Not all the light in galaxy groups and clusters comes from stars that are bound to galaxies. A significant fraction of it constitutes the so-called intracluster or diffuse light (ICL), a low surface brightness component of groups/clusters generally found in the surroundings of the brightest cluster galaxies and intermediate/massive satellites. In this review, I will describe the mechanisms responsible for its formation and evolution, considering the large contribution given to the topic in the last decades by both the theoretical and observational sides. Starting from the methods that are commonly used to isolate the ICL, I will address the remarkable problem given by its own definition, which still makes the comparisons among different studies not trivial, to conclude by giving an overview of the most recent works that take advantage of the ICL as a luminous tracer of the dark matter distribution in galaxy groups and clusters.

We study the growth pathways of brightest central galaxies (BCGs) and intracluster light (ICL) by means of a semi-analytic model. We assume that ICL forms by stellar stripping of satellite galaxies and violent processes during mergers, and implement two independent models: One considers both mergers and stellar stripping (named STANDARD model), and the other considers only mergers (named MERGERS model). We find that BCGs and ICL form, grow, and overall evolve at different times and with different time-scales, but they show a clear coevolution after redshift $$z$$ ∼ 0.7−0.8. Around 90 per cent of the ICL from stellar stripping is built up in the innermost 150 kpc from the halo centre and the dominant contribution comes from disc-like galaxies (B/T < 0.4) through a large number of small/intermediate stripping events (M_strip/M_sat < 0.3). The fractions of stellar mass in BCGs and in the ICL over the total stellar mass within the virial radius of the halo evolve differently with time. At high redshift, the BCG accounts for the bulk of the mass, but its contribution gradually decreases with time and stays constant after $$z$$ ∼ 0.4−0.5. The ICL, instead, grows very fast and its contribution keeps increasing down to the present time. The STANDARD and MERGERS models make very similar predictions in most of the cases, but predict different amounts of ICL associated with galaxies within the virial radius of the group/cluster other than the BCG, at $$z$$ = 0. We then suggest that this quantity is a valid observable that can shed light on the relative importance of mergers and stellar stripping for the formation of ICL.

We study the colors and metallicities of the brightest cluster galaxies (BCGs) and intracluster light (ICL) in galaxy groups and clusters, as predicted by a semi-analytic model of galaxy formation, coupled with a set of high-resolution N-body simulations. The model assumes stellar stripping and violent relaxation processes during galaxy mergers to be the main channels for the formation of the ICL. We find that BCGs are more metal-rich and redder than the ICL, at all redshifts once the ICL starts to form (z ∼ 1). In good agreement with several observed data, our model predicts negative radial metallicity and color gradients in the BCG+ICL system. By comparing the typical colors of the ICL with those of satellite galaxies, we find that the mass and metals in the ICL come from galaxies of different mass, depending on the redshift. Stripping of low-mass galaxies, , is the most important contributor in the early stages of ICL formation, but the bulk of the mass/metals contents are given by intermediate/massive galaxies, , at lower redshift. Our analysis supports the idea that stellar stripping is more important than galaxy mergers in building up the ICL, and highlights the importance of colors/metallicity measurements for understanding the formation and evolution of the ICL.

To understand the galaxy population in clusters today, we should also consider the impact of previous environments prior to cluster infall, namely preprocessing. We use the Yonsei Zoom-in Cluster Simulation, a hydrodynamic high-resolution zoom-in simulation of 15 clusters, and focus on the tidal stripping suffered by the dark matter halos of cluster members due to preprocessing. We find that ∼48% of today’s cluster members were once satellites of other hosts. This is slightly higher than previous estimates, in part because we consider not just group-mass hosts but hosts of all masses. Thus, we find that the preprocessed fraction is poorly correlated with cluster mass and is instead related to each cluster’s recent mass growth rate. Hosts less massive than groups are significant contributors, providing more than one-third of the total preprocessed fraction. We find that halo mass loss is a clear function of the time spent in hosts. However, two factors can increase the mass-loss rate considerably: the mass ratio of a satellite to its host and the cosmological epoch when the satellite was hosted. The latter means we may have previously underestimated the role of high-redshift groups. From a sample of heavily tidally stripped members in clusters today, nearly three-quarters were previously in a host. Thus, visibly disturbed cluster members are more likely to have experienced preprocessing. Being hosted before cluster infall enables cluster members to experience tidal stripping for extended durations compared to direct cluster infall and at earlier epochs when hosts were more destructive.

We take advantage of an analytic model of galaxy formation coupled to the merger tree of an N-body simulation to study the roles of environment and stellar mass in the quenching of galaxies. The model has been originally set in order to provide the observed evolution of the stellar mass function as well as reasonable predictions of the star formation rate–stellar mass relation, from high redshift to the present time. We analyze the stellar mass and environmental quenching efficiencies and their dependence on stellar mass, halo mass (taken as a proxy for the environment), and redshift. Our analysis shows that the two quenching efficiencies are dependent on redshift and stellar and halo mass, and that the halo mass is also a good proxy for the environment. The environmental quenching increases with decreasing redshift and is inefficient below log M _* ∼ 9.5, reaches the maximum value at log M _* ∼ 10.5, and decreases again, becoming poorly efficient at very high stellar mass (log M _* ≳ 11.5). Central and satellites galaxies are mass quenched differently: for the former, the quenching efficiency depends very weakly on redshift but strongly on stellar mass; for the latter, it strongly depends on both stellar mass and redshift in the range . According to the most recent observational results, we find that the two quenching efficiencies are not separable: intermediate-mass galaxies, as well as intermediate/massive galaxies in more massive halos, are environmentally quenched faster. At stellar masses lower than log M _* ≲ 9.5, both quenching mechanisms become inefficient, independently of the redshift.

Intracluster light (ICL) in observations is usually identified through the surface brightness limit (SBL) method. In this paper, for the first time we produce mock images of galaxy groups and clusters, using a cosmological hydrodynamical simulation to investigate the ICL fraction and focus on its dependence on observational parameters, e.g., the SBL, the effects of cosmological redshift-dimming, point-spread function (PSF), and CCD pixel size. Detailed analyses suggest that the width of the PSF has a significant effect on the measured ICL fraction, while the relatively small pixel size shows almost no influence. It is found that the measured ICL fraction depends strongly on the SBL. At a fixed SBL and redshift, the measured ICL fraction decreases with increasing halo mass, while with a much fainter SBL, it does not depend on halo mass at low redshifts. In our work, the measured ICL fraction shows a clear dependence on the cosmological redshift-dimming effect. It is found that there is more mass locked in the ICL component than light, suggesting that the use of a constant mass-to-light ratio at high surface brightness levels will lead to an underestimate of ICL mass. Furthermore, it is found that the radial profile of ICL shows a characteristic radius that is almost independent of halo mass. The current measurement of ICL from observations has a large dispersion due to different methods, and we emphasize the importance of using the same definition when observational results are compared with theoretical predictions.