A census of ultraluminous X-ray sources in the local Universe

Abstract

ABSTRACT Using the Chandra Source Catalog 2.0 and a newly compiled catalogue of galaxies in the local Universe, we deliver a census of ultraluminous X-ray source (ULX) populations in nearby galaxies. We find 629 ULX candidates in 309 galaxies with distance smaller than 40 Mpc. The foreground/background contamination is $\sim \! 20{{\ \rm per\ cent}}$. The ULX populations in bona fide star-forming galaxies scale on average with star formation rate and stellar mass (M⋆) such that the number of ULXs per galaxy is $0.45^{+0.06}_{-0.09}\times \frac{\rm SFR}{\rm M_\odot \, yr^{-1}}{+}3.3^{+3.8}_{-3.2}\times \frac{M_\star }{\rm M_\odot }$. The scaling depends strongly on the morphological type. This analysis shows that early spiral galaxies contain an additional population of ULXs that scales with M⋆. We also confirm the strong anticorrelation of the ULX rate with the host galaxy’s metallicity. In the case of early-type galaxies, we find that there is a non-linear dependence of the number of ULXs with M⋆, which is interpreted as the result of star formation history differences. Taking into account age and metallicity effects, we find that the predictions from X-ray binary population synthesis models are consistent with the observed ULX rates in early-type galaxies, as well as spiral/irregular galaxies.