JWST spectroscopy of $z\sim 5-8$ UV-selected galaxies: New constraints on the evolution of the Ly$\alpha$ escape fraction in the reionization era

Abstract

We describe {\it JWST}/NIRSpec prism measurements of Ly$\alpha$ emission in $z\gtrsim 5$ galaxies. We identify Ly$\alpha$ detections in 10 out of 69 galaxies with robust rest-optical emission line redshift measurements at $5\le z<7$ in the CEERS and DDT-2750 observations of the EGS field. Galaxies at $z\simeq 6$ with faint continuum (F150W $=$ 27--29 mag) are found with extremely large rest-frame Ly$\alpha$ equivalent widths (ranging up to 286 A). 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$\alpha$ equivalent widths that likely indicate significant transmission through the IGM. We measure high Ly$\alpha$ escape fractions in the 12 Ly$\alpha$ emitters in our sample (median 0.28), two of which show $f_{\mathrm{esc}}^{\mathrm{Ly}\alpha }$ near unity ($>0.80$). We find that $50_{-11}^{+11}$% of $z\simeq 6$ galaxies with [OIII]+H$\beta$ EW $>$ 1000 A have $f_{\mathrm{esc}}^{\mathrm{Ly}\alpha }$ $>0.2$, consistent with the fractions found in lower-redshift samples with matched [OIII]+H$\beta$ EWs. While uncertainties are still significant, we find that only $10_{-5}^{+9}$% of $z>7$ galaxies with similarly strong rest optical emission lines show such large $f_{\mathrm{esc}}^{\mathrm{Ly}\alpha }$, as may be expected if IGM attenuation of Ly$\alpha$ increases towards higher redshifts. We identify photometric galaxy overdensities near the $z\gtrsim 7$ 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.

main text 20 pages, 13 figures; accepted for publication by MNRAS