The Halo Boundary of Galaxy Clusters in the SDSS
  Mass around dark matter halos can be divided into "infalling" material and
"collapsed" material that has passed through at least one pericenter.
Analytical models and simulations predict a rapid drop in the halo density
profile associated with the transition between these two regimes. Using data
from SDSS, we explore the evidence for such a feature in the density profiles
of galaxy clusters and investigate the connection between this feature and a
possible phase space boundary. We first estimate the steepening of the outer
galaxy density profile around clusters: the profiles show an abrupt steepening,
providing evidence for truncation of the halo profile. Next, we measure the
galaxy density profile around clusters using two sets of galaxies selected
based on color. We find evidence of an abrupt change in the galaxy colors that
coincides with the location of the steepening of the density profile. Since
galaxies are likely to be quenched of star formation and turn red inside of
clusters, this change in the galaxy color distribution can be interpreted as
the transition from an infalling regime to a collapsed regime. We also measure
this transition using a model comparison approach which has been used recently
in studies of the "splashback" phenomenon, but find that this approach is not a
robust way to quantify the significance of detecting a splashback-like feature.
Finally, we perform measurements using an independent cluster catalog to test
for potential systematic errors associated with cluster selection. We identify
several avenues for future work: improved understanding of the small-scale
galaxy profile, lensing measurements, identification of proxies for the halo
accretion rate, and other tests. With upcoming data from the DES, KiDS and HSC
surveys, we can expect significant improvements in the study of halo
boundaries.
