James Webb Space Telescope Feed Post

IOP Science: Early Results from GLASS-JWST. XXIII. The Transmission of Lyα from UV-faint z ∼ 3–6 Galaxies

Paper abstract: Lya emission from galaxies can be used to trace neutral hydrogen in the epoch of reionization, however, there is a degeneracy between the attenuation of Lya in the intergalactic medium (IGM) and the line profile emitted by the galaxy. Large shifts of Lya redward of systemic due to scattering in the interstellar medium can boost Lya transmission in the IGM during reionization. The relationship between the Lya 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 Lya at z > 3. We present a sample of 12 UV-faint galaxies (-20 ? MUV ? -16) at 3 ? z ? 6, with Lya velocity offsets, ?vLya, measured from the Very Large Telescope/MUSE and JWST/NIRSpec from the GLASS-JWST Early Release Program. We find a median ?vLya of 205 km s-1 and standard deviation of 75 km s-1, compared to 320 and 170 km s-1, respectively, for MUV < -20 galaxies in the literature. Our new sample demonstrates the previously observed trend of decreasing Lya velocity offset with decreasing UV luminosity and optical line velocity dispersion, which extends to MUV ? -20, consistent with a picture where the Lya profile is shaped by gas close to the systemic redshift. Our results imply that during reionization Lya from UV-faint galaxies will be preferentially attenuated, but that detecting Lya with low ?vLya can be an indicator of large ionized bubbles.