A Panchromatic Study of the X-Ray Binary Population in NGC 300 on Subgalactic Scales

Abstract

The population-wide properties and demographics of extragalactic X-ray binaries (XRBs) correlate with the star formation rates (SFRs), stellar masses ( M _⋆ ), and environmental factors (such as metallicity, Z ) of their host galaxy. Although there is evidence that XRB scaling relations ( L _X /SFR for high-mass XRBs (HMXBs) and L _X / M _⋆ for low-mass XRBs) may depend on metallicity and stellar age across large samples of XRB-hosting galaxies, disentangling the effects of metallicity and stellar age from stochastic effects, particularly on subgalactic scales, remains a challenge. We use archival X-ray through IR observations of the nearby galaxy NGC 300 to self-consistently model the broadband spectral energy distribution and examine radial trends in its XRB population. We measure a current (<100 Myr) SFR of 0.18 ± 0.08 M _⊙ yr ^−1 and stellar mass M _⋆ = $({2.15}_{-0.14}^{+0.26})\times {10}^{9}$ M _⊙ . Although we measure a metallicity gradient and radially resolved star formation histories that are consistent with the literature, there is a clear excess in the number of X-ray sources below ∼10 ^37 erg s ^−1 that are likely a mix of variable XRBs and additional background active galactic nuclei. When we compare the subgalactic L _X /SFR ratios as a function of Z to the galaxy-integrated L _X -SFR- Z relationships from the literature, we find that only the regions hosting the youngest (≲30 Myr) HMXBs agree with predictions, hinting at time evolution of the L _X –SFR– Z relationship.

Keywords

• X-ray binary stars
• High mass x-ray binary stars
• Compact objects
• Spiral galaxies