On the Spatially Resolved Star-Formation History in M51 II: X-ray Binary Population Evolution

TL;DR

This paper introduces a new empirical method to calibrate the evolution of X-ray binary luminosity functions with stellar population age in M51, revealing how the XLF shape and normalization change over time.

Contribution

It develops a novel technique combining stellar population synthesis and X-ray data to study XRB evolution on subgalactic scales, improving understanding of their age-dependent properties.

Findings

XRB XLF normalization declines by ~3-3.5 dex from 10 Myr to 10 Gyr.

XRB XLF slope steepens with increasing stellar age.

Method recovers known age-related XRB properties and offers a self-consistent evolution model.

Abstract

We present a new technique for empirically calibrating how the X-ray luminosity function (XLF) of X-ray binary (XRB) populations evolves following a star-formation event. We first utilize detailed stellar population synthesis modeling of far-UV to far-IR photometry of the nearby face-on spiral galaxy M51 to construct maps of the star-formation histories (SFHs) on subgalactic (~400 pc) scales. Next, we use the ~850 ks cumulative Chandra exposure of M51 to identify and isolate 2-7 keV detected point sources within the galaxy, and we use our SFH maps to recover the local properties of the stellar populations in which each X-ray source is located. We then divide the galaxy into various subregions based on their SFH properties (e.g., star-formation rate [SFR] per stellar mass [M*] and mass-weighted stellar age) and group the X-ray point sources according to the characteristics of the regions in which they are found. Finally, we construct and fit a parameterized XLF model that quantifies how the XLF shape and normalization evolves as a function of the XRB population age. Our best-fit model indicates the XRB XLF per unit stellar mass declines in normalization, by ~3-3.5 dex, and steepens in slope from ~10 Myr to ~10 Gyr. We find that our technique recovers results from past studies of how XRB XLFs and XRB luminosity scaling relations vary with age and provides a self-consistent picture for how the XRB XLF evolves with age.