The X-ray Luminosity Functions of Field Low Mass X-ray Binaries in Early-Type Galaxies: Evidence for a Stellar Age Dependence

TL;DR

This study investigates how the population of low-mass X-ray binaries in early-type galaxies depends on stellar age, using combined Chandra and HST data to compare young and old galaxies and test population synthesis models.

Contribution

It provides direct observational constraints on the age dependence of field LMXB populations in early-type galaxies, supporting model predictions with new data.

Findings

Younger galaxy NGC 3384 has more luminous LMXBs per unit stellar mass.

Older galaxies NGC 3115 and 3379 show fewer luminous LMXBs.

Results align with population synthesis model predictions, but sample size is limited.

Abstract

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.