Evolution in the orbital structure of quiescent galaxies from MAGPI, LEGA-C and SAMI surveys: direct evidence for merger-driven growth over the last 7 Gy

TL;DR

This study provides direct evidence that quiescent galaxies have undergone merger-driven growth over the last 7 billion years, as indicated by changes in their stellar kinematic properties across cosmic time.

Contribution

It presents the first analysis of spatially integrated higher-order stellar kinematics over cosmic time, revealing evolution consistent with merger-driven growth in quiescent galaxies.

Findings

Increase in the $h_4$ parameter over time indicating more radial anisotropy.

Decrease in the rotation-to-dispersion ratio over 7 Gyr.

Massive galaxies continue to grow and become more dispersion-supported after quenching.

Abstract

We present the first study of spatially integrated higher-order stellar kinematics over cosmic time. We use deep rest-frame optical spectroscopy of quiescent galaxies at redshifts z=0.05, 0.3 and 0.8 from the SAMI, MAGPI and LEGA-C surveys to measure the excess kurtosis $h_4$ of the stellar velocity distribution, the latter parametrised as a Gauss-Hermite series. Conservatively using a redshift-independent cut in stellar mass ($M_\star = 10^{11}\,{\rm M}_\odot$), and matching the stellar-mass distributions of our samples, we find 7 $\sigma$ evidence of $h_4$ increasing with cosmic time, from a median value of 0.019$\pm$0.002 at z=0.8 to 0.059$\pm$0.004 at z=0.06. Alternatively, we use a physically motivated sample selection, based on the mass distribution of the progenitors of local quiescent galaxies as inferred from numerical simulations; in this case, we find 10 $\sigma$ evidence. This evolution suggests that, over the last 7 Gyr, there has been a gradual decrease in the rotation-to-dispersion ratio and an increase in the radial anisotropy of the stellar velocity distribution, qualitatively consistent with accretion of gas-poor satellites. These findings demonstrate that massive galaxies continue to accrete mass and increase their dispersion support after becoming quiescent.