From top to bottom: morphological type distributions (left) and B absolute magnitudes of LGG 93, LGG 127, LGG 225 and of the Local Group from Pasetto & Chiosi (2007).
Some important physical parameters mark the difference between the possible evolution within groups and clusters. Among them, the galaxy velocity dispersion which in groups is comparable to the inner velocity dispersion of individual galaxies. Processes such as galaxy merging and accretion, e.g. of HI-rich high-velocity clouds (Zabludoff 1998; Blitz et al. 1999), are much more effective in groups than in clusters. Conversely, mechanisms regulating galaxy
evolution in the cluster environment, such as ram-pressure stripping (Vollmer et al. 2002) and galaxy harassment (Moore et al. 1999), are less relevant in groups. In the group environment, galaxy encounters may then completely reshape the member's morphology
and trigger secondary star formation episodes even in early-type galaxies (see e.g. Annibali et al. 2007).
In the context of group evolution, the Local Group (LG hereafter) offers clear examples of ongoing interactions and/or merging episodes. Both observations and models suggest that the life in the LG is highly dynamic. Although the Milky Way, M 31 and M 33 do not appear severely distorted by the mutual gravitational interaction, the disruption and accretion of small galaxies by the spiral members seems very frequent. These accretions give rise to gaseous and stellar tidal streams that continue to orbit the accreting galaxy as fossil relics of the mass transfer activity. Ongoing accretion events have been detected around the Milky Way. We quote some cases discovered in the recent years: the Pal 5 (see e.g Odenkirchen et al. 2001), the Sagittarius stream (see e.g. Ibata et al. 2001), the Monoceros stream ( see e.g.Yanny et al. 2003), the orphan stream ( see e.g. Belokurov et al. 2006) and the anticenter stream (see e.g. Grillmair et al. 2006). The most prominent and the earliest discovered is the Magellanic stream (Brunset al. 2005) that dynamical models suggest to be about 1.7 Gyr old (Nidever et al. 2008).
In order to place the observed properties of the LG in the general evolutionary framework of loose groups dominated by late-type galaxies, we need to compare its basic parameters (e.g presence of tidal phenomena, star formation activity, etc.) with nearby, very similar systems, possibly so similar to be considered basically analogs of the galaxy system we inhabit. In this context, we started performing a study of a sample of late-type galaxy dominated groups performed with GALEX. Our pilot sample includes three nearby groups at approximately the same distance, namely LGG 93, LGG 127 and LGG 225. LGG 93 and LGG 125 groups are completely dominated by spiral galaxies while by LGG 225 starts to contain early-type galaxies.
This study aims, through a detailed analysis of each member in the UV and, where possible in optical, combined with a luminosity-weighted dynamical study of each group as a whole, at providing elements addressed to infer the evolutionary state of the group in a hierarchical evolutionary scenario. GALEX wide field of view allowed us to obtain a snapshot of the entire groups, of tidal features and to map the recent star formation in late-type galaxies (see e.g. Thilker et al. 2005).
Method
We imaged in the far (FUV, 1528 A) and near (NUV, 2271 A) ultraviolet (UV) bands of GALEX three nearby groups. We obtained the UV galaxy surface photometry and,
for LGG 225, the only one group covered by the SDSS, the photometry in all SLOAN bands. We discuss galaxy morphologies looking for interaction signatures. Intrinsic and reddened synthetic colors were used to investigate color profiles and population synthesis code to investigate the spectral energy distribution of the two eliptical galaxies in the LGG 225 group infer their luminosity--weighted ages. The UV and optical photometric information was used to perform a luminosity-weighted kinematical and dynamical analysis of each group.
Results
Few member galaxies located in LGG 225, show a disturbed UV morphology due to ongoing interactions. (FUV-NUV) colors suggest that spirals in LGG 93 and LGG 225 groups 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 star formation (few Myr old) than the average of the other irregular galaxies in LGG 225 suggesting that the encounter triggered star formation. The erly-type members of LGG 225, NGC 3522 and NGC 3457 have masses of the order of a few 109 solar masses comparable to the Local Group of galaxies. For the most massive spiral in LGG 225, we estimate a stellar mass of ~4x109 solar masses, comparable to M 33 in the Local Group. Ages of stellar populations range from a few to ~7 Gyr for 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 a dynamical relaxation, while LGG 93 group which is likely virialized. Both the photometric and the dynamical analysis suggest that LGG 225 group is in a more active evolution phase then LGG 93 and LGG 127.
more details in
Marino A., Bianchi, L., Rampazzo R., Buson L.M., Bettoni D.
Galaxy evolution in Local Group Analogs I.
A GALEX study of nearby groups dominated by late-type galaxies
2010, A&A, A29