The Galaxy Evolution Explorer (GALEX). Its legacy of UV surveys, and science highlights Luciana Bianchi Abstract FUV (λeff ∼ 1528A,˚ 1344-1786A)˚ and NUV (λeff ∼ The Galaxy Evolution Explorer (GALEX) imaged 2310A,˚ 1771-2831A).˚ It was operated with NASA sup- the sky in the Ultraviolet (UV) for almost a decade, port until 2012, then for a short time with support delivering the first sky surveys at these wavelengths. from private funding from institutions; it was decom- 1 Its database contains far-UV (FUV, λeff ∼ 1528A)˚ missioned on June 28, 2013. and near-UV (NUV, λeff ∼ 2310A)˚ images of most of GALEX's instrument consisted of a Ritchey-Chr´etien−type the sky, including deep UV-mapping of extended galax- telescope, with a 50 cm primary mirror and a focal ies, over 200 million source measurements, and more length of 299.8cm. Through a dichroic beam splitter, than 100,000 low-resolution UV spectra. The GALEX light was fed to the FUV and NUV detectors simul- archive will remain a long-lasting resource for statisti- taneusly. The FUV detector stopped working in May cal studies of hot stellar objects, QSOs, star-forming 2009; subsequent GALEX observations have only NUV galaxies, nebulae and the interstellar medium. It pro- imaging (Figure 1). vides an unprecedented road-map for planning future The GALEX field of view is ≈1.2◦ diameter (1.28/1.24◦, UV instrumentation and follow-up observing programs FUV/NUV), and the spatial resolution is ≈ 4.2/5.300 (Morrissey in the UV and at other wavelengths. et al. 2007). For each observation, the photon list We review the characteristics of the GALEX data, recorded by the two photon-counting micro-channel and describe final catalogs and available tools, that fa- plate detectors is used to reconstruct an FUV and an cilitate future exploitation of this database. We also NUV image, sampled with virtual pixels of 1.500 . From recall highlights from the science results uniquely en- the reconstructed image, the pipeline then derives a abled by GALEX data so far. sky background image, and performs source photome- Keywords Ultraviolet: surveys; Astronomical Data try. Sources detected in FUV and NUV images of the Bases: catalogs; Stars: post-AGB; Stars: White same observation are matched by the pipeline with a 00 Dwarfs; Galaxies: Milky Way; Ultraviolet: galaxies; 3 radius, to produce a merged list for each observation Ultraviolet: QSOs (Morrissey et al. 2007). To reduce localized response variations, in order to maximize photometric accuracy, each observation was 1 Introduction. GALEX instrument and data carried out with a 10 spiral dithering pattern. The sur- veys were accumulated by painting the sky with con- The Galaxy Evolution Explorer (GALEX), a NASA tiguous tiles, with series of such observations. A trailing Small Explorer Class mission, was launched on April 28, mode was instead used for the latest, privately-funded 2003 to perform the first sky-wide Ultraviolet surveys, observations, to cover some bright areas near the MW with both direct imaging and grism in two broad bands, plane. These latest data currently are not in the public archive. Luciana Bianchi Dept. of Physics & Astronomy, The Johns Hopkins University, 1GALEX was developed by NASA with contributions from the 3400 N. Charles St., Baltimore, MD 21218, USA Centre National d'Etudes Spatiales of France and the Korean http://dolomiti.pha.jhu.edu Ministry of Science and Technology. 2 At the end of the GALEX mission, the AIS survey were now reprocessed as part of the MAST effort to fi- was extended towards the Galactic plane, largely inac- nalize the database, therefore the final archive will con- cessible during the prime mission phase because of the tain a few more entries. Figure 1 shows the sky coverage many bright stars that violated high-countrate safety of all GALEX observations performed in both FUV and limits. A survey of the Magellanic Clouds (MC), also NUV (right), and in NUV regardless of FUV-detector previously unfeasible due to brightness limits, was com- status (left). The map does not include the last NUV pleted at the end, relaxing the initial count-rate saftey trailed observations (CLAUSE dataset). threshold. Because of the FUV detector's failure, these extensions include only NUV measurements (Figure 1). Much of this short review concerns GALEX's final 3 GALEX database and science catalogs data products (from the point of view of science applica- tions; technical documentation is available elsewhere), GALEX data can be obtained from the NASA MAST This choice, at the price of confining science results to archive at http://galex.stsci.edu. They include images a few highlights (Section 7), responds to various in- (direct imaging or grism) and associated photometry quiries and requests, timely as the almost entire (and from the pipeline, or