Within the next five years, the space-based Euclid and Roman missions and the ground-based Rubin Observatory will be operational, and Euclid and Rubin will both be producing data within a year. The combination of wide field-of-view and sensitivity afforded by these surveys will greatly expand the area for which deep imaging is available (from a thousand sq degrees to half of the sky). This step-change in data will enable a revolution in our understanding of the low surface brightness (LSB) Universe. We propose a series of ISSI workshops focused on using these new facilities to study one class of LSB structure — intracluster light (ICL) from galaxy clusters. Our diverse group of investigators from 6 countries, including both observers and theorists, will map out the best path forward to exploit data from these new missions to advance our understanding of ICL.
While the existence of the ICL has been known for over 75 years (Zwicky 1957), it is only with new facilities such as Euclid, Rubin and Roman, and improvements in the capabilities of computational methods, that it is now possible to solve the key challenges in the measurement and interpretation of the ICL. Our team’s investigations of the ICL are driven by two scientific questions.
Science question 1: What physical processes dominate ICL production over cosmic time?
One major uncertainty in our understanding of galaxy clusters is the relative importance of different environmental processes (e.g., ram pressure stripping, tidal stripping, tidal disruption, merging) in driving the evolution of the cluster galaxy population and the formation of the ICL. However, measurements of the high-surface brightness Universe, such as the galaxy stellar mass function, only give an incomplete view of cluster galaxy evolution because they ignore
the appreciable fraction of stellar mass that no longer resides in galaxies – the ICL.
Understanding how the ICL assembles, through measurements of its spatial distribution and stellar populations and their evolution, can help elucidate which environmental processes dominate over cosmic time. For example, if the ICL is formed early during cluster assembly, present day observations should show a well-mixed, smooth light profile and old stellar populations. Conversely, if ICL formation happens actively throughout the life of a cluster, via ongoing tidal stripping and destruction of infalling galaxies, the ICL should have significant spatial substructure, with a much more heterogeneous mix of stellar populations (see Contini, 2021 for a recent review). Meanwhile, the presence of very young stellar populations in the ICL would be the signature of in-situ formation mechanisms such as star formation triggered
by ram-pressure stripping of galaxies that have recently fallen into the cluster. This also connects to the dynamical state of the cluster. In relaxed clusters that have not recently undergone a major merger, the ICL will likely form early, with a similar stellar composition to the cluster galaxies and hence a radial age gradient from old in the core to young in the
outskirts. On the other hand, in dynamically active clusters, violent processes such as galaxy mergers or the disruption of dwarf galaxies likely dominate the assembly of the ICL (Jiménez-Teja et al 2018). These processes result in significant spatial substructures, and younger intracluster stars.
Science question 2: How well does the ICL trace dark matter?
The second driver for our project is to advance the use of the ICL as a cosmological tool. Pioneering work by Montes & Trujillo (2019) demonstrated that the ICL distribution traces the strong lensing mass distribution within the current observational uncertainties (~25 kpc) in cluster cores. On the theoretical side, while some simulations confirm a strong
correspondence between the ICL and total mass distributions, even out to radii of 1 Mpc from the cluster centre (Alonso Asensio et al. 2020), others find that the 1-D slopes of both distributions are different (Sampaio-Santos et al. 2021, Diego et al. 2023). Based on this possible correspondence between ICL and dark matter, other recent work has focused upon the measurement of cluster splashback radii via the ICL (Deason et al. 2021, Gonzalez et al. 2021). The splashback radius is the apogee of the orbit for infalling material after the first core passage. Unlike other typically-used cluster radii, such as R200, this radius is physically observable as a break in the density profile (Diemer & Kravtsov 2014, Adhikari et
al. 2014). If cluster masses are measured within the splashback radius, there is the expectation that scatter in mass-observable relations will be reduced, enabling improved cosmological constraints from cluster abundance measurements. The concept of using ICL as a potential tracer of dark matter has only emerged in the last 4 years (Montes & Trujillo 2019; Huang et al. 2021; Kluge et al. 2021; Sampaio-Santos et al, 2021). We still do not know how well the ICL traces the dark matter distribution, and if it does, how far from the centre of the cluster we should expect to see this similarity. A particular complication is that observations must rely on comparison to a proxy of the dark matter distribution (the gravitational lensing signal). Only simulations can compare the ICL directly to dark matter. A dialogue between observers and simulators is therefore critical to make progress here, exactly what our ISSI workshops will enable. Now is the time for this dialogue to occur since the upcoming facilities will provide the first opportunity to observationally compare ICL and lensing maps for large samples.