RELICS: ICL Analysis of the $z=0.566$ merging cluster WHL
  J013719.8-08284
  We present a pilot study of the intracluster light (ICL) in massive clusters
using imaging of the $z=0.566$ cluster of galaxies WHL J013719.8-08284 observed
by the RELICS project with the HST. We measure the ICL fraction in four optical
ACS/WFC filters (F435W, F475W, F606W, and F814W) and five infrared WFC3/IR
bands (F105W, F110W, F125W, F140W, and F160W). The ICL maps are calculated
using the free of a priori assumptions algorithm CICLE, and the cluster
membership is estimated from photometric properties. We find optical ICL
fractions that range between $\sim$6\% and 19\% in nice agreement with the
values found in previous works for merging clusters. We also observe an ICL
fraction excess between 3800 \AA and 4800 \AA, previously identified as a
signature of merging clusters at $0.18<z<0.55$. This excess suggests the
presence of an enhanced population of young/low-metallicity stars in the ICL.
All indicators thus point to WHL J013719.8-08284 as a disturbed cluster with a
significant amount of recently injected stars, bluer than the average stars
hosted by the cluster members and likely stripped out from infalling galaxies
during the current merging event. Infrared ICL fractions are $\sim$50\% higher
than the optical ones, which could be signature of an older and/or
higher-metallicity ICL population that can be associated with the build-up of
the BCG, the passive evolution of young stars, previously injected, or
preprocessing in infalling groups. Finally, investigating the photometry of the
cluster members, we tentatively conclude that WHL J013719.8-08284 fulfills the
expected conditions for a fossil system progenitor.
