Compact Size, High $Σ$SFR: Defining Morphological Features of Ly$α$-Emitters

TL;DR

Ly$\alpha$ emitters (LAEs) offer insight into early star formation and cosmic reionization, but Ly$\alpha$ escape depends on ISM geometry and dust. The authors extend UV-continuum size measurements to faint LAEs at $1.7<z<3.3$ by leveraging gravitational lensing to reach $M_{\rm UV}\simeq -14$, and compare their morphologies to low-redshift Green Pea galaxies. They report a mean effective radius $r_{\rm eff}=170\pm140$ pc and show that the $M_{\rm UV}\approx -21$ intercept of the size–luminosity relation is about three times smaller than typical star-forming galaxies, with the relation matching Green Pea galaxies, and that $EW(\mathrm{Ly}\alpha)$ increases as $r_{\rm eff}$ decreases. The compact sizes yield high $\Sigma$SFR, ($\Sigma$SFR) = 1–600 $M_{\odot}$ yr$^{-1}$ kpc$^{-2}$, separating LAEs from typical SFGs in $\Sigma$SFR$-$\,r_{\rm eff}$ space and implying outflows clear under-dense ISM channels to facilitate Ly$\alpha$ escape; these results suggest compact morphology and high $\Sigma$SFR are effective selectors for Ly$\alpha$ emitters and illuminate their escape physics.

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 ($M_{\rm UV} \simeq -14$) using gravitational lensing. Our results show that these LAEs are unusually small for their luminosity, with a mean effective radius ($r_{\rm eff}$) of $170 \pm 140$ pc. They follow a distinct size-luminosity relation, with an intercept at $M_{\rm UV} = -21$ 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 (EW(Ly$α$)) increases with decreasing $r_{\rm eff}$, confirming previous findings. The small sizes of LAEs lead to high star formation surface densities ($Σ$SFR $= 1-600 M_{\odot} \ \rm{yr}^{-1} \ \rm{kpc^{-2}}$), clearly separating them from typical SFGs in the $Σ$SFR vs. $r_{\rm eff}$ space. Given that high $Σ$SFR 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 under-dense channels in the ISM. Thus, these results demonstrate that compact size and high $Σ$SFR can help identify Ly$α$-emitters.