X-shooter Spectroscopy and HST Imaging of 15 Ultra Massive Quiescent Galaxies at $z\gtrsim2$

TL;DR

This study analyzes 15 ultra-massive quiescent galaxies at redshift around 2 using spectroscopy and imaging, revealing their compactness, merger activity, and evolutionary processes dominated by dry minor mergers over the past 10 billion years.

Contribution

First detailed spectroscopic and imaging analysis of a large sample of ultra-massive quiescent galaxies at z>2, confirming their properties and evolutionary mechanisms.

Findings

Galaxies are compact and follow the high-mass end of the mass-size relation at z=2.

Stellar velocity dispersions are consistent with local massive early-type galaxies.

Approximately 40% show signs of mergers or close companions, indicating merger activity.

Abstract

We present a detailed analysis of a large sample of spectroscopically confirmed ultra-massive quiescent galaxies (${\rm{log}}(M_{\ast}/M_{\odot})\sim11.5$) at $z\gtrsim2$. This sample comprises 15 galaxies selected in the COSMOS and UDS fields by their bright K-band magnitudes and followed up with VLT/X-shooter spectroscopy and HST/WFC3 $H_{F160W}$ imaging. These observations allow us to unambiguously confirm their redshifts ascertain their quiescent nature and stellar ages, and to reliably assess their internal kinematics and effective radii. We find that these galaxies are compact, consistent with the high mass end of the mass-size relation for quiescent galaxies at $z=2$. Moreover, the distribution of the measured stellar velocity dispersions of the sample is consistent with the most massive local early-type galaxies from the MASSIVE Survey showing that evolution in these galaxies, is dominated by changes in size. The HST images reveal, as surprisingly high, that $40\ \%$ of the sample have tidal features suggestive of mergers and companions in close proximity, including three galaxies experiencing ongoing major mergers. The absence of velocity dispersion evolution from $z=2$ to $0$, coupled with a doubling of the stellar mass, with a factor of four size increase and the observed disturbed stellar morphologies support dry minor mergers as the primary drivers of the evolution of the massive quiescent galaxies over the last 10 billion years.