Abstract
Abstract We have matched the ultraviolet (UV) sources in GUVcat_AIS with optical databases that have similar depth and wide sky coverage. GUVcat_AIS has Galaxy Evolution Explorer (GALEX) far-UV (FUV, λ eff ∼ 1528 Å) and near-UV (NUV, λ eff ∼ 2310 Å) photometry of ≈83 million sources, covering 24,788 square degrees of the sky, with a typical depth of FUV = 19.9 and NUV = 20.8 AB mag. Matches with Gaia and the Sloan Digital Sky Survey (SDSS) databases are presented here. Gaia data release 2 (DR2), covering the entire GUVcat footprint, detected about one-third of the GUVcat_AIS sources. We found 31,925,294 Gaia DR2 counterparts to 30,024,791 GUVcat_AIS unique sources, with photometry in the Gaia G band and often also in Gaia BP and RP bands; 26,275,572 matches have a parallax measurement, 21,084,628, 18,588,140, and 16,357,505 have a parallax error less than 50%, 30%, and 20%, respectively. The match with SDSS data release 14 (DR14) yields 23,310,532 counterparts to 22,207,563 unique GUVcat_AIS sources, 10,167,460 of which are pointlike, over a total overlap area of ≈11,100 square degrees (Bianchi et al. 2019). SDSS adds five optical magnitudes to the UV photometry : u, g, r, i, z, and optical spectra of 860,224 matched sources. We used a match radius of 3″, consistent with previous works, although the positions agree to ≲15 for the majority of (pointlike) matched sources, in order to identify possible multiple matches whose UV flux could be unresolved in GALEX imaging. The catalogs can be trimmed to a tighter match radius using the provided separation. The multiband photometry is used to identify classes of astrophysical objects that are prominent in UV, to characterize the content of the GUVmatch catalogs, where stars in different evolutionary stages, quasi-stellar objects, and galaxies can be separated.
Highlights
Due to the higher spatial resolution of the optical databases used here with respect to GALEX, when more than one optical counterpart to a UV source is found within the match radius, the UV flux could be composite of these optical counterparts, that are resolved e.g. by SDSS or Gaia but unresolved in GALEX imaging
Because the FUV-NUV color requires a detection in both filters, and the fraction of sources with FUV detections is about one tenth of the GU V cat total NUV sample overall, the fraction of “hotter” and “cooler” stars shown in Figure 2 is a small subset of the total GU V match AISxGaiaDR2 sample; the number of sources with good parallax measurements increases further for NUV sources undetected in FUV, that are cooler yet
GU V match AISxGaiaDR2. 30,024,791 GU V cat All-sky Imaging Survey (AIS) UV sources have 31,925,294 Gaia data release 2 (DR2) counterparts over the entire GU V cat AIS footprint, 26,275,572 of them have a parallax measurement, 18,588,139 with a parallax error ≤30%
Summary
To facilitate statistical studies of UV sources, Bianchi et al (2017)(revised 2020) constructed GU V cat AIS, a catalog of ∼83 million GALEX1 unique UV sources, by removing duplicate measurements from repeated observations of the same source. Due to the higher spatial resolution of the optical databases used here with respect to GALEX, when more than one optical counterpart to a UV source is found within the match radius, the UV flux could be composite of these optical counterparts, that are resolved e.g. by SDSS or Gaia but unresolved in GALEX imaging. To keep track of multiple matches, in order to enable a correct, unbiased scientific exploitation of the matched catalogs, we used a slightly generous match radius of 3 , which is larger than the typical coordinate offsets between the matched databases, but allows us to identify the cases where two or more nearby optical sources may be unresolved by GALEX.
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