geometrical beaming model

“…We find that in only a small fraction (∼1%) of galaxies do the ULXs compete with the underlying stellar populations in He ii emission, which is in agreement with recent studies (e.g., Senchyna et al 2020), while they are significant in the case of Ne v (e.g., Simmonds et al 2021). On the other hand, observational studies may also put constraints on the contribution of the hot gas component in star-forming galaxies, which is found to be comparable to the stellar component in ionizing the He ii in lowmetallicity galaxies (e.g., Lehmer et al 2022). From our modeling, the Ne v-ionizing power of ULXs points at the possibility of constraints for the EUV emission from ULXs with the use of emission lines associated with high-ionization potentials.…”

“…One of the most important difficulties in understanding the contribution of X-ray sources, which are typically observed at energy bands 0.2 − 10 keV, is that their spectra are poorly constrained at the extreme-ultraviolet (EUV) regime (He ii: ∼54 eV, Ne v: ∼97.1 eV; see Lehmer et al 2022). However, ULXs are strong candidates for four reasons: (i) ULXs exhibit softer spectra than typical XRBs and can potentially produce a high rate of He-ionizing photons (e.g., Simmonds et al 2021); (ii) they have been shown to dominate the X-ray emission from normal galaxies due to the shallow slope of the high-mass XRB (HMXB) luminosity function (XLF; e.g., Lehmer et al 2019); (iii) they are abundant in low-metallicity environments (e.g., Mapelli et al 2010); and (iv) there is evidence of geometrical beaming (e.g., King 2009), which indicates a larger underlying population of ULXs, and hence a stronger contribution in the ionization of the ISM than what is expected by the observed population (e.g., Rickards Vaught et al 2021).…”