Evidence for a correlation between the sizes of quiescent galaxies and local environment to z ~ 2

TL;DR

This study finds a strong correlation between galaxy sizes and local environment for quiescent galaxies at redshifts 1 to 2, showing larger sizes in denser regions, with implications for galaxy evolution.

Contribution

It provides the first robust evidence linking galaxy size to environment for quiescent galaxies at high redshift, using a large dataset and rigorous statistical analysis.

Findings

Quiescent galaxies at z~1.4 are ~50% smaller than today.

Passive galaxies in dense environments are significantly larger at fixed stellar mass.

Size-environment correlation is statistically significant with >4.8 sigma confidence.

Abstract

We present evidence for a strong relationship between galaxy size and environment for the quiescent population in the redshift range 1 < z < 2. Environments were measured using projected galaxy overdensities on a scale of 400 kpc, as determined from ~ 96,000 K-band selected galaxies from the UKIDSS Ultra Deep Survey (UDS). Sizes were determined from ground-based K-band imaging, calibrated using space-based CANDELS HST observations in the centre of the UDS field, with photometric redshifts and stellar masses derived from 11-band photometric fitting. From the resulting size-mass relation, we confirm that quiescent galaxies at a given stellar mass were typically ~ 50 % smaller at z ~ 1.4 compared to the present day. At a given epoch, however, we find that passive galaxies in denser environments are on average significantly larger at a given stellar mass. The most massive quiescent galaxies (M_stellar > 2 x 10^11 M_sun) at z > 1 are typically 50 % larger in the highest density environments compared to those in the lowest density environments. Using Monte Carlo simulations, we reject the null hypothesis that the size-mass relation is independent of environment at a significance > 4.8 sigma for the redshift range 1 < z < 2. In contrast, the evidence for a relationship between size and environment is much weaker for star-forming galaxies.