GALEX catalogs of UV sources: statistical properties and sample science applications: hot white dwarfs in the Milky Way

We describe the content and properties of UV source catalogs from GALEX's All-Sky Imaging Survey (AIS, 5σ depth ≈19.9(FUV)/20.8(NUV) mag, in the AB system) and Medium-depth Imaging Survey (MIS, 5σ depth ≈22.6(FUV)/22.7(NUV) mag), constructed by Bianchi L., et al.: Mon. Not. R. Astron. Soc. (2010, in press). The catalogs contain 65.3/12.6 million (AIS/MIS) unique UV sources with photometric error in NUV less than 0.5 mag, over 21 435(AIS)/1579(MIS) square degrees. Matched optical data from GSC-II provide additional B, R, I photometry for the brightest sources, and SDSS provides u g r i z photometry over 7325(AIS)/1103(MIS) square degrees overlap areas. We discuss statistical properties that are relevant for understanding sample selection biases and completeness, in potential science applications of these catalogs. The FUV (1344-1786 Å) and NUV (1771-2831 Å) photometry uniquely enable selection of the hottest stellar objects, in particular hot white dwarfs (WD), which are elusive at optical wavelengths because of their hot temperatures and faint luminosities. From the GALEX-SDSS matched sources we selected ˜40 000 Milky Way (MW) stars hotter than about 18 000 K (FUV-NUV < -0.13). Their density increases towards low Galactic latitudes, but drops in the MW disk due to dust extinction. The hot-WD density at different Galactic latitudes, analyzed with Milky Way models, constrains the Initial-Final Mass Relation (IFMR), relevant for understanding the yield of chemical elements from intermediate-mass stars and the chemical enrichment of the Galaxy.