Old and Young X‐Ray Point Source Populations in Nearby Galaxies

We analyzed 1441 Chandra X-ray point sources in 32 nearby galaxies. The totalpoint-source X-ray luminosity L_XP is well correlated with B, K, and FIR+UVluminosities of spiral host galaxies, and with the B and K luminosities forellipticals. This suggests an intimate connection between L_XP and both the oldand young stellar populations, for which K and FIR+UV luminosities are proxiesfor the galaxy mass M and star-formation rate SFR. We derive proportionalityconstants 1.3E29 erg/s/Msol and 0.7E39 erg/s/(Msol/yr), which can be used toestimate the old and young components from M and SFR, respectively. Thecumulative X-ray luminosity functions for the point sources have quitedifferent slopes for the spirals (gamma ~= 0.5-0.8) and ellipticals (gamma ~=1.4), implying *the most luminous point sources dominate L_XP* for the spirals.Most of the point sources have X-ray colors that are consistent with eitherLMXBs or Ultraluminous X-ray sources (ULXs a.k.a. IXOs) and we rule outclassical HMXBs (e.g. neutron-star X-ray pulsars) as contributing much to L_XP.Thus, for spirals, the ULXs dominate L_XP. We estimate that >~20% of all ULXsfound in spirals originate from the older (pop II) stellar populations,indicating that many of the ULXs that have been found in spiral galaxies are infact pop II ULXs, like those in elliptical galaxies. The linear dependence ofL_XP on the SFR argues for either a steepening in the X-ray luminosity functionof the young (pop I) X-ray source population at L_X >~10^(38.5-39) erg/s, or adecreasing efficiency for producing all types of young X-ray point sources asthe galaxy SFR increases.