J. Kim, A. Alavi, C. Snap-Kolas, B. Siana, J. Richard, H. I. Teplitz, J. Colbert, V. Mehta, A. Paulino-Afonso, E. Vanzella

Abstract
The mechanisms of Lyα photon escape are key to understanding galaxy evolution and cosmic reionization, yet remain poorly understood. We investigate the UV-continuum sizes of 23 Lyα emitters (LAEs) at Cosmic Noon (1.7 < z < 3.3), extending previous size analyses to include fainter galaxies (MUV ≃ −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 Lyα equivalent width increases with decreasing reff, 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 Lyα 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.

Keywords
Lyman-alpha galaxies / Reionization / Strong gravitational lensing / Emission line galaxies / Starburst galaxies / Galaxy structure

The Astrophysical Journal
Volume 996, Number 2
2025 January

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