The Ultraviolet Sky: An Overview from the GALEX Surveys

The Galaxy Evolution Explorer (GALEX) has performed the first surveys of the sky in the Ultraviolet (UV). Its legacy is an unprecedented database with more than 200 million source measurements in far-UV (FUV) and near-UV (NUV), as well as wide-field imaging of extended objects, filling an important gap in our view of the sky across the electromagnetic spectrum. The UV surveys offer unique sensitivity for identifying and studying selected classes of astrophysical objects, both stellar and extra-galactic. We examine the overall content and distribution of UV sources over the sky, and with magnitude and color. For this purpose, we have constructed final catalogs of UV sources with homogeneous quality, eliminating duplicate measurements of the same source. Such catalogs can facilitate a variety of investigations on UV-selected samples, as well as planning of observations with future missions. We describe the criteria used to build the catalogs, their coverage and completeness. We included observations in which both the far-UV and near-UV detectors were exposed; 28,707 fields from the All-Sky Imaging survey (AIS) cover a unique area of 22,080 square degrees (after we restrict the catalogs to the central 1-degree diameter of the field), with a typical depth of about 20/21 mag (FUV/NUV, in the AB mag system), and 3,008 fields from the Medium-depth Imaging Survey (MIS) cover a total of 2,251 square degrees at a depth of about 22.7mag. The catalogs contain about 71 and 16.6 million sources respectively. The density of hot stars reflects the Galactic structure, and the number counts of both Galactic and extra-galactic sources are modulated by the Milky Way dust extinction, to which the UV data are very sensitive.Comment: J. Adv. Space Res. (2013), Full resolution figures can be found in the original published article (open access) at : http://www.sciencedirect.com/science/article/pii/S0273117713004742 or from http://dolomiti.pha.jhu.edu/publgoto.html ; catalogs are posted on MAS

Galaxy evolution in nearby galaxy groups. III. A GALEX view of NGC 5846, the largest group in the local universe

We explore the co-evolution of galaxies in nearby groups (V < 3000 km/s) with a multi-wavelength approach. We analyze GALEX far-UV (FUV) and near-UV (NUV) imaging and SDSS u,g,r,i,z data of groups spanning a large range of dynamical phases. We characterize the photometric properties of spectroscopically-confirmed galaxy members and investigate the global properties of the groups through a dynamical analysis. Here we focus on NGC 5846, the third most massive association of Early-Type Galaxies (ETG) after the Virgo and Fornax clusters. The group, composed of 90 members, is dominated by ETGs (about 80 per cent), and among ETGs about 40\% are dwarfs. Results are compared with those obtained for three groups in the LeoII cloud, which are radically different both in member-galaxy population and dynamical properties. The FUV-NUV cumulative colour distribution and the normalized UV luminosity function (LF) significantly differ due to the different fraction of late-type galaxy population. The UV LF of NGC 5846 resembles that of the Virgo cluster, however our analysis suggests that star-formation episodes are still occurring in most of the group galaxies, including ETGs. The NUV-i colour distribution, the optical-UV colour-colour diagram, and NUV-r vs. Mr colour-magnitude relation suggest that the gas contribution cannot be neglected in the evolution of ETG-type group members. Our analysis highlights that NGC~5846 is still in an active phase of its evolution, notwithstanding the dominance of dwarf and bright ETGs and its virialized configuration.Comment: 26 pages, 13 figures, accepted for publication in MNRA

The Ultraviolet View of the Magellanic Clouds from GALEX: A First Look at the LMC Source Catalog

