The Morphology-Density relationship in 1<z<2 clusters
  The morphology-density relationship states that dense cosmic environments
such as galaxy clusters have an overabundance of quiescent elliptical galaxies,
but it is unclear at which redshift this relationship is first established. We
study the morphology of 4 clusters with $1.2<z<1.8$ using HST imaging and the
morphology computation code statmorph. By comparing median morphology of
cluster galaxies to CANDELS field galaxies using Monte Carlo analysis, we find
that 2 out of 4 clusters (at z=1.19 and z=1.75) have an established
morphology-density relationship with more than $3\sigma$ significance.
$\sim$50% of galaxies in these clusters are bulge-dominated compared to
$\sim$30% in the field, and they are significantly more compact. This result is
more significant for low-mass galaxies with $\log M/M_\odot \lessapprox 10.5$,
showing that low-mass galaxies are affected the most in clusters. We also find
an intriguing system of two z $\approx$ 1.45 clusters at a unusually small
separation 2D separation of $3'$ and 3D separation of $\approx73$ Mpc that
exhibit no morphology-density relationship but have enhanced merger signatures.
We conclude that the environmental mechanism responsible for the
morphology-density relationship is 1) already active as early as z=1.75, 2)
forms compact, bulge-dominated galaxies and 3) affects primarily low-mass
galaxies. However, there is a significant degree of intracluster variance that
may depend on the larger cosmological environment in which the cluster is
embedded.
