The coordinated key role of wet, mixed, and dry major mergers in the buildup of massive early-type galaxies at z<~1

TL;DR

This study uses a semi-analytical model to demonstrate that wet, mixed, and dry major mergers collectively explain the observed buildup of massive early-type galaxies since redshift 1, aligning hierarchical models with observational data.

Contribution

The paper introduces a semi-analytical model that accounts for observed major mergers, showing their coordinated role in galaxy evolution and reconciling hierarchical and downsizing observations.

Findings

Wet mergers dominate galaxy assembly since z~1

Dry and mixed mergers are also significant in the process

Major mergers drive the migration from blue to red galaxy populations

Abstract

Hierarchical models predict that massive early-type galaxies (mETGs) derive from the most massive and violent merging sequences occurred in the Universe. However, the role of wet, mixed, and dry major mergers in the assembly of mETGs is questioned by some recent observations. We have developed a semi-analytical model to test the feasibility of the major-merger origin hypothesis for mETGs, just accounting for the effects on galaxy evolution of the major mergers strictly reported by observations. The model proves that it is feasible to reproduce the observed number density evolution of mETGs since z~1, just accounting for the coordinated effects of wet/mixed/dry major mergers. It can also reconcile the different assembly redshifts derived by hierarchical models and by mass downsizing data for mETGs, just considering that a mETG observed at a certain redshift is not necessarily in place since then. The model predicts that wet major mergers have controlled the mETGs buildup since z~1, although dry and mixed mergers have also played an essential role in it. The bulk of this assembly took place at 0.7<z<1, being nearly frozen at z<~0.7 due to the negligible number of major mergers occurred per existing mETG since then. The model suggests that major mergers have been the main driver for the observational migration of mass from the massive end of the blue galaxy cloud to that of the red sequence in the last ~8 Gyr.