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
Bret Lehmer, Rafael T. Eufrasio, Antara Basu‐Zych, Kristen Garofali, Woodrow Gilbertson, Andrei Mesinger, Mihoko Yukita
Abstract
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 ≈ 0.3 Z ⊙ , 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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>log</mml:mi> <mml:msubsup> <mml:mrow> <mml:mi>L</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>0.5</mml:mn> <mml:mo>–</mml:mo> <mml:mn>8</mml:mn> <mml:mspace width="0.33em"/> <mml:mi>keV</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>HMXB</mml:mi> </mml:mrow> </mml:msubsup> </mml:math> /SFR = 40.19 ± 0.06 and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>log</mml:mi> <mml:msubsup> <mml:mrow> <mml:mi>L</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>0.5</mml:mn> <mml:mo>–</mml:mo> <mml:mn>2</mml:mn> <mml:mspace width="0.33em"/> <mml:mi>keV</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>gas</mml:mi> </mml:mrow> </mml:msubsup> </mml:math> /SFR <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mo>=</mml:mo> <mml:mspace width="0.50em"/> <mml:msubsup> <mml:mrow> <mml:mn>39.58</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.28</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>0.17</mml:mn> </mml:mrow> </mml:msubsup> <mml:mo>;</mml:mo> </mml:math> significantly elevated compared to local relations. The HMXB scaling is also somewhat higher than <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mrow> <mml:mi>L</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>0.5</mml:mn> <mml:mo>–</mml:mo> <mml:mn>8</mml:mn> <mml:mspace width="0.33em"/> <mml:mi>keV</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>HMXB</mml:mi> </mml:mrow> </mml:msubsup> </mml:math> –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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mrow> <mml:mi>L</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>0.5</mml:mn> <mml:mo>–</mml:mo> <mml:mn>2</mml:mn> <mml:mspace width="0.33em"/> <mml:mi>keV</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>gas</mml:mi> </mml:mrow> </mml:msubsup> </mml:math> –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.