The KMOS Cluster Survey (KCS) II - The Effect of Environment on the Structural Properties of Massive Cluster Galaxies at Redshift $1.39 < z <1.61$

TL;DR

This study investigates how the environment influences the structural evolution of massive passive galaxies at redshifts 1.39 to 1.61, revealing that cluster galaxies are larger and exhibit different color gradients compared to field galaxies, supporting minor mergers as a key process.

Contribution

It provides new insights into the environmental effects on galaxy sizes and stellar population gradients at high redshift, emphasizing the role of minor mergers during cluster assembly.

Findings

Cluster galaxies are 2-3 times smaller than local counterparts.

Median size of cluster galaxies is 24% larger than field galaxies.

Color gradients are twice as steep as local values, indicating age and metallicity variations.

Abstract

We present results on the structural properties of massive passive galaxies in three clusters at $1.39<z<1.61$ from the KMOS Cluster Survey. We measure light-weighted and mass-weighted sizes from optical and near-infrared Hubble Space Telescope imaging and spatially resolved stellar mass maps. The rest-frame $R$-band sizes of these galaxies are a factor of $\sim2-3$ smaller than their local counterparts. The slopes of the relation between the stellar mass and the light-weighted size are consistent with recent studies in clusters and the field. Their mass-weighted sizes are smaller than the rest frame $R$-band sizes, with an average mass-weighted to light-weighted size ratio that varies between $\sim0.45$ and $0.8$ among the clusters. We find that the median light-weighted size of the passive galaxies in the two more evolved clusters is $\sim24\%$ larger than for field galaxies, independent of the use of circularized effective radii or semi-major axes. These two clusters also show a smaller size ratio than the less evolved cluster, which we investigate using color gradients to probe the underlying $M_{*}/L_{H_{160}}$ gradients. The median color gradients are $\nabla{z-H} \sim-0.4$ mag dex$^{-1}$, twice the local value. Using stellar populations models, these gradients are best reproduced by a combination of age and metallicity gradients. Our results favor the minor merger scenario as the dominant process responsible for the observed galaxy properties and the environmental differences at this redshift. The environmental differences support that clusters experience accelerated structural evolution compared to the field, likely via an epoch of enhanced minor merger activity during cluster assembly.