MOSFIRE Spectroscopy of Quiescent Galaxies at 1.5 < z < 2.5. I - Evolution of Structural and Dynamical Properties

TL;DR

This study uses deep near-infrared spectroscopy to analyze the structural and dynamical evolution of quiescent galaxies from redshift 1 to 2.5, revealing size growth, increased velocity dispersions, and rotational contributions over cosmic time.

Contribution

It provides the largest homogeneously-selected sample of quiescent galaxies across 3 Gyr, analyzing their kinematic and structural evolution with new velocity dispersion and size growth measurements.

Findings

Galaxies show larger velocity dispersions and smaller sizes at higher redshifts.

Disk-like galaxies with flatter shapes exhibit higher dynamical-to-stellar mass ratios, indicating rotational motion.

Size growth rate is faster at earlier cosmic times, linked to decreasing systematic rotation.

Abstract

We present deep near-infrared spectra for a sample of 24 quiescent galaxies in the redshift range 1.5 < z < 2.5 obtained with the MOSFIRE spectrograph at the W. M. Keck Observatory. In conjunction with a similar dataset we obtained in the range 1 < z < 1.5 with the LRIS spectrograph, we analyze the kinematic and structural properties for 80 quiescent galaxies, the largest homogeneously-selected sample to date spanning 3 Gyr of early cosmic history. Analysis of our Keck spectra together with measurements derived from associated HST images reveals increasingly larger stellar velocity dispersions and smaller sizes to redshifts beyond z~2. By classifying our sample according to Sersic indices, we find that among disk-like systems the flatter ones show a higher dynamical to stellar mass ratio compared to their rounder counterparts which we interpret as evidence for a significant contribution of rotational motion. For this subset of disk-like systems, we estimate that V/sigma, the ratio of the circular velocity to the intrinsic velocity dispersion, is a factor of two larger than for present-day disky quiescent galaxies. We use the velocity dispersion measurements also to explore the redshift evolution of the dynamical to stellar mass ratio, and to measure for the first time the physical size growth rate of individual systems over two distinct redshift ranges, finding a faster evolution at earlier times. We discuss the physical origin of this time-dependent growth in size in the context of the associated reduction of the systematic rotation.