Lyα Escape from Low-mass, Compact, High-redshift Galaxies

We investigate the effects of stellar populations and sizes on Lyα escape in 27 spectroscopically confirmed and 35 photometric Lyα emitters (LAEs) at z ≈ 2.65 in seven fields of the Boötes region of the NOAO Deep Wide-Field Survey. We use deep HST/WFC3 imaging to supplement ground-based observations and infer key galaxy properties. Compared to typical star-forming galaxies (SFGs) at similar redshifts, the LAEs are less massive (M _⋆ ≈ 10⁷-10⁹ M _⊙), younger (ages ≲1 Gyr), smaller (r _e < 1 kpc), and less dust-attenuated (E(B-V) ≤ 0.26 mag) but have comparable star formation rates (SFRs ≈ 1-100 M _⊙ yr^-1). Some of the LAEs in the sample may be very young galaxies having low nebular metallicities (Z _neb ≲ 0.2 Z _⊙) and/or high ionization parameters ( $\mathrm{log}({\rm{U}})\,\gtrsim \,-2.4$ ). Motivated by previous studies, we examine the effects of the concentration of star formation and gravitational potential on Lyα escape by computing SFR surface density, Σ_SFR, and specific SFR surface density, Σ_sSFR. For a given Σ_SFR, the Lyα escape fraction is higher for LAEs with lower stellar masses. The LAEs have a higher Σ_sSFR, on average, compared to SFGs. Our results suggest that compact star formation in a low gravitational potential yields conditions amenable to the escape of Lyα photons. These results have important implications for the physics of Lyα radiative transfer and for the type of galaxies that may contribute significantly to cosmic reionization.