Catalogues of hot white dwarfs in the Milky Way from GALEX's ultraviolet sky surveys: constraining stellar evolution

We present comprehensive catalogues of hot star candidates in the Milky Way (MW), selected from Galaxy Evolution Explorer (GALEX) far-UV (FUV; 1344-1786 Å) and near-UV (NUV; 1771-2831 Å) imaging. The FUV and NUV photometry allows us to extract the hottest stellar objects, in particular hot white dwarfs (WD), which are elusive at other wavelengths because of their high temperatures and faint optical luminosities. We generated catalogues of UV sources from two GALEX's surveys: All-Sky Imaging Survey (AIS; depth AB magnitude ∼19.9/20.8 in FUV/NUV) and Medium-depth Imaging Survey (MIS; depth ∼22.6/22.7 mag). The two catalogues (from GALEX fifth data release) contain 65.3/12.6 million (AIS/MIS) unique UV sources with error_NUV≤ 0.5 mag, over 21 435/1579 deg². We also constructed subcatalogues of the UV sources with matched optical photometry from Sloan Digital Sky Survey (SDSS; seventh data release): these contain 0.6/0.9 million (AIS/MIS) sources with errors ≤0.3 mag in both FUV and NUV, excluding sources with multiple optical counterparts, over an area of 7325/1103 deg². All catalogues are available online. We then selected 28 319 (AIS)/9028 (MIS) matched sources with FUV - NUV < -0.13; this colour cut corresponds to stellar T_eff hotter than ∼18 000 K (the exact value varying with gravity). An additional colour cut of NUV-r > 0.1 isolates binaries with largely differing T_effs, and some intruding quasi-stellar objects (QSOs; more numerous at faint magnitudes). Available spectroscopy for a subsample indicates that hot-star candidates with NUV-r < 0.1 (mostly 'single' hot stars) have negligible contamination by non-stellar objects. We discuss the distribution of sources in the catalogues, and the effects of error and colour cuts on the samples. The density of hot-star candidates increases from high to low Galactic latitudes, but drops on the MW plane due to dust extinction. Our hot-star counts at all latitudes are better matched by MW models computed with an initial-final mass relation (IFMR) that favours lower final masses. The model analysis indicates that the brightest sample is likely composed of WDs located in the thin disc, at typical distances between 0.15 and 1 kpc, while the fainter sample comprises also a fraction of thick disc and halo stars. Proper motion distributions, available only for the bright sample (NUV < 18 mag), are consistent with the kinematics of a thin-disc population.