The contribution of X-ray binaries to the evolution of late-type galaxies: Evolutionary population synthesis simulations

TL;DR

This study models the X-ray evolution of late-type galaxies over 14 billion years, revealing a decreasing X-ray luminosity-to-mass ratio consistent with stellar evolution, but shows a discrepancy with observed X-ray-to-optical ratios at higher redshifts.

Contribution

It introduces a population synthesis simulation of galaxy X-ray evolution that accounts for stellar and binary evolution over cosmic time.

Findings

X-ray luminosity-to-mass ratio decreases with time, matching observations.

X-ray-to-optical luminosity ratio remains nearly constant over time.

Discrepancy between model predictions and observed increase in X-ray-to-optical ratio at higher redshifts.

Abstract

X-ray studies of normal late-type galaxies have shown that non-nuclear X-ray emission is typically dominated by X-ray binaries, and provides a useful measure of star formation activity. We have modeled the X-ray evolution of late-type galaxies over the $\sim$ 14 Gyr of cosmic history, with an evolutionary population synthesis code developed by Hurley et al. Our calculations reveal a decrease of the X-ray luminosity-to-mass ratio $L_{\rm X}/M$ with time, in agreement with observations (Fig.~7$a$). We show that this decrease is a natural consequence of stellar and binary evolution and mass accumulating process in galaxies. The X-ray-to-optical luminosity ratio $L_{\rm X}/L_{\rm B}$ is found to be fairly constant (around $\sim 10^{30}$ erg\,s$^{-1}$$L_{\rm B,\odot}^{-1}$, Fig.~7$b$), and insensitive to the star formation history in the galaxies. The nearly constant value of $L_{\rm X}/L_{\rm B}$ is in conflict with the observed increase in $L_{\rm X}/L_{\rm B}$ from $z=0$ to 1.4. The discrepancy may be caused by intense obscured star formation activity that leads to nonlinear relationship between X-ray and B-band emission.