The X-ray Luminosity Functions of Field Low Mass X-ray Binaries in
  Early-Type Galaxies: Evidence for a Stellar Age Dependence
  We present direct constraints on how the formation of low-mass X-ray binary
(LMXB) populations in galactic fields depends on stellar age. In this pilot
study, we utilize Chandra and Hubble Space Telescope (HST) data to detect and
characterize the X-ray point source populations of three nearby early-type
galaxies: NGC 3115, 3379, and 3384. The luminosity-weighted stellar ages of our
sample span 3-10 Gyr. X-ray binary population synthesis models predict that the
field LMXBs associated with younger stellar populations should be more numerous
and luminous per unit stellar mass than older populations due to the evolution
of LMXB donor star masses. Crucially, the combination of deep Chandra and HST
observations allows us to test directly this prediction by identifying and
removing counterparts to X-ray point sources that are unrelated to the field
LMXB populations, including LMXBs that are formed dynamically in globular
clusters, Galactic stars, and background AGN/galaxies. We find that the "young"
early-type galaxy NGC 3384 (~2-5 Gyr) has an excess of luminous field LMXBs
(L_X > (5-10) x 10^37 erg/s) per unit K-band luminosity (L_K; a proxy for
stellar mass) than the "old" early-type galaxies NGC 3115 and 3379 (~8-10 Gyr),
which results in a factor of ~2-3 excess of LX/LK for NGC 3384. This result is
consistent with the X-ray binary population synthesis model predictions;
however, our small galaxy sample size does not allow us to draw definitive
conclusions on the evolution field LMXBs in general. We discuss how future
surveys of larger galaxy samples that combine deep Chandra and HST data could
provide a powerful new benchmark for calibrating X-ray binary population
synthesis models.
