While Virgo's proximity gives us a detailed view of intracluster stellar populations, to gain a wider census of ICL in galaxy clusters we must move beyond Virgo. Going to greater distances opens up the ability to study ICL in a wider sample of clusters which span a range of mass, dynamical state, and redshift, allowing us to connect ICL properties with cluster evolution. This comes at a cost, however; beyond Virgo, current generation telescopes cannot directly image intracluster stars, and even studies of more luminous tracers such as PNe and GC become more difficult. At higher redshifts, one becomes limited to broadband imaging, where the strong cosmological (1 + z)4 surface brightness dimming makes the already diffuse ICL even more difficult to observe.
Aside from these observational difficulties, a second major problem is the ambiguous definition of intracluster light itself. Since much of the ICL is formed via the mergers that build up the central BCG, there is often no clear differentiation between the BCG halo and the extended ICL – the two components blend smoothly together (and indeed may not be conceptually distinct components at all). In attempts to separate BCG halos from extended ICL, a variety of photometric definitions have been proposed, which typically adopt different functional forms (such as multiple r1/4 or Sersic profiles) for each component when fitting the total profile (e.g. Gonzalez et al. 2005, Krick & Bernstein 2007, Seigar et al. 2007). However, such definitions are very sensitive to the functional forms adopted for the profiles. For example, M87's profile is reasonably well-fit by either a single Sersic or a double r1/4 model (Janowiecki et al. 2010); the former fit would imply little additional ICL, while the latter fit puts equal light into the inner and outer profiles. To avoid this ambiguity, alternate non-parametric measures have also been employed to characterize the ICL luminosity, defining the ICL as diffuse light fainter than some characteristic surface brightness (e.g. Feldmeier et al. 2004, Burke et al. 2015). While these present systematic uncertainties of their own, simulations suggest that thresholds of µV ≳ 26.5 do a reasonable job of separating out an extended and perhaps unrelaxed ICL component from the central BCG light (Rudick et al. 2011, Cooper et al. 2015).
Still other methods propose kinematic separation of the ICL from the central galaxy light. Dolag et al. (2010) used simulated clusters to show that separate kinematic populations exist in cluster cores, well-characterized by distinct Maxwellian distributions. These kinematic populations then separate out spatially into two Sersic-like profiles plausibly identified as the BCG galaxy and the cluster ICL (perhaps reflecting different accretion events as well; Cooper et al. 2015). And indeed these definitions have some observational support. Longslit spectroscopy of the BCG galaxy in Abell 2199 shows a velocity dispersion profile that first falls with radius, then increases in the outer halo to join smoothly onto the cluster velocity dispersion (Kelson et al. 2002). Meanwhile in Virgo the velocities of the PNe around M87 show a double Gaussian distribution (Longobardi et al. 2015b), suggesting distinct BCG and ICL components. However, observational constraints make accessing kinematic information for the ICL in distant clusters a daunting task.
A comparison of these different metrics is shown in Figure 3 (from Rudick et al. 2011), which shows that the inferred ICL fraction in simulated clusters can vary by factors of 2−3 depending on the adopted metric (see also Puchwein et al. 2010). Kinematic separation leads to higher ICL fractions, as a significant amount of starlight found within the BCG galaxy belongs to the high-velocity ICL component. In contrast, density-based estimates yield systematically lower ICL fractions, as material at high surface brightness is typically assigned to the cluster galaxies independent of its kinematic properties.
Figure 3. ICL fractions in simulated galaxy clusters, taken from Rudick et al. (2011). Five simulated clusters of varying mass are shown; at a given mass, the symbols show ICL fractions calculate for a single cluster using different ICL definitions: stars at low surface brightness (µV > 26.5, green squares), stars in low density intracluster space now (open triangles) or ever (blue stars), stars unbound from galaxies (filled red circles), including stars kinematically separated from the central cD galaxy (open red circle). See Rudick et al. (2011) for details.
Given both the ambiguity in defining the ICL and the observational difficulties in studying it, attempts to characterize ICL in samples of clusters spanning a range of mass and redshift have led to varying results. An early compilation of results for local clusters by Ciardullo et al. (2004) showed ICL fractions ranging from ∼ 15−40%, with no clear dependence on cluster velocity dispersion or Bautz-Morgan type. Recent imaging of more distant clusters probes the connection between cluster evolution and ICL more directly, but again yields mixed results. While Guennou et al. (2012) find no strong difference between the ICL content of clusters between at z ∼ 0.5 and today, Burke et al. (2015) find rapid evolution in the ICL fraction of massive clusters over a similar redshift range. Other studies of clusters at z ∼ 0.3−0.5 yield ICL fractions of 10−25% (Presotto et al. 2014, Montes & Trujillo 2014, Giallongo et al. 2014), similar to z = 0 results. However, these studies use different ICL metrics and are limited to only a handful of clusters; clearly a large sample of clusters with ICL fractions measured in a consistent manner is needed to tackle the complex question of ICL evolution.
A similar story holds for recent attempts to constrain ICL stellar populations as well. Using HST imaging of distant CLASH clusters, DeMaio et al. (2015) infer moderately low metallicities ([Fe/H] ∼ −0.5) from the ICL colors, in contrast to the case of Abell 2744, where Montes & Trujillo (2014) use colors to argue for a dominant population of intermediate age stars with solar metallicity. Meanwhile, spectroscopic population synthesis studies show similarly diverse results. For example, in the Hydra I cluster, Coccato et al. (2011) find old ICL populations with sub-solar metallicities, while in the massive cluster RX J0054.0−2823, Melnick et al. (2012) find similarly old but metal-rich ICL stars ([Fe/H] ≳ 0). However, while intriguing, all these studies are subject to strong photometric biases, limited largely to the brightest portions of the ICL which may not be representative of the ICL as a whole and may also include substantial fraction of what would normally be considered BCG light as well.