Star-Forming Compact Groups (SFCGs): An ultraviolet search for a local sample

Abstract

We present a local sample (z<0.15) of 280 Star-Forming Compact Groups (SFCGs) of galaxies identified in the ultraviolet Galaxy Evolution EXplorer (GALEX) All-sky Imaging Survey (AIS). So far, just one prototypical example of SFCG, the Blue Infalling Group, has been studied in detail in the Local Universe. The sample of SFCGs is mainly the result of applying a Friends-of-Friends group finder in the space of celestial coordinates with a maximum linking-length of 1.5 arcmin and choosing groups with a minimum number of four members of bright UV-emitting 17<FUV<20.5 sources (mostly galaxies) from the GALEX/AIS catalogue. The result from the search are 280 galaxy groups composed by 226, 39, 11 and 4 groups of four, five, six and seven bright ultraviolet (UV) members, respectively. Only 59 of these 280 newly identified SFCGs have a previous catalogued group counterpart. Group redshifts are available for at least one member in 75% of the SFCGs, and over 40% of the SFCGs have redshifts measured for two or more galaxies. Twenty-six of the SFCGs appear to be located in the infalling regions of clusters with known redshift. The SFCG sample presents a combination of properties different from the group samples studied up to now, such as low velocity dispersions of $\sigma_{\rm{}l-o-s}$$\sim$120 km/s, small crossing-times ($H_{0}$$t_{c}$$\sim$0.05) and high star-formation content (95% of star-forming galaxies). This points to the SFCGs being in an evolutionary stage distinct from those groups selected in the optical and near-infrared ranges (see Fig. 6).