Compact Size, High ΣSFR: Defining Morphological Features of Ly α Emitters

Abstract

The mechanisms of Ly α photon escape are key to understanding galaxy evolution and cosmic reionization, yet remain poorly understood. We investigate the UV-continuum sizes of 23 Ly α emitters (LAEs) at Cosmic Noon (1.7 < z < 3.3), extending previous size analyses to include fainter galaxies ( M UV ≃ −14) using gravitational lensing. Our results show that these LAEs are unusually small for their luminosity, with a mean effective radius ( r eff ) of 170 ± 140 pc. They follow a distinct size–luminosity relation, with an intercept at M UV = −21 approximately three times smaller than typical star-forming galaxies (SFGs) at similar redshifts. This relation, however, is consistent with that of low-redshift Green Pea galaxies, suggesting that LAEs maintain compact sizes across redshifts. We also find that Ly α equivalent width increases with decreasing r eff , confirming previous findings. The small sizes of LAEs lead to high star formation surface densities (ΣSFR =1–600 M ⊙ yr −1 kpc −2 ), clearly separating them from typical SFGs in the ΣSFR versus r eff space. Given that high ΣSFR is linked to strong galactic outflows, our findings imply that compact morphology plays a key role in Ly α escape, likely facilitated by outflows that clear underdense channels in the intersetellar medium (ISM). Thus, these results demonstrate that compact size and high ΣSFR can help identify LAEs.