Fast evolving size of early-type galaxies at z>2 and the role of dissipationless (dry) merging

TL;DR

This study investigates the size evolution of early-type galaxies from redshift 0 to 3, comparing observations with dry merging models, and finds accelerated size growth at z>2 that models struggle to fully explain.

Contribution

It provides the first comprehensive analysis of early-type galaxy size evolution over a broad redshift range, highlighting discrepancies with pure dry merging models at high redshift.

Findings

Size evolution follows R_e(M) ∝ (1+z)^β with β ~ -1 at lower redshifts.

Most massive ETGs show faster size evolution, β ~ -1.4.

Dry merging models fit well at 0<z<1.7 but not at z>2.

Abstract

We present the analysis of a large sample of early-type galaxies (ETGs) at 0<z<3 aimed at tracing the cosmic evolution of their size and compare it with a model of pure dissipationless (dry) merging in the LambdaCDM framework. The effective radius R_e depends on stellar mass M as R_e(M) \propto M}^{alpha} with alpha ~ 0.5 at all redshifts. The redshift evolution of the mass- or SDSS-normalized size can be reproduced as \propto (1+z)^beta with beta ~ -1, with the most massive ETGs possibly showing the fastest evolutionary rate (beta ~ -1.4). This size evolution slows down significantly to beta ~ -0.6 if the ETGs at z>2 are removed from the sample, suggesting an accelerated increase of the typical sizes at z>2, especially for the ETGs with the largest masses. A pure dry merging LambdaCDM model is marginally consistent with the average size evolution at 0<z<1.7, but predicts descendants too compact for z>2 progenitor ETGs. This opens the crucial question on what physical mechanism can explain the accelerated evolution at z>2, or whether an unclear observational bias is partly responsible for that.