Connecting Environment, Star Formation History, and Morphology of Massive Quiescent Galaxies at 3 &lt; <i>z</i> &lt; 4 with JWST

Abstract We present the morphological properties of 17 spectroscopically confirmed massive quiescent galaxies ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mn>10.2</mml:mn> <mml:mo>&lt;</mml:mo> <mml:mi>log</mml:mi> <mml:mo stretchy="false">(</mml:mo> <mml:msub> <mml:mrow> <mml:mi>M</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>*</mml:mo> </mml:mrow> </mml:msub> <mml:mo>/</mml:mo> <mml:msub> <mml:mrow> <mml:mi>M</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>⊙</mml:mo> </mml:mrow> </mml:msub> <mml:mo stretchy="false">)</mml:mo> <mml:mo>&lt;</mml:mo> <mml:mn>11.2</mml:mn> </mml:math> ) at 3.0 &lt; z &lt; 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 × 10 10 M ⊙ , 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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mi mathvariant="normal">log</mml:mi> </mml:mrow> <mml:mo stretchy="false">(</mml:mo> <mml:mn>1</mml:mn> <mml:mo>+</mml:mo> <mml:msubsup> <mml:mi>δ</mml:mi> <mml:mrow> <mml:mn>3</mml:mn> </mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">′</mml:mi> </mml:mrow> </mml:msubsup> <mml:mo stretchy="false">)</mml:mo> </mml:math> 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.