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

We present the morphological properties of 17 spectroscopically confirmed massive quiescent galaxies ($10.2<\log (M_{\ast }\mathrm{/}{M}_{\odot })<11.2$) at $3.0<z<4.3$, observed with JWST/NIRSpec and NIRCam. Using Sérsic 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 $\log (1+{\delta }_3^{\mathrm{\prime }})$ from the three nearest neighbors, as the strongest predictor of bulge-to-total ratio ($B\mathrm{/}T$), which spans 0.25-1. In the {\sc IllustrisTNG} simulation, the ex-situ stellar mass fraction ($f_{\ast ,\mathrm{e}\mathrm{x}\text{-}\mathrm{s}\mathrm{i}\mathrm{t}\mathrm{u}}$) – a proxy for mergers – is instead the dominant predictor of $B\mathrm{/}T$. Galaxies with high $B\mathrm{/}T$ in dense environments show bursty star formation and short quenching timescales ($\lesssim 0.4$ 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\mathrm{/}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.