The Evolution of Early-type Galaxies Selected by Their Spatial Clustering

TL;DR

This paper introduces a clustering-based method to trace the evolution of early-type galaxies over cosmic time, revealing that mergers are necessary to reconcile the changing number densities without significant star formation.

Contribution

The study develops a novel clustering-based approach to identify galaxy progenitors and descendants, providing new insights into galaxy evolution and merger history from z~1 to today.

Findings

ETGs at a given redshift evolve into brighter, more massive galaxies at lower redshifts.

Progenitors are about 5.5 times more numerous than their descendants, indicating mergers reduce their number density.

No significant star formation is needed in ETGs since z=1, suggesting dry mergers dominate.

Abstract

Aims: We present a new method that uses luminosity or stellar mass functions combined with clustering measurements to select samples of galaxies at different redshifts likely to follow a progenitor-to-descendant relationship. As the method uses clustering information, we refer to galaxy samples selected this way as clustering-selected samples. We apply this method to infer the number of mergers during the evolution of MUSYC early-type galaxies (ETGs) from z~1 to the present-day. Methods: The method consists in using clustering information to infer the typical dark-matter halo mass of the hosts of the selected progenitor galaxies. Using LambdaCDM predictions, it is then possible to follow these haloes to a later time where the sample of descendants will be that with the clustering of these descendant haloes. Results: This technique shows that ETGs at a given redshift evolve into brighter galaxies at lower redshifts (considering rest-frame, passively evolved optical luminosities). This indicates that the stellar mass of these galaxies increases with time and that, in principle, a stellar mass selection at different redshifts does not provide samples of galaxies in a progenitor-descendant relationship. Conclusions: The comparison between high redshift ETGs and their likely descendants at z=0 points to a higher number density for the progenitors by a factor 5.5+-4.0, implying the need for mergers to decrease their number density by today. Because the luminosity densities of progenitors and descendants are consistent, our results show no need for significant star-formation in ETGs since z=1, which indicates that the needed mergers are dry, i.e. gas free.