Galaxy evolution in Local Group analogs

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, λ eff = 1539 Å) and near ultraviolet (NUV, λ eff = 2316 Å) 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.

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

“…For all 11 galaxies in U268 and for all but 3 galaxies in U376, the best-fit solution confirms the low foreground extinction values (< 0.02 mag) found in previous compilations (Table 1). For most Ellipticals/S0s and Spirals the models with the SFH assumed for their respective type (see Table 5 in Marino et al, 2010) give good fits. The SEDs of three Spirals in U376 (NGC 3681, NGC 3684, and NGC 3686) and two in U268 (UGC 5393, NGC 3021), are also well fitted with the SFH assumed in our grid for Irregulars.…”

“…We computed broad-band model magnitudes in the GALEX and SDSS bands, after progressively reddening the models by increasing amounts of dust. We adopted a Salpeter IMF and the typical parameters regulating the star-formation rate for spiral, irregular and elliptical types respectively, as given by Silva et al (1998) 2 , and Marino et al (2010, their Table 5). An important feature of GRASIL is that the code includes the effect of age-dependent extinction with young stars being more affected by dust.…”

“…The resulting SEDs at some representative ages have been shown in Marino et al (2010); their Figure 10 illustrates the effects of evolution (left panels) and extinction (right panels).…”

“…In this context, Marino et al (2010) combined ultraviolet data from the Galaxy Evolution Explorer (GALEX, Morrissey et al, 2007) and optical data from the Sloan Digital Sky Survey (SDSS) to study the galaxies of the "Local Group analog" LGG 225 in the Leo II cloud. Fitting the observed SEDs from far-UV (FUV) to near-IR (NIR) with stellar population models, the total stellar mass of the group was estimated between 5 and 35 × 10 10 M ⊙ and the ages of the stellar populations from a few to ≈ 7 Gyr.…”

“…In this work we derive the age of the stellar population (for an assumed SFH), the stellar mass, and the additional foreground extinction of galaxies in these two groups by fitting their SED with a grid of models computed with GRASIL code (Silva et al, 1998), adopting SFH for elliptical, spiral and irregular galaxies as defined in Marino et al (2010, their Table 5). Together with the characterization of the galaxy and group properties, the objective of the present study is to compare results from the analysis with GRASIL models with those obtained from SPH simulations with chemophotometric implementation, matching not only the integrated-SED but also the galaxy's kinematical and morphological structure.…”