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

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 $L_{\rm X}/M$withtime,inagreementwithobservations(Fig\mathrm{.}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 $L_{\rm X}/L_{\rm B}$isfoundtobefairlyconstant(around$\sim 10^{30}$ergs$^{-1}$$L_{\mathrm{B},\odot }^{-1}$, Fig.~7$b$), and insensitive to the star formation history in the galaxies. The nearly constant value of $L_{\rm X}/L_{\rm B}$isinconflictwiththeobservedincreasein$L_{\rm X}/L_{\rm B}$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.