With the aim of investigating galaxy evolution in nearby galaxy groups, we analysed the spectral energy distribution of 24 galaxies, members of two groups in the Leo cloud, USGC U268 and USGC U376. We estimated the ages and stellar masses of the gala
xies by fitting their total apparent magnitudes from far-ultraviolet to near-infrared with population synthesis models. The comparison of the results for a subsample of galaxies with smooth particle hydrodynamic (SPH) simulations with chemo-photometric implementation, shows that in most cases the estimated stellar masses obtained with the two different approaches are in good agreement. The kinematical and dynamical analysis indicates that USGC U268 is in a pre-virial collapse phase while USGC U376 is likely in a more evolved phase towards virialization.
We present GALEX far-ultraviolet (FUV, $lambda_{eff}$=1538 AA) and near-ultraviolet (NUV, $lambda_{eff}$=2316 AA) surface photometry of 40 early-type galaxies (ETGs) selected from a wider sample of 65 nearby ETGs showing emission lines in their optic
al spectra. We derive FUV and NUV surface brightness profiles, (FUV-NUV) colour profiles and D$_{25}$ integrated magnitudes. We extend the photometric study to the optical {it r} band from SDSS imaging for 14 of these ETGs. In general, the (FUV-NUV) radial colour profiles become redder with galactocentric distance in both rejuvenated ($leq 4$ Gyr) and old ETGs. Colour profiles of NGC 1533, NGC 2962, NGC 2974, NGC 3489, and IC 5063 show rings and/or arm-like structures, bluer than the body of the galaxy, suggesting the presence of recent star formation. Although seven of our ETGs show shell systems in their optical image, only NGC 7135 displays shells in the UV bands. We characterize the UV and optical surface brightness profiles, along the major axis, using a Sersic law. The Sersic law exponent, $n$, varies from 1 to 16 in the UV bands. S0 galaxies tend to have lower values of $n$ ($leq5$). The Sersic law exponent $n=4$ seems to be a watershed: ETGs with $n>4$ tend to have [$alpha$/Fe] greater than 0.15, implying a short star-formation time scale. We find a significant correlation between the FUV$-$NUV colour and central velocity dispersions $sigma$, with the UV colours getting bluer at larger $sigma$. This trend is likely driven by a combined effect of `downsizing and of the mass-metallicity relation.
Understanding the astrophysical processes acting within galaxy groups and their effects on the evolution of the galaxy population is one of the crucial topic of modern cosmology, as almost 60% of galaxies in the Local Universe are found in groups. We
imaged in the far (FUV 1539 A) and near ultraviolet (NUV 2316 A) with GALEX three nearby groups, namely LGG93, LGG127 and LGG225. We obtained the UV galaxy surface photometry and, for LGG225, the only group covered by the SDSS, the photometry in u, g, r, i, z bands. We discuss galaxy morphologies looking for interaction signatures and we analyze the SED of galaxies to infer their luminosity-weighted ages. The UV and optical photometry was also used to perform a kinematical and dynamical analysis of each group and to evaluate the stellar mass. A few member galaxies in LGG225 show a distorted UV morphology due to ongoing interactions. (FUV-NUV) colors suggest that spirals in LGG93 and LGG225 host stellar populations in their outskirts younger than that of M31 and M33 in the LG or with less extinction. The irregular interacting galaxy NGC3447A has a significantly younger stellar population (few Myr old) than the average of the other irregular galaxies in LGG225 suggesting that the encounter triggered star formation. The early-type members of LGG225, NGC3457 and NGC3522, have masses of the order of a few 10^9 Mo, comparable to the Local Group ellipticals. For the most massive spiral in LGG225, we estimate a stellar mass of ~4x10$^{10}$ Mo, comparable to M33 in the LG. Ages of stellar populations range from a few to ~7 Gyr for the galaxies in LGG225. The kinematical and dynamical analysis indicates that LGG127 and LGG225 are in a pre-virial collapse phase, i.e. still undergoing dynamical relaxation, while LGG93 is likely virialized. (Abridged)
We discuss selection of QSO candidates from the combined SDSS and GALEX catalogues. We discuss properties of QSOs within the combined sample, and note uncertainties in number counts and completeness, compared with other SDSS-based samples. We discuss
colour and other properties with redshift within the sample and the SEDs for subsets. We estimate the numbers of faint QSOs that are classified as extended objects in the SDSS, and consequent uncertainties that follow.
The bright galaxy population of the Local Group Analog (LGA) LGG 225 has been imaged with the Galaxy Evolution Explorer (GALEX) through its Far- and Near-UV wavebands. A significant fraction of the group members appear to underwent recent/on-going in
teraction episodes that strongly disturbed overall galaxy morphology. UV-bright regions, sites of intense star formation activity accompanied by intense dust extinction, mark the galaxy outskirts forming irregular structures and tails. Compared to the Local Group, LGG 225 seems thus to be experiencing a more intense and active evolutionary phase.
We summarize the main properties of the extended UV (XUV) emission found in roughly 30% of the nearby spiral galaxies observed by the GALEX satellite. Two different classes of XUV disks are identified, the Type 1 XUV disks where significant, structur
ed UV-bright features are found beyond the classical azimuthally-averaged star-formation threshold, and the Type 2 XUV disks, which are characterized by very extended (seven times the area where most of the stellar mass is found), blue [(FUV-K)<5mag] outer disks. These latter disks are extreme examples of galaxies growing inside-out. The few XUV disks studied in detail to date are rich in HI but relatively poor in molecular gas, have stellar populations with luminosity-weighted ages of ~1 Gyr, and ionized-gas metal abundances of ~Zsun/10. As part of the CAHA-XUV project we are in the process of obtaining deep multi-wavelength imaging and spectroscopy of 65 XUV-disk galaxies so to determine whether or not these properties are common among XUV disks.
We describe HST imaging of recent star formation complexes located in the extended UV disk (XUV-disk) component of NGC 5236 (M 83), NGC 5055 (M 63), and NGC 2090. Photometry in four FUV--visible bands permits us to constrain the type of resolved star
s and effective age of clusters, in addition to extinction. The preliminary results given herein focus on CMD analysis and clustering properties in this unique star-forming environment.
We present color-magnitude diagrams of the open cluster NGC 2420, obtained from Galaxy Evolution Explorer (GALEX) ultraviolet images in FUV and NUV bands and Sloan Digital Sky Survey (SDSS) u,g,r,i,z photometry. Our goal is to search for and characte
rize hot evolved stars and peculiar objects in this open cluster, as part of a larger project aimed to study a number of open clusters in the Milky Way with GALEX and ground-based data.
