We can use the nearby Virgo Cluster to illustrate the wealth of information locked in the ICL. Figure 2 shows deep, wide-field imaging of the Virgo Cluster taken using CWRU Astronomy's Burrell Schmidt telescope (Mihos in prep). Covering 16 square degrees down to a surface brightness of µV ∼ 28.5, the imaging reveals the complex web of diffuse light spread throughout the core of Virgo. A number of tidal streams are visible, most notably two long (> 100 kpc) thin streams NW of M87 (Mihos et al. 2005, Rudick et al. 2010). Smaller streams are also found around the Virgo ellipticals M86 and M84, likely due to stripping of low mass satellite galaxies, as well as a system of shells and plumes around M89 suggestive of one or more major mergers (Malin 1979, Janowiecki et al. 2010). However, the total luminosity contained in these discrete streams is only ∼ 1−2 × 109 L⊙ ; the bulk of the ICL is likely found in more diffuse form, locked in the extended halo of M87 or strewn throughout the cluster at lower surface brightness.
Figure 2. Diffuse light in the Virgo Cluster. The center panel shows deep (µV,lim ≈ 28.5) wide-field imaging of Virgo taken using CWRU's Burrell Schmidt telescope (Mihos in prep); the inset moon shows a 30′ scale. Panels show M87's extended halo (upper left; Mihos et al. 2005), tidal streams in the Virgo core (upper right; Mihos et al. 2005, 2015), M49's system of accretion shells (lower left; Janowiecki et al. 2010, Mihos et al. 2013) and the diffuse intragroup light surrounding NGC 4365 (lower right; Bogdán et al. 2012, Mihos in prep).
Indeed the deep imaging reveals not only the thin ICL streams but also the large radial extent of the halos of Virgo ellipticals. In particular, M87's halo is traced beyond 150 kpc, where a variety of signatures indicative of past accretion events can be seen. The outermost regions of M87's halo are extremely boxy (Mihos in prep), a behavior reflected in the spatial distribution of its GC system as well (Durrell et al. 2014). This combination of boxy isophotes and low halo rotation (Romanowsky et al. 2012) hints at a major merger event in M87's past, and indeed, both the GC and PNe systems around M87 show kinematic substructure (Romanowsky et al. 2012 and Longobardi et al. 2015a, respectively), suggesting the recent accretion of one or more ∼ 1010 L⊙ systems.
Signatures of past accretion are also found in other Virgo ellipticals as well. Located south of the Virgo core, M49 has long been known to have a dynamically complex halo, as traced by kinematic substructure in its GC system (Côté et al. 2003). The deep imaging in Figure 2 reveals the cause: after subtraction of a smooth isophotal model for M49, an extensive set of accretion shells (Janowiecki et al. 2010, Arrigoni Battaia et al. 2012, Capaccioli et al. 2015) can be seen, spanning ∼ 150 kpc in extent and containing close to 109 L⊙ of light (Janowiecki et al. 2010). The shells are morphologically similar to those formed during the radial accretion of a low mass satellite, and may be linked to the tidally disturbed dwarf companion VCC 1249 (Arrigoni Battaia et al. 2012). The shells are also distinctly redder than M49's surrounding halo (Mihos et al. 2013), suggesting that the accretion event is building up both the mass and metallicity of M49's outer halo.
Figure 2 also illustrates the efficacy of the group environment in driving ICL formation. Lying 5.3∘ to the SW of the Virgo core (and ∼ 7 Mpc behind; Mei et al. 2007) is the infalling Virgo W′ group, with the massive elliptical NGC 4365 at its core. Our deep imaging shows an extended, diffuse tidal tail emanating SW from the galaxy (Bogdán et al. 2012; Mihos in prep), and GC kinematics clearly link the tail to an interaction with its companion NGC 4342 (Blom et al. 2014). The tail contains ∼ 1.5 × 109 L⊙ , and a number of other streams are visible in NGC 4365's halo as well (including the loop visible to the NE of the galaxy), all indicative of cold tidal stripping in the group environment. Once the W′ group eventually falls into the main body of Virgo, this diffuse and extended intragroup light will be easily mixed into Virgo's diffuse ICL.
Finally, the imaging contains a dramatic example of the complex dynamical interplay between tidal stripping, ICL formation, and the destruction and formation of cluster galaxy populations. Lying at the center of the “Tidal streams” panel of Figure 2 is a large and extremely dim ultra-diffuse galaxy; with a half light radius of 9.7 kpc and central surface brightness µV = 27.0 it is the most extreme ultradiffuse cluster galaxy yet discovered (Mihos et al. 2015). The galaxy also sports a long tidal tail arcing ∼ 100 kpc to the north, as well as a compact nucleus whose photometric properties are well-matched to those of ultracompact dwarf galaxies (UCDs) found in Virgo (e.g. Zhang et al. 2015, Liu et al. 2015). In this object, we are clearly seeing the tidal destruction of a low mass, nucleated galaxy which is both feeding Virgo's ICL population and giving rise to a new Virgo UCD.
To go beyond morphology and study the stellar populations in Virgo's ICL in detail, a variety of tools are available. The colors of the streams around M87 (B − V = 0.7 − 1.0; Rudick et al. 2010) are well-matched to those of the Virgo dE population and of M87's halo itself, suggesting M87's halo may be built at least in part from low mass satellite accretion. HST imaging of discrete RGB populations in Virgo intracluster fields shows the ICL to be predominantly old and metal-poor (t > 10 Gyr, [Fe/H] ≈ −1; Williams et al. 2007), but with an additional population of stars with intermediate ages and higher metallicities (t ≈ 4−8 Gyr, [Fe/H] ≳ −0.5). These younger populations may arise either from stripped star forming galaxies or from ICL formed in-situ. The inference that stripping of late-type galaxies has contributed to the Virgo ICL is also supported by the luminosity function of PNe in M87's outer halo, which shows a “dip” characteristic of lower mass galaxies with extended star formation histories (Longobardi et al. 2015b). The diversity of stellar populations seen in Virgo's ICL almost certainly reflects the diversity of processes that create diffuse light in clusters.