Luminous Supersoft X-Ray Sources in External Galaxies

We use a set of conservative criteria to identify luminous supersoft X-ray sources(SSSs) in external galaxies. We test this approach on blackbody models, and then apply it to {\it Chandra} data from 4 galaxies: an elliptical (NGC 4967), 2 face-on spirals (M101 and M83), and an interacting galaxy (M51). We find SSSs in every galaxy, and estimate that the total SSS population of sources with $L > 10^{37}$ erg s$^{-1}$ in each galaxy is at least several hundred and could be significantly larger. In addition, we discover two intriguing features of galactic populations of SSSs. First, there are significant subpopulations of high-luminosity sources; the bolometric luminosities can exceed $10^{39}$ erg s$^{-1}.$ Second, in the spiral galaxies M101, M83 and M51, SSSs appear to be associated with the spiral arms. This may indicate that some SSSs are young systems, possibly younger than $10^8$ years.