Search

We present a new empirical framework modeling the metallicity and star formation history (SFH) dependence of X-ray luminous ( L ≳ 10 ^36 erg s ^−1 ) point-source population X-ray luminosity functions (XLFs) in normal galaxies. We expect that the X-ray point-source populations are dominated by X-ray binaries (XRBs), with contributions from supernova remnants near the low luminosity end of our observations. Our framework is calibrated using the collective statistical power of 3731 X-ray detected point sources within 88 Chandra-observed galaxies at D ≲ 40 Mpc that span broad ranges of metallicity ( Z ≈ 0.03–2 Z _⊙ ), SFH, and morphology (dwarf irregulars, late types, and early types). Our best-fitting models indicate that the XLF normalization per unit stellar mass declines by ≈2–3 dex from 10 Myr to 10 Gyr, with a slower age decline for low-metallicity populations. The shape of the XLF for luminous X-ray sources ( L ≳ 10 ^38 erg s ^−1 ) significantly steepens with increasing age and metallicity, while the lower-luminosity XLF appears to flatten with increasing age. Integration of our models provides predictions for X-ray scaling relations that agree very well with past results presented in the literature, including, e.g., the L _X –SFR– Z relation for high-mass XRBs in young stellar populations as well as the L _X / M _⋆ ratio observed in early-type galaxies that harbor old populations of low-mass XRBs. The model framework and data sets presented in this paper further provide unique benchmarks that can be used for calibrating binary population synthesis models.

Drawing from the Chandra archive and using a carefully selected set of nearby dwarf galaxies, we present a calibrated high-mass X-ray binary (HMXB) luminosity function in the low-mass galaxy regime and search for an already hinted at dependence on metallicity. Our study introduces a new sample of local dwarf galaxies ( D < 12.5 Mpc and M _* < 5 × 10 ^9 M _⊙ ), expanding the specific star formation rates (sSFR) and gas-phase metallicities probed in previous investigations. Our analysis of the observed X-ray luminosity function indicates a shallower power-law slope for the dwarf galaxy HMXB population. In our study, we focus on dwarf galaxies that are more representative in terms of sSFR compared to prior work. In this regime, the HMXB luminosity function exhibits significant stochastic sampling at high luminosities. This likely accounts for the pronounced scatter observed in the galaxy-integrated HMXB population’s L _X /SFR versus metallicity for our galaxy sample. Our calibration is necessary to understand the active galactic nuclei content of low-mass galaxies identified in current and future X-ray survey fields and has implications for binary population synthesis models, as well as X-ray-driven cosmic heating in the early Universe.