Recent Structural Evolution of Early-Type Galaxies: Size Growth from z=1 to z=0

TL;DR

This study confirms significant size growth of early-type galaxies from redshift 1 to now, with observed evolution nearly doubling their size, exceeding previous model predictions and supporting ongoing structural evolution.

Contribution

It provides a model-independent analysis of size evolution using dynamically measured masses and consistent size measurements, confirming rapid growth since z=1.

Findings

Size at fixed mass increases by a factor of 1.97 from z=1 to present

Observed evolution is faster than semianalytic model predictions

Results align with previous photometric studies showing up to fivefold size increase since z~2

Abstract

Strong size and internal density evolution of early-type galaxies between z~2 and the present has been reported by several authors. Here we analyze samples of nearby and distant (z~1) galaxies with dynamically measured masses in order to confirm the previous, model-dependent results and constrain the uncertainties that may play a role. Velocity dispersion measurements are taken from the literature for 50 morphologically selected 0.8<z<1.2 field and cluster early-type galaxies with typical masses 2e11 Msol. Sizes are determined with ACS imaging. We compare the distant sample with a large sample of nearby (0.04<z<0.08) early-type galaxies extracted from the SDSS for which we determine sizes, masses, and densities in a consistent manner, using simulations to quantify systematic differences between the size measurements of nearby and distant galaxies. We find a highly significant structural difference between the nearby and distant samples, regardless of sample selection effects. The implied evolution in size at fixed mass between z=1 and the present is a factor of 1.97(0.15). This is in qualitative agreement with semianalytic models; however, the observed evolution is much faster than the predicted evolution. Our results reinforce and are quantitatively consistent with previous, photometric studies that found size evolution of up to a factor of 5 since z~2. A combination of structural evolution of individual galaxies through the accretion of companions and the continuous formation of early-type galaxies through increasingly gas-poor mergers is one plausible explanation of the observations.