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The evolution shown in Figure 1 argues that interactions in the group environment should be particularly effective at stripping stars from galaxies and redistributing them into the diffuse intragroup light, an important precursor to the ICL in massive clusters. Curiously, though, these arguments have not always been borne out observationally. In the nearby Leo I group, searches for intragroup light using both PNe (Castro-Rodriguez et al. 2003) and broadband imaging (Watkins et al. 2014) have come up empty, particularly notable given that the system is contains a large (∼ 200 kpc) HI ring thought to be collisional in origin (Michel-Dansac et al. 2010). Similarly, the M101 group also show little sign of extended diffuse light (Mihos et al. 2013), despite the tidal disturbances evident in M101 and its nearby companions. Even in the clearly interacting M81 / M82 group, early searches for orphaned RGB stars (Durrell et al. 2004) and PNe (Feldmeier et al. 2003) could only place upper limits on the intragroup light fraction (≲ 2%).

In contrast, intragroup light is quite evident in dense, strongly interacting groups. The ability for these strong interactions to expel diffuse material to large distances is shown in recent deep imaging of the M51 system by Watkins et al. (2015; Fig 4a), where several extremely low surface brightness plumes extend nearly 50 kpc from the center. Similarly, many compact groups are awash in diffuse light (e.g. Da Rocha et al. 2005, 2008), including the archetypal groups Seyfert's Quintet (Mendes de Oliveira et al. 2001) and Seyfert's Sextet (Durbala et al. 2008, Figure 4b). The contrast between the copious diffuse light seen in these dense systems and the dearth of light in loose groups is striking, arguing either that the tidal debris is rapidly dispersed to even lower (undetectable) surface brightnesses, or that close interactions in loose groups are relatively uncommon.

Figure 4

Figure 4. Left: Deep imaging of M51 showing diffuse tidal debris extending to nearly 50 kpc (Watkins et al. 2015). Right: Diffuse light in Seyfert's Sextet (Durbala et al. 2008).

More recently, the ability to probe discrete stellar populations in external galaxies provides a powerful new tool for studying intragroup light. Probing stellar densities far below the capabilities of wide-area surface photometry, these techniques are now revealing the diffuse light contained even in loose groups. Deep imaging by Okamoto et al. (2015) has uncovered the previously undetected and very extended stellar tidal debris field in the M81 group, while imaging of M31 and M33 by the PAndAS team (e.g. Ibata et al. 2014) has mapped the myriad of tidal streams that characterize Andromeda's extended stellar halo and trace its past interaction with M33. While at low surface brightness and containing only a small amount of the total light of their parent groups, the diffuse starlight found in these studies of nearby loose groups represent the important first step in building the intragroup and intracluster light in dense galaxy environments.

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