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

摘要

X-ray studies of normal late-type galaxies have shown that non-nuclear X-rayemission is typically dominated by X-ray binaries, and provides a usefulmeasure of star formation activity. We have modeled the X-ray evolution oflate-type galaxies over the $\sim$ 14 Gyr of cosmic history, with anevolutionary population synthesis code developed by Hurley et al. Ourcalculations reveal a decrease of the X-ray luminosity-to-mass ratio $L_{\rmX}/M$ with time, in agreement with observations (Fig.~7$a$). We show that thisdecrease is a natural consequence of stellar and binary evolution and massaccumulating process in galaxies. The X-ray-to-optical luminosity ratio $L_{\rmX}/L_{\rm B}$ 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 starformation history in the galaxies. The nearly constant value of $L_{\rmX}/L_{\rm B}$ is in conflict with the observed increase in $L_{\rm X}/L_{\rmB}$ from $z=0$ to 1.4. The discrepancy may be caused by intense obscured starformation activity that leads to nonlinear relationship between X-ray andB-band emission.