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

Abstract

Abstract Many of the early-type galaxies (ETGs) observed so far at z > 1 turned out to have smaller radii with respect to that of a typical present day ETG with comparable mass. This has generated the conviction that in the past ETGs 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 ETGs at 0.9 < zspec < 1.92. We find a majority (62 per cent) of normal ETGs, similar to typical local ones, co-existing with compact early types from ∼ two to ∼ six times smaller in spite of the same mass and redshift. The co-existence of normal and compact ETGs at 〈z〈 ≃ 1.5 suggests that their build-up taken place in the first 3–4 Gyr, followed distinct paths. Furthermore, 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.