Galaxy Evolution in Local Group Analogs. I. A GALEX study of nearby groups

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)Comment: 20 pages, 13 figures, accepted for publication in Astronomy and Astrophysic

Galaxy evolution in groups. USGC U268 and USGC U376 in the Leo cloud

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 galaxies 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.Comment: 16 pages, 6 figures; accepted for publication in Advances in Space Research, Special Issue: Ultraviolet Astrophysic

A multi-wavelength study of the evolution of Early-Type Galaxies in Groups: the ultraviolet view

ABRIDGED- The UV-optical color magnitude diagram (CMD) of rich galaxy groups is characterised by a well developed Red Sequence (RS), a Blue Cloud (BC) and the so-called Green Valley (GV). Loose, less evolved groups of galaxies likely not virialized yet may lack a well defined RS. This is actually explained in the framework of galaxy evolution. We are focussing on understanding galaxy migration towards the RS, checking for signatures of such a transition in their photometric and morphological properties. We report on the UV properties of a sample of ETGs galaxies inhabiting the RS. The analysis of their structures, as derived by fitting a Sersic law to their UV luminosity profiles, suggests the presence of an underlying disk. This is the hallmark of dissipation processes that still must have a role in the evolution of this class of galaxies. SPH simulations with chemo-photometric implementations able to match the global properties of our targets are used to derive their evolutionary paths through UV-optical CDM, providing some fundamental information such as the crossing time through the GV, which depends on their luminosity. The transition from the BC to the RS takes several Gyrs, being about 3-5 Gyr for the the brightest galaxies and more long for fainter ones, if it occurs. The photometric study of nearby galaxy structures in UV is seriously hampered by either the limited FoV of the cameras (e.g in HST) or by the low spatial resolution of the images (e.g in the GALEX). Current missions equipped with telescopes and cameras sensitive to UV wavelengths, such as Swift-UVOT and Astrosat-UVIT, provide a relatively large FoV and better resolution than the GALEX. More powerful UV instruments (size, resolution and FoV) are obviously bound to yield fundamental advances in the accuracy and depth of the surface photometry and in the characterisation of the galaxy environment.Comment: 12 pages, 6 figures: accepted for publication in Astrophysics & Space Science as contributions to the workshop: "UV astronomy, the needs and the means

Early Type Galaxies in the Mid Infrared: a new flavor to their stellar populations

The mid infrared emission of early type galaxies traces the presence of intermediate age stellar populations as well as even tiny amounts of ongoing star formation. Here we discuss high S/N Spitzer IRS spectra of a sample of Virgo early type galaxies, with particular reference to NGC 4435. We show that, by combining mid infrared spectroscopic observations with existing broad band fluxes, it is possible to obtain a very clean picture of the nuclear activity in this galaxy.Comment: 4 pages; proceedings of IAU Symposium No. 241, "Stellar Populations as Building Blocks of Galaxies", editors A. Vazdekis and R. Peletie

Galaxy evolution in nearby groups. II. Galaxy evolution in nearby loose groups. II. Photometric and kinematic characterization of USGC U268 and USGC U376 group members in the Leo cloud

We present the photometric and kinematic characterization of two groups, USGC U268 and USGC U376 located in different regions of the Leo cloud. U268, composed of 10 catalogued members and 11 new added members, has a small fraction (~24%) of early-type galaxies (ETGs). U376 has 16 plus 8 new added members, with ~38% of ETGs. We find the presence of significant substructures in both groups suggesting that they are likely accreting galaxies. U268 is located in a more loose environment than U376. For each member galaxy, broad band integrated and surface photometry have been obtained in far-UV and near-UV with GALEX, and in u,g, r, i, z (SDSS) bands. H_alpha imaging and 2D high resolution kinematical data have been obtained using PUMA Scanning Fabry-Perot interferometer at the 2.12 m telescope in San Pedro M\'artir, (Baja California, M\'exico). We improved the galaxy classification and we detected morphological and kinematical distortions that may be connected to either on-going and/or past interaction/accretion events or environmental induced secular evolution. U268 appears more active than U376, with a large fraction of galaxies showing interaction signatures (60% vs. 13%). The presence of bars among late-type galaxies is ~10% in U268 and ~$29% in U376. The cumulative distribution of (FUV - NUV) colours of galaxies in U268 is significantly different than that in U376 with galaxies in U268 bluer than those in U376. In the (FUV-r vs. M_r) and (NUV-r vs. M_r) planes no members of U268 are found in the `red sequence', even early-type galaxies lie in the `blue sequence' or in the `green valley'. Most (80%) of the early-type members in U376 inhabits the `red sequence, a large fraction of galaxies, of different morphological types, are located in the `green valley', while the `blue sequence' is under-populated with respect to U268.Comment: 27 pages, 11 figures, 8 tables, accepted for publication in MNRAS (abridged abstract

The role of the synchrotron component in the mid infrared spectrum of M 87

We study in detail the mid-infrared Spitzer-IRS spectrum of M 87 in the range 5 to 20 micron. Thanks to the high sensitivity of our Spitzer-IRS spectra we can disentangle the stellar and nuclear components of this active galaxy. To this end we have properly subtracted from the M 87 spectrum, the contribution of the underlying stellar continuum, derived from passive Virgo galaxies in our sample. The residual is a clear power-law, without any additional thermal component, with a zero point consistent with that obtained by high spatial resolution, ground based observations. The residual is independent of the adopted passive template. This indicates that the 10 micron silicate emission shown in spectra of M 87 can be entirely accounted for by the underlying old stellar population, leaving little room for a possible torus contribution. The MIR power-law has a slope alpha ~ 0.77-0.82 (S$_\nu\propto\nu^{-\alpha}$), consistent with optically thin synchrotron emission.Comment: 5 pages, 4 figures, accepted for publication in ApJ main journa

Small Scale Systems of Galaxies. III. X-ray detected E+S galaxy pairs in low density environments

We present a comprehensive study of the local environments of four E+S galaxy pairs with the main goal to investigate their formation/evolution histories. New XMM-Newton data were obtained for two pairs (RR 143 and RR 242) that complements existing ROSAT data for the other two (RR 210 and RR 216). The new observations reveal diffuse X-ray emission in both pairs. The emission is asymmetric in both cases and extends out to 120 kpc and 160 kpc in RR 143 and RR 242 respectively. The nucleus of RR 242 hosts a low luminosity mildly absorbed AGN. We find that the early-type components of pairs with diffuse hot gas appear to be relaxed objects while those in RR 210 and RR 216, where no diffuse emission has been found, display unambiguous signatures of ongoing interaction. Wide-field V and R-band data are used to study the photometric properties of the early-type components and to search for a candidate faint galaxy populations around each of the pairs. While no diffuse optical light is found for any of the pairs, all of the early-type members show very extended and concentric luminous envelopes. We identify a faint galaxy sample in each field and we consider whether they could be physically associated with the luminous pairs based upon (V-R) colors and photometric properties. We find that the distribution of r_e and M_R for the candidates are similar in three of the fields (RR 143, 216 and 242). The same selection criteria applied to the field of RR 210 suggest a fainter and more compact population possibly suggesting a larger background fraction than in the other fields.Comment: 47 pages, 18 figures, Astron. J. in pres