Galaxy evolution in Local Group analogs - I. A GALEX study of nearby groups dominated by late-type galaxies

Abstract

Context. Understanding the astrophysical processes acting within galaxy groups and their effects on the evolution of the galaxy population is one of the crucial topics of modern cosmology, as almost 60% of galaxies in the Local Universe are found in groups. Aims. We aim at learning about galaxy evolution within nearby groups dominated by late-type galaxies, specifically by studying their ultraviolet-emitting stellar population. Methods. We imaged in the far (FUV, λeff = 1539 A) and near ultraviolet (NUV, λeff = 2316 A) with the Galaxy Evolution Explorer (GALEX) three nearby groups, namely LGG 93, LGG 127 and LGG 225. We obtained the UV galaxy surface photometry and, for LGG 225, 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 spectral energy distribution of galaxies to infer their luminosity-weighted ages. The UV and optical photometry was also used to perform a luminosity-weighted kinematical and dynamical analysis of each group and to evaluate the stellar mass. Results. A few member galaxies in LGG 225 show a distorted UV morphology due to ongoing interactions. (FUV − NUV) colors suggest that spirals in LGG 93 and LGG 225 host stellar populations in their outskirts younger than that of M 31 and M 33 in the Local Group or with less extinction. The irregular interacting galaxy NGC 3447A has a significantly younger stellar population (a few Myr old) than the average of the other irregular galaxies in LGG 225 suggesting that the encounter triggered star formation. The early-type members of LGG 225, NGC 3457 and NGC 3522, have masses of the order of a few 10 9 M� , comparable to the Local Group ellipticals. For the most massive spiral in LGG 225, we estimate a stellar mass of ≈4 × 10 10 M� , comparable to M 33 in the Local Group. Ages of stellar populations range from a few to ≈7 Gyr for the galaxies in LGG 225. The kinematical and dynamical analysis indicates that LGG 127 and LGG 225 are in a pre-virial collapse phase, i.e. still undergoing dynamical relaxation, while LGG 93 is likely virialized. Both the photometric and the dynamical analyses suggest that LGG 225 is in a more active evolution phase than LGG 93 and LGG 127.