The population of early-type galaxies at 1<z<2 - New clues on their formation and evolution

Abstract

[Abridged]We present the morphological analysis based on HST-NICMOS observations in the F160W filter of a sample of 32 early-type galaxies (ETGs) at 1<z<2 with spectroscopic confirmation of their redshift and spectral type. We find that ETGs at are composed of two distinct populations, an older population (oETGs) and a younger population (yETGs) whose mean ages differ by about 1.5-2 Gyr. Young ETGs are not denser than local ones since they follow the size-mass relation of local ETGs and luminosity evolution brings them onto the local Kormendy and size-luminosity relations. On the constrary, old ETGs do not follow the size-mass relation of local ETGs and luminosity evolution does not account for the discrepancy they show with respect to the local size-luminosity and Kormendy relations. An increase of their effective radius R_e by a factor 2.5-3 (a density decrease by a factor 15-30) from z~1.5-2 to z~0 is required to bring these galaxies onto the local scaling relations. The different behaviour shown by the two populations implies different formation and evolution scenarios. The older population must have formed at higher-z in a sort of dissipative gas-rich collapse ables to produce remnants which at z~2 are old and compact. The size evolution they must experience from z~2 to z~0 has to leave unchanged their mass to not exceed the local number of high-mass ETGs. Thus, major merging cannot fit this requirement. The younger population of ETGs can be formed later through subsequent episodes of merging which increased progressively their size and assembled their mass down to z~2. At z<2 they evolve purely in luminosity since episodes of major merging would bring them far from the local scaling relations.Comment: Accepted for publication in MNRAS; 16 pages, 10 figures 2 table