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 ^ { \prime } and 3D separation of \approx 73 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 .