Connecting Environment, Star Formation History, and Morphology of Massive Quiescent Galaxies at $3<z<4$ with JWST

TL;DR

This study investigates the morphology, formation history, and environment of massive quiescent galaxies at redshifts 3 to 4 using JWST data, revealing diverse growth pathways and the influence of environment and mergers.

Contribution

It provides the first detailed morphological analysis of massive quiescent galaxies at z=3-4 with JWST, linking their sizes, structures, and environments to formation processes and simulation predictions.

Findings

Typical galaxy sizes of 0.6-0.8 kpc at z=3-4

Environmental density predicts bulge-to-total ratio

High B/T galaxies show merger-driven starbursts and elevated ex-situ fractions

Abstract

We present the morphological properties of 17 spectroscopically confirmed massive quiescent galaxies ($10.2 < \log(M_{\ast}/M_{\odot}) < 11.2$) at $3.0 < z < 4.3$, observed with JWST/NIRSpec and NIRCam. Using S\'ersic profile fits to F277W and F444W imaging, we derive the size-mass relation and find typical sizes of $\sim$0.6--0.8 kpc at $M_{\ast} = 5 \times 10^{10}~M_{\odot}$, consistent with $\sim$7$\times$ growth from $z \sim 4$ to the present, including $\sim$2$\times$ by $z \sim 2$. We find tentative evidence that formation history and morphology jointly influence galaxy sizes: late-forming bulge-dominated galaxies appear more compact by $\sim$0.2-0.3 dex relative to the expected relation, while late-forming disk-dominated galaxies are larger. Using a random forest regressor, we identify local environmental density, quantified by $\log(1+\delta^{\prime}_{3})$ from the three nearest neighbors, as the strongest predictor of bulge-to-total ratio ($B/T$), which spans 0.25-1. In the {\sc IllustrisTNG} simulation, the ex-situ stellar mass fraction ($f_{\ast,\mathrm{ex\text{-}situ}}$) -- a proxy for mergers -- is instead the dominant predictor of $B/T$. Galaxies with high $B/T$ in dense environments show bursty star formation and short quenching timescales ($\lesssim0.4$ Gyr), consistent with bulge growth through merger-driven starbursts; in simulations, such systems exhibit elevated ex-situ fractions ($\sim$20-30%). In contrast, some high-$B/T$ galaxies in intermediate-density environments have low ex-situ fractions, suggesting that additional processes -- such as violent disk instabilities -- also contribute. These results point to multiple bulge growth pathways at high redshift, unified by rapid gas accretion, central starbursts, and AGN feedback, as predicted by cosmological simulations.