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

We describe JWST/NIRSpec prism measurements of Ly alpha emission in z greater than or similar to 5 galaxies. We identify Ly alpha detections 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 similar or equal to 6 with faint continuum (F150W=27-29 mag) are found with extremely large rest-frame Ly alpha equivalent widths (EWs; ranging up to 286 & Aring;). Likely Ly alpha detections 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 alpha EWs that likely indicate significant transmission through the intergalactic medium (IGM). We measure high Ly alpha escape fractions in the 12 Ly alpha emitters in our sample (median 0.28), two of which show f(esc)(Ly alpha) near unity (>0.80). We find that 50-11+11 per cent of z similar or equal to 6 galaxies with [O III] + H beta EW>1000 & Aring; have f(esc)(Ly alpha) >0.2, consistent with the fractions found in lower redshift samples with matched [O III] + H beta EWs. While uncertainties are still significant, we find that only 10-5+9 per cent of z > 7 galaxies with similarly strong rest optical emission lines show such large f(esc)(Ly alpha), as may be expected if IGM attenuation of Ly alpha increases towards higher redshifts. We identify photometric galaxy overdensities near the z greater than or similar to 7 Ly alpha emitters, potentially providing the ionizing flux necessary to create large ionized sightlines that facilitate Ly alpha transmission. Finally, we investigate the absence of Ly alpha emission in a comparable (and spectroscopically confirmed) galaxy overdensity at z = 7.88 in the Abell 2744 field, discussing new prism spectra of the field obtained with the UNCOVER program.