The multiwavelength structure of post-starburst galaxies at 0.5 < z < 3 with JWST PRIMER: compact morphologies and residual disturbances

We investigate the multiwavelength structure of recently-quenched post-starburst (PSB) galaxies at 0.5 < z < 3, using photometrically selected samples from the Ultra Deep Survey. Leveraging deep eight-band James Webb Space Telescope/NIRCam imaging from the PRIMER programme, we analyse ∼ 120 PSBs across the rest-frame optical–to–near-infrared, and compare with a reference sample of ∼ 3000 passive and star-forming galaxies. Structural parameters(effective radius R_e and Sérsic index n) are derived independently in each waveband, and reveal that PSBs exhibit minimal structural variation with wavelength, indicating negligible stellar population age gradients or internal dust obscuration. We confirm that PSBs follow the established redshift–mass trends: at z > 1, massive PSBs (M_∗ > 10¹⁰ M_☉) are compact spheroids resembling massive passive galaxies, albeit significantly more compact, whereas at 0.5 < z < 1, PSBs are typically low-mass (M_∗ < 10¹⁰ M_☉) compact, disc-dominated systems akin to low-mass passive discs. Furthermore, for the first time, we systematically quantify disturbance indicators (residual flux fraction RFF, asymmetry, residual asymmetry) across a large PSB sample. At all masses, PSBs exhibit low RFF and asymmetry values comparable to passive systems and consistent with smooth, largely undisturbed morphologies. However, at z > 1, massive PSBs (M_∗ > 10^10.25 M_☉) show enhanced residual asymmetry relative to the passive population, indicating a previously unrecognized level of structural disturbance masked beneath a smooth stellar distribution. These results suggest that, while structural transformation is largely complete by the PSB phase, residual disturbances persist at high redshift, supporting a scenario in which rapid quenching proceeds via two distinct pathways: highly disruptive events (e.g. major mergers) at high z and high mass, and comparatively gentle processes at later times.