The Size Evolution and the Size–Mass Relation of Ly α Emitters across 3 ≲ z < 7 as Observed by JWST

Abstract Understanding the morphological structures of Ly α emitters (LAEs) is crucial for unveiling their formation pathways and the physical origins of Ly α emission. However, the evolution of their sizes and structural scaling relations remains debated. In this study, we analyze a large sample of 876 spectroscopically confirmed LAEs at 3 ≲ z < 7, selected from the MUSE, VANDELS, and CANDELSz7 surveys in the GOODS-S, Ultra Deep Survey, and COSMOS fields. Utilizing James Webb Space Telescope NIRCam imaging data, we measure their rest-frame UV and optical V -band effective radii ( R e ) through two-dimensional Sérsic profile fitting. Our results show that these LAEs are generally compact, with a median R e,UV of 0.50 − 0.24 + 0.30 kpc and a median R e, V of 0.57 − 0.24 + 0.33 kpc. The size evolution follows R e,UV ∝ (1 + z ) −0.91±0.10 and R e, V ∝ (1 + z ) −0.93±0.18 , respectively. Their UV and optical sizes are statistically comparable, indicating negligible UV-to-optical color gradients. For the first time, we establish the rest-frame optical size–mass relation for LAEs at z > 3, finding slopes comparable to typical star-forming galaxies, but with slightly smaller sizes at a given stellar mass. These results provide important clues for understanding the structural evolution of LAEs in the early Universe.