JWST PRIMER Shows Massive Post-Starburst Galaxies at z>1 Retain Subtle Merger Signatures Beneath Smooth Profiles

{"The Maltby et al. study measures structural parameters independently across rest-frame optical to near-infrared bands for PSBs drawn from the UDS field. Effective radii and Sersic indices show little wavelength dependence, indicating that age gradients and dust effects are minimal once star formation has ceased. At z>1, massive PSBs (M*>10^10 Msun) are smaller than coeval passive galaxies of equal mass, while lower-redshift, lower-mass PSBs resemble compact discs.","Disturbance metrics (residual flux fraction, asymmetry, residual asymmetry) place most PSBs statistically close to the passive population. Only the highest-mass subset at z>1 deviates, displaying excess residual asymmetry that prior HST studies lacked the depth or wavelength coverage to detect. This points to structural transformation largely complete by the PSB phase, yet with lingering merger imprints at early times.","The findings support a mass- and redshift-dependent quenching dichotomy: major mergers dominate at high z and high mass, while gentler processes suffice later. This refines earlier merger-rate estimates that treated all PSBs as uniformly relaxed and highlights the need for kinematic follow-up to distinguish faded major-merger remnants from minor-interaction products.","Next steps include ALMA or JWST/NIRSpec integral-field spectroscopy to measure dynamical disturbances below the imaging threshold and test whether the observed asymmetry correlates with molecular gas fractions or AGN activity."}