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 $M_{500,c}=2times10^{13}-10^{15}$ M$_odot$ Specifically
, we measure the relationship between the BCG+ICL stellar mass $M_star$ 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 $M_starpropto M_{500,c}^{0.48pm0.06}$. Fixing the exponent of this power law for all redshifts, we constrain the normalization of this relation to be $2.08pm0.21$ times higher at z = 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 $M_{500,c}$, $M_star$ at 10 < r < 100 kpc increases from z = 1.55 to z = 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 a factor of 1.4, excluding evolution due to definition of $r_{500,c}$. We find that $M_star$ within the central 100 kpc increases by a factor of 3.8 over the same period. Thus, the growth of $M_star$ 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 redshift. 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.
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$times10^{13}<$M$_{500,c}$ [M$_odot$]$<9times10^{14}$ at 0.29$<$z$<
$0.89. The color gradients of the BCG+ICL become bluer with increasing radius out to 53-100 kpc for all but one system, suggesting that violent relaxation after major mergers with the BCG cannot be the dominant source of ICL. For clusters the BCG+ICL luminosity at r$<$100 kpc (0.08-0.13 r$_{500,c}$) is 1.2-3.5$times 10^{12}$L$_odot$; for the groups, BCG+ICL luminosities within 100 kpc (0.17-0.23 r$_{500,c}$) range between 0.7-1.3$times 10^{12}$ L$_odot$. The BCG+ICL stellar mass in the inner 100 kpc increases with total cluster mass as M$_bigstarpropto$M$_{500,c}$$^{0.37pm0.05}$. This steep slope implies that the BCG+ICL is a higher fraction of the total mass in groups than in clusters. The BCG+ICL luminosities and stellar masses are too large for the ICL stars to come from the dissolution of dwarf galaxies alone, implying instead that the ICL grows from the stripping of more massive galaxies. Using the colors of cluster members from the CLASH sample, we place conservative lower limits on the luminosities of galaxies from which the ICL could originate. We find that at 10 kpc the ICL has a color similar to massive, passive cluster galaxies ($>10^{11.6}$ M$_odot$), while by 100 kpc this colour is equivalent to that of a 10$^{10}$ M$_odot$ galaxy. Additionally, we find 75% of the total BCG+ICL luminosity is consistent in color of galaxies with L$>$0.2 L$_*$ (log(M$_bigstar$[M$_odot$])$>$10.4), assuming conservatively that these galaxies are completely disrupted. We conclude that tidal stripping of massive galaxies is the likely source of the intracluster light from 10-100 kpc (0.008-0.23 r$_{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 clu
sters we set an empirical limit on the amount of scatter in ICL surface brightness profiles of such clusters at z=0.5 and constrain the progenitor population and formation mechanism of the ICL by measuring the ICL surface brightness profile, the ICL color and color gradient, and the total ICL luminosity within 10<r<110 kpc. The observed scatter is physical, which we associate with differences in ICL assembly process, formation epoch, and/or ICL content. Using stellar population synthesis models we transform the observed colors to metallicity. For three of the four clusters we find clear negative gradients that, on average, decrease from super solar in the central regions of the BCG to sub-solar in the ICL. Such negative color/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 color comparable to m*+2 red sequence galaxies and a total luminosity between 10<r<110 kpc of 4-8 L*. This suggests that the ICL is dominated by stars liberated from galaxies with L>0.2 L* and that neither dwarf disruption nor major mergers with the BCG alone can explain the observed level of luminosity and remain consistent with either the observed evolution in the faint end slope of the luminosity function or predictions for the number of BCG major mergers since z=1. Taken together, the results of this pilot study are suggestive of a formation history for these clusters in which the ICL is built-up by the stripping of >0.2 L* galaxies, and disfavor significant contribution to the ICL by dwarf disruption or major mergers with the BCG.
