Old and young x-ray point source populations in nearby galaxies

We have analyzed Chandra ACIS observations of 32 nearby spiral and elliptical galaxies and present the results of 1441 X-ray point sources that were detected in these galaxies. The total point-source X-ray (0.3-8.0 keV) luminosity L-XP is well correlated with the B-band, K-band, and FIR+UV luminosities of spiral host galaxies and is well correlated with the B-band and K-band luminosities of elliptical galaxies. This suggests an intimate connection between L-XP and both the old and young stellar populations, for which K and FIR+UV luminosities are reasonable proxies for the galaxy mass M and star formation rate SFR. We derive proportionality constants alpha=1.3x10(29) ergs s(-1) M-circle dot(-1) and beta=0.7x10(39) ergs s(-1) (M-circle dot yr(-1))(-1), which can be used to estimate the old and young components from M and SFR, respectively. The cumulative X-ray luminosity functions for the point sources have significantly different slopes. For the spiral and starburst galaxies, gammaapproximate to0.6-0.8, and for the elliptical galaxies, gammaapproximate to1.4. This implies that the most luminous point sources-those with L(X)greater than or similar to10(38) ergs s(-1)-dominate L-XP for the spiral and starburst galaxies. Most of the point sources have X-ray colors that are consistent with soft-spectrum (photon index Gammasimilar to1-2) low-mass X-ray binaries, accretion-powered black hole high-mass X-ray binaries (BH HMXBs), or ultraluminous X-ray sources (ULXs, also known as IXOs). We rule out hard-spectrum neutron star HMXBs (e.g., accretion-powered X-ray pulsars) as contributing much to L-XP. Thus, for spirals, L-XP is dominated by ULXs and BH HMXBs. We find no discernible difference between the X-ray colors of ULXs (L(X)greater than or equal to10(39) ergs s(-1)) in spiral galaxies and point sources with LXapproximate to10(38)-10(39) ergs s(-1). We estimate that greater than or similar to20% of all ULXs found in spirals originate from the older (Population II) stellar populations, indicating that many of the ULXs that have been found in spiral galaxies are in fact Population II ULXs, like those in elliptical galaxies. We find that L-XP depends linearly (within uncertainties) on both M and SIR for our sample galaxies (Mless than or equal to10(11) M-circle dot and SFRless than or similar to10 M-circle dot yr(-1)).