The Galaxy Evolution Exporer (GALEX) has performed unprecedented imaging surveys of the Magellanic Clouds (MC) and their surrounding areas including the Magellanic Bridge (MB) in near-UV (NUV, 1771-2831\AA) and far-UV (FUV, 1344-1786\AA) bands at 5" resolution. Substantially more area was covered in the NUV than FUV, particularly in the bright central regions, because of the GALEX FUV detector failure. The 5$\sigma$ depth of the NUV imaging varies between 20.8 and 22.7 (ABmag). Such imaging provides the first sensitive view of the entire content of hot stars in the Magellanic System, revealing the presence of young populations even in sites with extremely low star-formation rate surface density like the MB, owing to high sensitivity of the UV data to hot stars and the dark sky at these wavelengths. The density of UV sources is quite high in many areas of the LMC and SMC. Crowding limits the quality of source detection and photometry from the standard mission pipeline processing. We performed custom-photometry of the GALEX data in the MC survey region ($<15^{\circ}$ from the LMC, $<10^{\circ}$ from the SMC). After merging multiple detections of sources in overlapping images, the resulting catalog we have produced for the LMC contains nearly 6 million unique NUV point sources within 15$^{\circ}$ and is briefly presented herein. This paper provides a first look at the GALEX MC survey and highlights some of the science investigations that the entire catalog and imaging dataset will make possible.Comment: 16 pages, 8 figures; J. Adv. Space Res. (2013

UV spectral analysis of very hot H-deficient [WCE]-type central stars of planetary nebulae: NGC 2867, NGC 5189, NGC 6905, Pb 6, and Sand 3

We analysed UV FUSE, IUE, and HST/STIS spectra of five of the hottest [WCE]-type central stars of planetary nebulae: NGC 2867, NGC 5189, NGC 6905, Pb 6, and Sand 3. The analysis leveraged on our grid of CMFGEN synthetic spectra, which covers the parameter regime of hydrogen deficient central stars of planetary nebulae and allows a uniform and systematic study of the stellar spectra. The stellar atmosphere models calculated by us include many elements and ionic species neglected in previous analyses, which allowed us to improve the fits to the observed spectra considerably and provided an additional diagnostic line: the Ne VII $\lambda$ 973 $\mathrm{\AA}$, which had not been modelled in [WCE] spectra and which presents, in these stars, a strong P-Cygni profile. We report newly derived photospheric and wind parameters and elemental abundances. The central stars of NGC 2867, NGC 5189, and Pb 6 had their temperatures revised upward in comparison with previous investigations and we found the carbon to helium mass ratio of the sample objects to span a wide range of values, 0.42$\leq$C:He$\leq$1.96. Modelling of the Ne VII $\lambda$ 973 $\mathrm{\AA}$ P-Cygni profile indicated strong neon overabundances for the central stars of NGC 2867, NGC 5189, NGC 6905, and Pb 6, with Ne mass fractions between 0.01 and 0.04. Nitrogen abundances derived by us for the central stars of NGC 5189, Pb 6, and Sand 3 are higher than previous determinations by factors of 3, 10, and 14, respectively.Comment: Accepted on MNRA

Extreme UV QSOs

We present a sample of spectroscopically confirmed QSOs with FUV-NUV color (as measured by GALEX photometry) bluer than canonical QSO templates and than the majority of known QSOs. We analyze their FUV to NIR colors, luminosities and optical spectra. The sample includes a group of 150 objects at low redshift (z $<$ 0.5), and a group of 21 objects with redshift 1.7$<$z$<$2.6. For the low redshift objects, the "blue" FUV-NUV color may be caused by enhanced Ly$\alpha$ emission, since Ly$\alpha$ transits the GALEX FUV band from z=0.1 to z=0.47. Synthetic QSO templates constructed with Ly$\alpha$ up to 3 times stronger than in standard templates match the observed UV colors of our low redshift sample. The H$\alpha$ emission increases, and the optical spectra become bluer, with increasing absolute UV luminosity. The UV-blue QSOs at redshift about 2, where the GALEX bands sample restframe about 450-590A (FUV) and about 590-940A(NUV), are fainter than the average of UV-normal QSOs at similar redshift in NUV, while they have comparable luminosities in other bands. Therefore we speculate that their observed FUV-NUV color may be explained by a combination of steep flux rise towards short wavelengths and dust absorption below the Lyman limit, such as from small grains or crystalline carbon. The ratio of Ly$\alpha$ to CIV could be measured in 10 objects; it is higher (30% on average) than for UV-normal QSOs, and close to the value expected for shock or collisional ionization. FULL VERSION AVAILABLE FROM AUTHOR'S WEB SITE: http://dolomiti.pha.jhu.edu/papers/2009_AJ_Extreme_UV_QSOs.pdfComment: Astronomical Journal, in pres