A census of ultraluminous X-ray sources in the local Universe
  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 ${\sim}20\%$. The ULX populations in bona-fide star-forming
galaxies scale on average with star-formation rate (SFR) and stellar mass
($M_\star$) 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
$M_\star$. We also confirm the strong anti-correlation 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 $M_\star$, 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.
