A Panchromatic Study of the X-ray Binary Population in NGC 300 on Sub-Galactic Scales

Abstract

The population-wide properties and demographics of extragalactic X-ray binaries (XRBs) correlate with the star formation rates (SFRs), stellar masses ($M_{\star }$), 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 $L_X$/$M_{\star }$ for low mass XRBs [LMXBs]) 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 ($<$100 Myr) SFR of 0.18$\pm$0.08 $M_{\odot }$ yr${}^{-1}$ and $M_{\star }$= $(2.15_{-0.14}^{+0.26})\times 10^9$ $M_{\odot }$. 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 $\sim 10^{37}$ erg s${}^{-1}$ that are likely a mix of variable XRBs and additional background AGN. 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 ($\lesssim$30 Myr) HMXBs agree with predictions, hinting at time evolution of the $L_X$-SFR-$Z$ relationship.

Accepted to AAS Journals; 22 pages, 12 figures, 6 tables