Ly$\hat{I}\pm$ emission in galaxies at $z\simeq 5-6$: new insight from JWST into the statistical distributions of Ly$\hat{I}\pm$ properties at the end of reionization

Abstract

JWST has recently sparked a new era of Ly$\hat{I}\pm$ spectroscopy, delivering the first measurements of the Ly$\hat{I}\pm$ escape fraction and velocity profile in typical galaxies at $z\simeq 6-10$. These observations offer new prospects for insight into the earliest stages of reionization. But to realize this potential, we need robust models of Ly$\hat{I}\pm$ properties in galaxies at $z\simeq 5-6$ when the IGM is mostly ionized. Here we use new JWST observations from the JADES and FRESCO surveys combined with VLT/MUSE and Keck/DEIMOS data to characterize statistical distributions of Ly$\hat{I}\pm$ velocity offsets, escape fractions, and EWs in $z\simeq 5-6$ galaxies. We find that galaxies with large Ly$\hat{I}\pm$ escape fractions (> 0.2) are common at $z\simeq 5-6$, comprising 30 per cent of Lyman break selected samples. Comparing to literature studies, our census suggests that Ly$\hat{I}\pm$ becomes more prevalent in the galaxy population toward higher redshift from $z\sim 3$ to $z\sim 6$, although we find that this evolution slows considerably between $z\sim 5$ and $z\sim 6$, consistent with modest attenuation from residual HI in the mostly ionized IGM at $z\simeq 5-6$. We find significant evolution in Ly$\hat{I}\pm$ velocity profiles between $z\simeq 2-3$ and $z\simeq 5-6$, likely reflecting the influence of resonant scattering from residual intergalactic HI on the escape of Ly$\hat{I}\pm$ emission near line center. This effect will make it challenging to use Ly$\hat{I}\pm$ peak offsets as a probe of Lyman continuum leakage at $z\simeq 5-6$. We use our $z\simeq 5-6$ Ly$\hat{I}\pm$ distributions to make predictions for typical Ly$\hat{I}\pm$ properties at $z\gtrsim 8$ and discuss implications of a recently-discovered Ly$\hat{I}\pm$ emitter at $z\simeq 8.5$ with a small peak velocity offset (156 km s${}^{-1}$).

31 pages, 12 figures, accepted by MNRAS