We have presented an imaging Atlas of 1034 galaxies observed in two UV bands by the GALEX satellite. From these we have derived surface brightness and color profiles in the FUV & NUV GALEX bands. Asymptotic magnitudes and colors along with concentration indices have also been obtained. A morphological classification of the profiles is also carried out. Despite a small but non-negligible excess of high-luminosity and paucity of low-luminosity spiral galaxies (compared with the luminosity distribution of ellipticals both in our and the NFGS samples) it is shown that this sample adequately matches the distribution and full range of properties of galaxies in the local Universe. We have augmented this data set with corollary data from the optical (RC3), NIR (2MASS), and far-infrared (IRAS). We emphasize here the special caution should be observed when comparing these results with those derived from a volume-limited sample. From a broad-based initial analysis of the UV properties of this sample we conclude:
The value of the integrated (FUV- K) color of galaxies provides an excellent criterion with which to discriminate elliptical/lenticular galaxies from spirals and irregulars. The best discrimination between these two classes of galaxies (quiescent vs. star-forming) is achieved if a cut-off color (FUV - K) = 8.8 mag is adopted. A reasonably good separation is also obtained by using a (FUV-NUV) cut-off color at 0.9 mag. These colors also allow for a continuous distinction (although with a significant dispersion) of spiral galaxies of different types.
Elliptical/lenticular galaxies with brighter FUV and K-band luminosities show bluer (FUV-NUV) colors than ellipticals with fainter luminosities but redder (NUV - K) colors. This is true for ellipticals galaxies specifically within the range of absolute magnitudes covered by this Atlas (i.e. MK < -21 mag). This behavior is probably a consequence of luminous elliptical galaxies having stronger UV upturns than their intermediate-mass counterparts (see Boselli et al. 2005).
We do not find a large dispersion in the intrinsic (corrected for internal extinction) (FUV-NUV) colors of the spiral/irregular galaxies in the Atlas ((FUV-NUV)0 = 0.05 mag) neither a strong dependence of it with the galaxy luminosity. Consequently, the variations in the observed (FUV-NUV) colors with the luminosity or morphological type of the spiral and irregular galaxies in the sample are plausibly due to variations in the dust content (due for example to changes in metallicity) with these magnitudes. In the case of the (FUV - K) color the star formation history necessarily contributes to its dependence on luminosity and morphological type.
The change in the observed (FUV-NUV) color with the TIR-to-FUV ratio also suggests that the attenuation law in these galaxies differs from a pure Milky-Way extinction law. In particular, attenuation laws with relatively steep FUV rise and no 2175Å bump, like those based on a SMC Bar extinction law or the Calzetti law in the case of the most luminous objects, are favored.
A significant fraction (28%) of the UV profiles show some degree of flattening in the inner regions. The galaxies showing this kind of profiles belong to a relatively small range of optical morphological types (compared with the pure-exponential profiles), 2 < T < 8, i.e. they are all truly spiral galaxies. We interpret this as a consequence of the high past SFR but comparatively low current gas infall rate in the inner disks of spiral galaxies, leading to an efficient consumption of the gas in these regions and, consequently, to a flattening of the UV profiles compared with the outer disks, where the gas supply is still abundant. This is, indeed, expected to be particularly important in intermediate-type spirals.
The GALEX and corollary photometry data along with the profiles and UV
images of galaxies in the sample can be accessed through a dedicated
web page at
GALEX (Galaxy Evolution Explorer) is a NASA Small Explorer, launched in April 2003. We gratefully acknowledge NASA's support for construction, operation, and science analysis for the GALEX mission, developed in cooperation with the Centre National d'Etudes Spatiales of France and the Korean Ministry of Science and Technology. AGdP is partially financed by the MAGPOP EU Marie Curie Research Training Network and the Spanish Programa Nacional de Astronomía y Astrofísica under grant AYA2003-01676. We thank Cren Frayer and Olga Pevunova for preparing the online version of the Atlas. We are also thankful to the referee for his/her valuable comments which helped to improve the paper.