The number density of superdense early-type galaxies at 1<z<2 and the local cluster galaxies

TL;DR

This study shows that at redshifts 1<z<2, the population of early-type galaxies included both normal and compact types, with the number density of compact galaxies comparable to local clusters, challenging the idea of significant size evolution over time.

Contribution

It provides a complete analysis of early-type galaxy sizes and densities at 0.9<z<1.92, revealing the coexistence of normal and compact galaxies and linking high-z compact ETGs to local cluster galaxies.

Findings

62% of early-type galaxies at 1<z<2 are similar to local ones.

The number density of compact ETGs at z~1.5 matches local cluster densities.

Most high-z compact ETGs are progenitors of local cluster compact galaxies.

Abstract

Many of the early-type galaxies observed so far at z>1 turned out to have smaller radii with respect to that of a typical present-day early-type galaxy with comparable mass. This has generated the conviction that in the past early-type galaxies were more compact, hence denser, and that as a consequence, they should have increased their radius across the time to reconcile with the present-day ones. However, observations have not yet established whether the population of early-types in the early Universe was fully represented by compact galaxies nor if they were so much more numerous than in the present-day Universe to require an evolution of their sizes. Here we report the results of a study based on a complete sample of 34 early-type galaxies at 0.9<z_{spec}<1.92. We find a majority (62 per cent) of normal early-type galaxies similar to typical local ones, co-existing with compact early-types from ~2 to ~6 times smaller in spite of the same mass and redshift. The co-existence of normal and compact early-type galaxies at <z>~1.5 suggests that their build-up taken place in the first 3-4 Gyr, followed distinct paths. Also, we find that the number density of compact early-types at <z>~1.5 is consistent with the lower limits of the local number density of compact early-types derived from local clusters of galaxies. The similar number of compact early-types found in the early and in the present day Universe frustrates the hypothesized effective radius evolution while provides evidence that also compact ETGs were as we see them today 9-10 Gyr ago. Finally, the fact that (at least) most of the compact ETGs at high-z are accounted for by compact early-types in local cluster of galaxies implies that the former are the direct progenitors of the compact early-type cluster galaxies establishing a direct link between environment and early phases of assembly of ETGs.