Ultramassive dense early-type galaxies: velocity dispersions and number density evolution since z=1.6

TL;DR

This study traces the evolution of ultramassive dense early-type galaxies over the last 9 billion years, revealing a slight decrease in their number density and suggesting most formed before z=1.4, with minimal size evolution.

Contribution

It provides new measurements of velocity dispersions for high-redshift ultramassive dense ETGs and analyzes their structural evolution, offering insights into their assembly history.

Findings

Number density decreases by ~25% since z=1.6

Most ultramassive dense ETGs are already assembled by z=1.4

High-z ETGs have local counterparts, indicating early formation

Abstract

In this paper we investigate the mass assembly history of ultramassive (Mstar > 10^11Msun) dense (Sigma = Mstar/(2*pi*Re^2) > 2500 Msun/pc^2) early-type galaxies (ETGs) over the last 9 Gyr. We have traced the evolution of the number density rho of ultramassive dense ETGs and have compared their structural (effective radius Re and stellar mass Mstar) and dynamical (velocity dispersion sigma_e) parameters over the redshift range 0 < z < 1.6. We have derived the number density at 1.6 < z < 1 from the MUNICS and GOODS-South surveys, while we have used the COSMOS and SDSS spectroscopic surveys to probe the intermediate and local redshift range. For the comparison of the dynamical and structural parameters, we have collected the ultramassive dense ETGs at 1.2 < z < 1.6 for which velocity dispersion measurements are available (11 ETGs). For 4 of them we present unpublished estimates of sigma_e. We probe the intermediate redshift range, and the local universe using the samples of ETGs by Saglia et al. (2010), Zahid et al. (2015), and by Thomas et al. (2010). We find that the number density of ultramassive dense ETGs evolves as rho(z) = K*(1 + z)^(0.3\pm0.8) implying a decrease of ~ 25% of the population since z = 1.6. By comparing the values of Re, Mstar, and sigma_e of ultramassive dense ETGs over the range 0 < z < 1.6 we find that all the high-z ETGs have a counterpart in the local universe. This implies either that the majority (~70%) of ultramassive dense ETGs has already completed its assembly and its shaping at <z> = 1.4, or that, if a significant fraction of them evolves in size, new ultramassive dense ETGs must form at z < 1.5 to maintain their number density almost constant. The difficulty into identify good progenitors for these new dense ETGs at z < 1.5, and the stellar populations properties of local ultramassive dense ETGs point toward the first hypothesis.