Why study groups of galaxies?

Despite the small number of galaxies a typical group contains compared to a cluster, the role of galaxy groups in the construction of large-scale structures is fundamental, as in time, galaxies get assembled into groups, and in turn groups may merge together to form richer groups and clusters, in the hierarchical nature of structure formation in the Universe. The study of groups is therefore essential in order to acquire a complete understanding of the evolution of galaxies along with the environmental processes involved. There are several reasons that support the importance of such studies (Forbes et al., 2006). To begin with, with approximately 70% of galaxies located in groups (Tully, 1987), they are the most common environment in the local Universe (Eke et al., 2005; Geller & Huchra, 1983). Taking into

account the dominant scenario of structure formation, the groups that are seen at the present are the precursors of future galaxy clusters. For this reason, the early evolution of rich cluster galaxies (e.g. Bekki et al., 1999; Moss & Whittle, 2000) is closely connected with the evolution of galaxies in groups. As groups come together, collapse and merge to form in time larger systems, the logical result would be that they are a scaled-down picture of galaxy clusters, but in-depth studies of individual systems, based on the multi-wavelength observations, show that indeed this is not the case (e.g. Mulchaey, 2000; Ponman et al., 2003; Voit et al., 2005).

Another reason for the study of groups to be interesting is that they are the ideal labora- tory to study the most efficient processes in the morphological transformation of galaxies: merging (e.g. Toomre & Toomre, 1972). Mamon (2000) showed that groups merge a couple of orders of magnitude times more often than rich clusters do. As groups represent less deep potential wells, gravity plays an important role due to the close proximity of members, given the reasonably high galaxy density of groups. More crucial in this context is that the velocity dispersion of these systems is lower than in clusters, and thus groups are better environments for enhanced tidal interactions and mergers, and it is in these environments that most of the galaxy metamorphosis in morphology and star formation rates will occur (e.g. Hashimoto & Oemler, 2000; Mulchaey & Zabludoff, 1998).

Recent large surveys, such as the 2dFGRS (Lewis et al., 2002) have shown that star formation is suppressed on group level densities. The increased incidence of quenched star formation in galaxy groups can be strongly linked with the reduction of the mean star formation rate of the Universe moving to redshift 0 (Laganá et al., 2013). As noted in Hou et al. (2013) the interactions between galaxies, which are more common in a group environment, initially give rise to star formation (Cox et al., 2006; Sanders et al., 1988;

Teyssier et al., 2010), that consumes the available supply of cold gas and prompts the ceasing of star formation, if no more gas is accreted into the galaxy. For that reason, processes such as merging and interactions rely on the stage of the evolution at which a galaxy is observed and play a decisive role in boosting or cutting off star formation. Considering only the galaxies that reside in rich groups or clusters with masses of M_halo> 1013 h−1 M⊙, Hou et al. (2013) reports that approximately 40% is believed to be preprocessed (i.e.

with star formation suppressed) before their accretion into the system (De Lucia et al., 2012; McGee et al., 2009). The physical mechanism for this ‘pre-processing’ of galaxies in groups is currently an open issue and not properly understood (e.g. Fujita et al., 2003), but this scenario provides a plausible explanation for the observed continuous reduction of the global star forming rate as a function of the time of the Universe (e.g. Cowie et al., 1999; Cooper et al., 2008).

Another significant characteristic of galaxy groups is that they contain a large fraction of the hot gas that is seen in the Universe (Forbes et al., 2006). Compared to clusters, most groups appear to have a similar amount of hot gas. Given the abundance of groups of galaxies, these systems thus significantly contribute to the baryonic component of the Universe (Fukugita et al., 1998). However, the matter content in groups differs significantly from one system to another. If we examine groups in more detail, the baryon composition in groups and clusters can be divided into two major components: i) the hot gas between galaxies, and ii) the stars in them. The amount between hot gas and stars in groups of galaxies can either be equal (Laganá et al., 2011) or in some cases the amount of hot gas can be lower than the stellar mass component (e.g. Giodini et al., 2009). In clusters, the baryon component which is always dominant is the hot gas. Hence, groups give the very rare opportunity to study the origin and nature of this important mass component and its

very close relation with galaxies and their evolution (Forbes et al., 2006).

Finally, our own environment in which our Galaxy exists, is a small group with a few tens of galaxies. As the environments that correspond to galaxy groups range from systems that have collapsed very early (fossil groups) to those with density only a little higher than that in the ‘field’, in order to understand what are the implications of the evolution in our Local Group environment we need to study in more detail systems of nearby galaxy groups. While in clusters of galaxies early-type galaxies had the majority of stars created at very early epochs, (e.g. Tanaka et al., 2005; Terlevich & Forbes, 2002), star formation occurs in a wider variety of systems in groups of galaxies (Terlevich & Forbes, 2002). For this reason, the detailed examination and multi-wavelength study of nearby groups of galaxies, taking advantage of their proximity, is essential in order to examine the extent to which star formation in galaxies is related to the state of evolution in these abundant systems, keeping in mind that much of the early evolution of galaxies occurs in the group environment (Forbes et al., 2006).