Evolution of blue E/S0 galaxies from z~1: merger remnants or disk rebuilding galaxies?

TL;DR

This study investigates the evolution of blue early-type galaxies from z~1.4 to z~0.2, revealing a mass-dependent transition in their nature, possibly indicating different formation or evolutionary pathways such as mergers or disk rebuilding.

Contribution

It identifies a turn-over mass in blue E/S0 galaxies and analyzes their evolution, providing insights into their origins and migration within the galaxy population.

Findings

Threshold mass for blue E/S0 abundance increases from z~0.3 to z~1.

High-mass blue E/S0s resemble merger remnants migrating to red sequence.

Lower-mass blue E/S0s are similar to late-type galaxies, possibly rebuilding disks.

Abstract

Studying outliers from the bimodal distribution of galaxies in the color-mass space, such as morphological early-type galaxies residing in the blue cloud, can help to better understand the physical mechanisms that lead galaxy migrations in this space. In this paper we study the evolution of the properties of 210 M*/Msol>10^10 blue E/S0s between z~1.4 and z~0.2 in the COSMOS field with confirmed spectroscopic redshifts from the zCOSMOS 10k release. We first observe that the threshold mass, defined at z=0 in previous studies as the mass below which the population of blue early-type galaxies starts to be abundant relative to passive E/S0s, evolves from log(M*/Msol)~10.1 at z~0.3 to log(M*/Msol)~10.9 at z~1. Second, there seems to be a turn-over mass in the nature of blue E/S0 galaxies. Above log(M*/Msol)~10.8 blue E/S0 resemble to merger remnants probably migrating to the red-sequence in a time-scale of ~3 Gyr. Below this mass, they seem to be closer to normal late-type galaxies as if they were the result of minor mergers which triggered the central star-formation and built a central bulge component or were (re)building a disk from the surrounding gas, suggesting that they are moving back or staying in the blue-cloud. This turn-over mass does not seem to evolve significantly from z~1 in contrast with the threshold mass and therefore does not seem to be linked with the relative abundance of blue E/S0s.