The Structure and Formation Histories of Low-mass Quiescent Galaxies in the A2744 Cluster Environment

Low-mass quiescent galaxies are thought to predominantly reside in overdense regions, as environmental effects are often invoked to explain their shutdown of star formation. These longer-timescale quenching mechanisms—such as interactions with hot gas in the intracluster medium and dynamical encounters with other cluster galaxies —leave imprints on galaxy morphologies, emphasizing the importance of quantifying the structures of low-mass quiescent galaxies in galaxy clusters at z < 0.5. Using spectrophotometric data from the UNCOVER and MegaScience programs, we present the first measurement of the quiescent size–mass relation between 7 < log(M /M ) < 10 using JWST imaging, based on a sample of 1531 galaxies in the z = 0.308 A2744 galaxy cluster. The resulting size–mass relation has a significantly higher scatter than similar-redshift field samples, despite comparable best-fit relations in both the dwarf and intermediate-mass regimes. Both “progenitor bias,” where larger, diskier low-mass galaxies enter the cluster at later epochs, and a general expansion of galaxy structure from dynamical interactions could be at play. This evolutionary framework is further supported by the tentative evidence that older low-mass quiescent galaxies in the cluster are more spheroidal. The star formation histories derived for our cluster sample imply their formation and quenching occurs relatively late, at z < 1.5. In this scenario, the progenitor population would have disky axis-ratio distributions at cosmic noon, in agreement with recent observations. While this leaves ample time for dynamical interactions to maintain quiescence and drive the observed subsequent morphological evolution postquenching, the data disfavor an onset of quenching due to the environment.