Stellar-mass compact object evolution from the deepest X-ray surveys of the extragalactic Universe*

The ever-increasing depth of X-ray surveys raises the possibility of detecting extremely X-ray faint source populations, including the X-ray faint early-type galaxy population. Such a population of galaxies presents the opportunity to study the long-term evolution of low-mass X-ray binary (LMXB) populations. To this end, we have assembled a sample of $\sim 400$ low-luminosity early-type galaxies over $0.05 < z < 1.2$ in the three deep {\it Chandra} surveys (the CDF-S, E-CDF-S and CDF-N). Even with the 4~Ms {Chandra} Deep Field coverage currently available, the deepest survey of the extragalactic sky ever conducted at X-ray wavelengths, the vast majority of these galaxies ($>90$\%) are undetected, so our work relies heavily on stacking analysis. We compare our observational constraints with new theoretical models and discuss possibilities for future deep X-ray observations.