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

Abstract

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 deg 2 . 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 deg 2 . 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 eff s, and some intruding quasistellar 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.

Predicting dust extinction properties of star-forming galaxies from Hα/UV ratio

“…We use unique GALEX sources selected from All-sky Imaging Survey (AIS) of GALEX fifth data release (GR5), reaching down to the depth of ∼19.9 and ∼20.8 [AB mag] at far-UV (FUV; λ eff =1516Å) and near-UV (NUV; λ eff =2267Å), respectively. Here we use the catalogue published by Bianchi et al (2011), available on the MAST web site 4 . We cross-identified our SDSS (spectroscopic) galaxy sample with GALEX sources using 3 ′′ search radius.…”

The Expected Stellar Populations in the Kepler and CoRoT Fields

“…More recently, the code has become a fundamental tool to test stellar evolutionary tracks from the Padova-Trieste group (e.g. Girardi et al, 2009Girardi et al, , 2010Bianchi et al, 2011;Rosenfield & al., 2014). With the recent/ongoing addition of new quantities in the code, like the surface chemical composition and asteroseismic parameters, it is ready to be applied in the simulation and interpretation of large databases like those provided by CoRoT and Kepler.…”

A stellar population synthesis model for the study of ultraviolet star counts of the Galaxy

“…We found most of the real matched sources within this radius, with a very small fraction (<1%) having multiple matches, which were removed from the final catalogue. We also estimated the possible contamination by spurious matches (random coincidences) for the matched sources following the method of Bianchi et al (2011a). For this purpose, we used a match radius of 6.0 , which is equivalent to the resolution of WISE, to find the GALEX counterparts of WISE sources.…”

“…The GALEX has covered a large part of the sky, which provides an opportunity to explore and characterise these hot sources in the far-UV (FUV: 1344-1786 Å, λ eff = 1538.6 Å) and near-UV (NUV: 1771-2831 Å, λ eff = 2315.7 Å) wavebands with better resolution and greater sensitivity than the previous surveys. A vivid description of the source selection, FUV and NUV magnitude error cuts, and the statistical analysis of the GALEX catalogue is provided by Bianchi et al (2007), Bianchi (2009), and Bianchi et al (2011aBianchi et al ( , 2014. Detection of WDs and BHBs is one of the main achievements of GALEX as these sources are elusive in the other wavelength bands of the electromagnetic spectrum due to their high temperature.…”

“…Very few attempts had been made to study the star counts in ultraviolet (UV) prior to Galaxy Evolution Explorer (GALEX) because of lack of availability of UV photometric surveys (Brosch 1991;Cohen et al 1994). The advent of GALEX, which provided a wide sky coverage in UV, now allows for a new analysis of the UV sky (Xu et al 2005;Bianchi et al 2011aBianchi et al ,b, 2014). An attempt has also been made to predict the star counts in the X-ray band (Guillout et al 1996) by extending the Besançon model of stellar population synthesis (Robin & Crézé 1986) to the ROSAT position sensitive proportional counter energy bands.…”