extracted spectra. High-level sci- final) database is becoming available, and new tools, ence catalogs (HLSP) are also available, such as those which will support new investigations. described below, which greatly facilitate some investi- gations. Several objects have multiple measurements in the 2 GALEX surveys database, due to repeated observations of the same field, or overlap between fields. For studies involving GALEX has performed sky surveys with different depth UV-source counts, one needs to eliminate repeats, as and coverage (Morrissey et al. 2007, Bianchi 2009). well as artefacts. Therefore, we have constructed cata- The two detectors, FUV and NUV, observed simulta- logs of unique UV sources, eliminating duplicate mea- neously as long as the FUV detector was operational; surements of the same object. Separate catalogs were note however that there are occasional observations in which one of the two detectors was off (mostly FUV) constructed for AIS and MIS, because of the ∼2-3 mag due to brief shut-downs, even in the early part of the difference in depth. The most recent version of such mission, and in some observations the FUV and NUV catalogs is published by Bianchi et al. (2014a, here- exposure times differ (see Bianchi et al. 2014a, in par- after \BCS", see Appendix A); BCS also presented sky ticular their Table 1 and Fig. 2). maps showing density of UV sources with various cuts. The surveys with the largest area coverage are the A previous version, based on data release GR5, was All-Sky Imaging survey (AIS) and the Medium-depth published by Bianchi et al. (2011a, b), who extensively Imaging Survey (MIS). Exposure times slightly vary discuss criteria for constructing GALEX source cata- within each survey, around the respective nominal ex- logs and matched catalogs between GALEX and SDSS, posures of 100 sec for AIS (corresponding to a depth GSC2, and more. Earlier work on source classification of FUV∼20/NUV∼21 ABmag) and 1500 sec for MIS was presented by Bianchi et al. (2007, 2005), Bianchi (corresponding to a depth of ∼22.7 ABmag in both (2009). FUV and NUV). The Deep Imaging Survey (DIS) ac- In the GALEX database, the non-detection value for cumulated exposures of the order of several tens of the FUV magnitude, FUV =-999, means either that thousand of seconds in selected fields (for a 30,000 sec the detector was on but the source was too faint to exposure, the depth reached is ∼24.8/24.4 ABmag in be measured in FUV, of the FUV detector was off. In FUV/NUV). The \Nearby Galaxies Survey" (Bianchi order to examine and classify sources by colors, and rel- et al. 2003, Gil de Paz et al. 2007) covered initially 436 ative fraction of sources with different colors, Bianchi fields at MIS depth, but hundreds of additional nearby et al. (2014a, 2011a) restricted the catalogs to those galaxies were mapped by GALEX, as part of MIS or observations in which both detectors were exposed. In other surveys. Other observations were obtained for addition, these catalogs were conservatively restricted guest investigator (GI) programs, and for other tar- to measurements within the central 1◦ diameter of the geted regions such as, for example, the Kepler field. field of view, to exclude the outer rim, where distor- The GALEX database at the end of the mission tions prevent position and photometry of sources to be (2013, data release GR7) contained 214,449,551 source derived accurately, and counts from rim's spikes cause measurements, most of which (210,691,504) from obser- numerous artefacts to intrude the source list. For more vations with both detectors on. However, a small num- details we refer to the original BCS publication. ber of additional datasets obtained throughout the mis- The BCS catalogs include 28,707 AIS fields with sion, which were not previously included in the archive, both FUV and NUV exposed, covering a unique area 3 Fig. 1 Sky coverage, in Galactic coordinates, of the GALEX imaging. The surveys with the largest area coverage are AIS (blue) and MIS (green). Observations from other surveys are shown in black (figure adapted from Bianchi et al. 2014a). Data from the privately-funded observations at the end of the mission are not shown. Left: fields observed with at least the NUV detector on; right: fields observed with both FUV and NUV detectors on. The latter constitute the BCScat's. Fig. 2 Portion of a GALEX field with the stellar cluster NGC2420 (de Martino et al. 2008). Green circles mark sources detected with our photometry, the purple contours mark sources as defined by the GALEX pipeline. The right-side image is an enlargement of the crowded region. The left image also includes a section of the field’s outer edge, showing how numerous rim artefacts intrude photometric source detections. The rim is excluded from the BCS catalogs. Fig.