The evolution of the morphological scale of early-type galaxies since z=2

TL;DR

This study analyzes the morphological evolution of early-type galaxies from redshift 2 to the present, revealing significant surface brightness and size evolution that challenge fixed-size models.

Contribution

It provides new evidence that early-type galaxies undergo size evolution since z=2, contradicting the fixed-size hypothesis.

Findings

Early-types at z~1.5 have higher surface brightness than local counterparts.

Surface brightness must fade by ~2.5 mag from z~1.5 to z=0.

Size evolution is necessary to explain observed properties.

Abstract

We present the morphological analysis based on HST-NICMOS observations in the F160W filter of a sample of 30 early-type galaxies spectroscopically confirmed at 1.2<z<2. We derive the effective radius R_e and the mean surface brightness mu_e of galaxies in the rest-frame R-band. We find that early-types at z~1.5 are characterized by a surface brightness (SB) much higher then their local counterparts with comparable R_e. In particular, we find that the mean SB of these early-types should get fainter by ~2.5 mag from z~1.5 to z=0 to match the SB of the local early-types with comparable R_e. This evolution exceeds by a factor two the luminosity evolution expected for early-types in this redshift range and more than a factor three the one derived from the observed luminosity function of galaxies. Consequently, an evolution of the effective radius R_e from the epoch of their formation towards z=0 has to be invoked and the hypothesis of fixed size rejected.