JWST spectroscopy of z ∼5-8 UV-selected galaxies: new constraints on the evolution of the Ly αescape fraction in the reionization era

Abstract

We describe JWST /NIRSpec prism measurements of Ly αemission in z ≳ 5 galaxies. We identify Ly αdetections in 10 out of 69 galaxies with robust rest-optical emission-line redshift measurements at 5 ≤z < 7 in the Cosmic Evolution Early Release Science (CEERS) and DDT-2750 observations of the Extended Groth Strip field. Galaxies at z ≃ 6 with faint continuum ( F 150 W = 27-29 mag) are found with extremely large rest-frame Ly αequi v alent widths (EWs; ranging up to 286 Å). Likely Ly αdetections are also seen in two new z > 7 galaxies ( z = 7.49 and 7.17) from the second epoch of CEERS observations, both showing large Ly αEWs that likely indicate significant transmission through the intergalactic medium (IGM). We measure high Ly αescape fractions in the 12 Ly αemitters in our sample (median 0.28), two of which show f Ly αesc near unity ( > 0.80). We find that 50 + 11 -11 per cent of z ≃ 6 galaxies with [O III ] + H βEW > 1000 Åhave f Ly αesc > 0.2, consistent with the fractions found in lower redshift samples with matched [O III ] + H βEWs. While uncertainties are still significant, we find that only 10 + 9 -5 per cent of z > 7 galaxies with similarly strong rest optical emission lines show such large f Ly αesc , as may be expected if IGM attenuation of Ly αincreases towards higher redshifts. We identify photometric galaxy o v erdensities near the z ≳ 7 Ly αemitters, potentially providing the ionizing flux necessary to create large ionized sightlines that facilitate Ly αtransmission. Finally, we investigate the absence of Ly αemission in a comparable (and spectroscopically confirmed) galaxy o v erdensity at z = 7.88 in the Abell 2744 field, discussing new prism spectra of the field obtained with the UNCOVER program.