With Hubble Space Telescope imaging, we investigate the progenitor population and formation mechanisms of the intracluster light (ICL) for 23 galaxy groups and clusters ranging from 3×10 13 1×10 14 M is not statistically different than that of the lower-mass groups. The BCG+ICL luminosity within 100 kpc increases with total cluster mass more steeply than within 10 kpc, implying a decoupling between in the inner and outer stellar components. For clusters the BCG+ICL luminosity at r<100 kpc (0.08-0.13 r 500,c ) is 1.2−3.5×10 12 L ; for the groups, BCG+ICL luminosities within 100 kpc (0.17-0.23 r 500,c ) range between 0.7-1.3×10 12 L . The BCG+ICL stellar mass in the inner 100 kpc increases with total cluster mass as M ∝M 500,c 0.37±0.05 . This steep slope implies that the BCG+ICL makes up a higher fraction of the total mass in groups than in clusters and that group environments are more efficient ICL producers within 100 kpc. The BCG+ICL luminosities and stellar masses are too large for the ICL stars to come from the dissolution of dwarf galaxies alone, given the observed evolution of the faint end of the cluster galaxy luminosity function, implying instead that the ICL grows from the stripping of more massive galaxies. Using the colours of cluster members from the CLASH high-mass sample, we place conservative lower limits on the luminosities of galaxies from which the ICL at r<100 kpc could originate via stripping. We find that the ICL has a colour similar to massive, passive cluster galaxies (>10 11.6 M ) at 10 kpc, while by 100 kpc this colour is equivalent to that of a 10 10.0 M galaxy. Additionally, the colour of the BCG+ICL light within 100 kpc is consistent with 75% of the total BCG+ICL luminosity originating in galaxies with L>0.2 L* (log(M [M ])>10.4), assuming conservatively that these galaxies are completely disrupted. We conclude that the tidal stripping of massive galaxies is the likely source of the intracluster light from 10−100 kpc (0.008-0.23 r 500,c ) for galaxy groups and clusters.

We present a pilot study on the origin and assembly history of the ICL for four galaxy clusters at 0.44 ≤ z ≤ 0.57 observed with the Hubble Space Telescope from the Cluster Lensing and Supernova Survey with Hubble (CLASH) sample. Using this sample of CLASH clusters we set an empirical limit on the amount of scatter in ICL surface brightness profiles of such clusters at z=0.5, a mean of 0.24 mag arcsec −2 for 1011 Gyrs. Such negative colour (and equivalently, metallicity) gradients can arise from tidal stripping of L* galaxies and/or the disruption of dwarf galaxies, but not major mergers with the BCG. We also find that the ICL at 110 kpc has a colour comparable to m*+2 red sequence galaxies, suggesting that out to this radius the ICL is dominated by stars liberated from galaxies with L>0.2 L*. Finally, we find ICL luminosities of 4-8 L* between 100.2L* galaxies, and disfavour significant contribution to the ICL by dwarf disruption or major mergers with the BCG.

We constrain the evolution of the brightest cluster galaxy plus intracluster light (BCG+ICL) using an ensemble of 42 galaxy groups and clusters that span redshifts of z = 0.05 − 1.75 and masses of M 500,c = 2 × 10 13 − 10 15 M . Specifically, we measure the relationship between the BCG+ICL stellar mass M and M 500,c at projected radii 10 < r < 100 kpc for three different epochs. At intermediate redshift (z = 0.40), where we have the best data, we find M ∝M 500,c 0.48±0.06 . Fixing the exponent of this power law for all redshifts, we constrain the normalization of this relation to be 2.08 ± 0.21 times higher atz = 0.40 than at high redshift (z = 1.55). We find no change in the relation from intermediate to low redshift (z = 0.10). In other words, for fixed M 500,c , M at 10 < r < 100 kpc increases fromz = 1.55 toz = 0.40 and not significantly thereafter. Theoretical models predict that the physical mass growth of the cluster from z = 1.5 to z = 0 within r 500,c is 1.4×, excluding evolution due to definition of r 500,c . We find that M within the central 100 kpc increases by ∼ 3.8× over the same period. Thus, the growth of M in this central region is more than a factor of two greater than the physical mass growth of the cluster as a whole. Furthermore, the concentration of the BCG+ICL stellar mass, defined by the ratio of stellar mass within 10 kpc to the total stellar mass within 100 kpc, decreases with increasing M 500,c at all z. We interpret this result as evidence for inside-out growth of the BCG+ICL over the past ten Gyrs, with stellar mass assembly occuring at larger radii at later times.

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