The Morphology-Density relationship in 1<z<2 clusters

TL;DR

This study investigates the emergence of the morphology-density relationship in galaxy clusters at redshifts 1.2 to 1.8, finding it established in some clusters and affecting mainly low-mass galaxies, with environmental effects evident as early as z=1.75.

Contribution

It provides the first evidence that the morphology-density relationship is already active at z=1.75 in some clusters and highlights the impact on low-mass, compact, bulge-dominated galaxies.

Findings

Two clusters show a significant morphology-density relationship.

Low-mass galaxies are more affected by environmental factors.

Clusters with no relationship show enhanced merger signatures.

Abstract

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.