2026ApJ...997...29K

We present the morphological properties of 17 spectroscopically confirmed massive quiescent galaxies ($10.2\lt {\mathrm{log}}({M}_{* }/{M}_{\odot })\lt 11.2$) 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 ∼0.6-0.8 kpc at M_* = 5 × 10¹⁰M_⊙, consistent with ∼7× growth from z ∼ 4 to the present, including ∼2× by z ∼ 2. We find tentative evidence that formation history and morphology jointly influence galaxy sizes: late-forming bulge-dominated galaxies appear more compact by ∼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 ${\rm{log}}(1+{\delta }_{3}^{{\rm{{\prime} }}})$ from the three nearest neighbors, as the strongest predictor of bulge-to-total ratio (B/T), which spans 0.25-1. In the ILLUSTRISTNG simulation, the ex situ stellar mass fraction (f_*,ex 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 (≲0.4 Gyr), consistent with bulge growth through merger-driven starbursts; in simulations, such systems exhibit elevated ex situ fractions (∼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 active galactic nucleus feedback, as predicted by cosmological simulations.