Early Results from GLASS-JWST. XXIII. The Transmission of Ly from UV-faint z 3-6 Galaxies

Ly alpha emission from galaxies can be used to trace neutral hydrogen in the epoch of reionization, however, there is a degeneracy between the attenuation of Ly alpha in the intergalactic medium (IGM) and the line profile emitted by the galaxy. Large shifts of Ly alpha redward of systemic due to scattering in the interstellar medium can boost Ly alpha transmission in the IGM during reionization. The relationship between the Ly alpha velocity offset from systemic and other galaxy properties is not well established at high redshift or low luminosities, due to the difficulty of observing emission lines which trace the systemic redshift. Rest-frame optical spectroscopy with JWST/NIRSpec has opened a new window into understanding Ly alpha at z > 3. We present a sample of 12 UV-faint galaxies (-20 less than or similar to M-UV less than or similar to -16) at 3 less than or similar to z less than or similar to 6, with Ly alpha velocity offsets, Delta v(Ly alpha), measured from the Very Large Telescope/MUSE and JWST/NIRSpec from the GLASS-JWST Early Release Program. We find a median Delta v(Ly alpha) of 205 km s(-1) and standard deviation of 75 km s(-1), compared to 320 and 170 km s(-1), respectively, for M-UV < -20 galaxies in the literature. Our new sample demonstrates the previously observed trend of decreasing Ly alpha velocity offset with decreasing UV luminosity and optical line velocity dispersion, which extends to M-UV greater than or similar to -20, consistent with a picture where the Ly alpha profile is shaped by gas close to the systemic redshift. Our results imply that during reionization Ly alpha from UV-faint galaxies will be preferentially attenuated, but that detecting Ly alpha with low Delta v(Ly alpha) can be an indicator of large ionized bubbles.