Reconstructing the Assembly of Massive Galaxies. III: Quiescent Galaxies Loose Angular Momentum as They Evolve in a Mass-dependent Fashion

TL;DR

This study uses unresolved stellar kinematics from LEGA-C and Prospector-derived stellar ages to investigate how massive quiescent galaxies evolve dynamically after quenching at redshifts around 0.8. By analyzing the relationship between the integrated velocity dispersion σ'_{ m star} and morphological axis ratio q across mass and age bins, the authors find that the youngest quiescent galaxies retain significant rotation, while the most massive systems show a progressive loss of angular momentum over time, consistent with cumulative dry-merger effects. A simple toy model converts σ'_{ m star}–q trends into an inferred V/σ, revealing mass-dependent declines in rotational support and linking these changes to post-quenching evolution. A stochastic merger-based framework further explains the findings, predicting stronger angular-momentum loss for higher-mass galaxies and offering qualitative agreement with both local fast/slow rotator behavior and high-redshift resolved kinematics. The results imply that the fast/slow rotator dichotomy is established, at least in part, by z ~ 0.8, with angular-momentum transformation continuing within the quiescent population and driven largely by dry mergers; future spatially resolved kinematic studies will be essential to map V/σ and slow-rotator fractions at high redshift more precisely.

Abstract

We study the evolution of stellar kinematics of a sample of 952 massive quiescent galaxies with $M_*>10^{10.5}M_\odot$ at $0.6<z<1$. Utilizing spatially integrated spectroscopy from the LEGA-C survey, we focus on the relationship between the observed integrated stellar velocity dispersion ($σ^\prime_{star}$) and the morphological axial ratio ($q$), and its variation with the stellar age and mass of quiescent galaxies. For the youngest quiescent galaxies, regardless of stellar mass, $σ^\prime_{star}$ decreases with increasing $q$, a trend that is consistent with a system having significant rotation and hence suggests that massive galaxies still retain significant amount of angular momentum in the aftermath of quenching. As they continue to evolve, the variation of the $σ^\prime_{star}$-$q$ relationship depends on stellar mass. For quiescent galaxies with $M_*<10^{11.3}M_\odot$, $σ^\prime_{star}$ decreases with $q$ in all stellar-age bins, suggesting that the quiescent populations of this mass regime retain significant rotation even long time after quenching. In contrast, for more massive quiescent galaxies with $M_*>10^{11.3}M_\odot$, the relationship between $σ^\prime_{star}$ and $q$ becomes significantly flattened with increasing stellar age. This indicates that, as the very massive galaxy populations continue to evolve after quenching, angular momentum gradually reduces, which eventually transforms them into velocity-dispersion supported systems. We suggest that incoherent, continuous merging and accretion events onto the galaxies are the main drivers of the observed mass-dependent, posting-quenching dynamical evolution, because more massive galaxies are more likely to undergo such interactions. We are witnessing the early formation epoch of fast and slow rotators at $z \sim 0.8$, when the Universe was only half of its age nowadays.

Figures (8)

• Figure 1: Illustration of the main idea of this work (see Section \ref{['sec:intro']} for details). We investigate the dynamical transformation of massive quiescent galaxies at high redshifts with spatially integrated/unresolved stellar kinematics by studying the empirical relationship between $\sigma^\prime_{\rm{star}}$ ($y$-axis) and $q$ ($x$-axis).
• Figure 2: Left: Distribution of the final sample of 952 massive quiescent galaxies in the plane of SFR vs $M_*$. The black solid line shows the star-forming main sequence of Leja2022, and the black dashed lines mark the range of $\pm0.3$ dex. Right: Distribution of the final sample in the plane of $\rm{H\delta_A}$ vs. $\rm{D_{N}4000}$. The background grey contours show the distribution of all galaxies with $M_*>10^{10.5}M_\odot$ from the LEGA-C survey. Each one of the quiescent galaxies in our final sample is color coded according to its mass-weighted stellar age derived from SED fitting assuming non-parametric SFH. Our quiescent sample selected via UVJ technique also occupies the region of the parameter space of quiescent galaxies in these two planes.
• Figure 3: $\sigma^\prime_{\rm{star}}$ vs. $q$. The first and second rows show the results of the quiescent galaxies with $M_*<10^{11.3}M_\odot$ and $M_*>10^{11.3}M_\odot$, respectively. For each stellar-mass bin, galaxies are further divided into three age bins, each of which roughly contains 1/3 of the entire sample. In each panel the solid line and shaded region show the median trend and the corresponding 1-$\sigma$ uncertainty derived using the LOWESS algorithm. The dashed line shows the best-fit from our toy model (see Section \ref{['sec:model']}). In the fourth column, all LOWESS median trends are shown together.
• Figure 4: $V/\sigma$ as a function of stellar age. Blue ($M_*<10^{11.3}M_\odot$) and red ($M_*>10^{11.3}M_\odot$) squares show the $V/\sigma$, inferred from our toy model (Section \ref{['sec:model']}), of $z\sim0.8$ quiescent populations of different ages. The horizontal dashed line marks $V/\sigma=1$. In the left-most grey shaded region, we also plot three strongly lensed massive ($\gtrsim10^{11.3}M_\odot$) quiescent galaxies at $z\sim2$ with direct, spatially resolved measures of stellar kinematics from Newman2018b.
• Figure 5: Change of the Spearman's rank correlation coefficient as older quiescent galaxies are included to the correlation test (dashed lines). The $x$-axis shows the maximum mass-weighted age of the quiescent galaxies included in the test. The dark and light shaded regions mark the 0.5- and 1-$\sigma$ uncertainties.