An Hα Impression of Lyα Galaxies at $z\simeq 6$ with Deep JWST/NIRCam Imaging

Abstract

We present a study of seven spectroscopically confirmed (Ly$\alpha$ emitting) galaxies at redshift $z\simeq 6$ using the $JWST$/NIRCam imaging data. These galaxies, with a wide range of Ly$\alpha$ luminosities, were recently observed in a series of NIRCam broad- and medium-bands. We constrain the rest-frame UV/optical continua and measure the H$\alpha$ line emission of the galaxies using the combination of the $JWST$/NIRCam and archival $HST$/WFC3 infrared photometry. We further estimate their escape fractions of Ly$\alpha$ photons ($f_{\mathrm{esc}}^{\mathrm{Ly}\alpha }$) and the production efficiency of ionizing photons (${\xi }_{\mathrm{ion}}$). Among the sample, 6/7 galaxies have Ly$\alpha$ escape fractions of $\lesssim 10\%$, which might be the status for most of star-forming galaxies at $z\simeq 6$. One UV-faint Ly$\alpha$ galaxy with an extremely blue UV slope owns a large value of $f_{\mathrm{esc}}^{\mathrm{Ly}\alpha }$ reaching $\simeq 50\%$. These galaxies spread a broad range of ${\xi }_{\mathrm{ion}}$ over log${}_{10}$ ${\xi }_{\mathrm{ion},0}$ (Hz erg${}^{-1}$) $\sim 25.0-26.5$. We find that UV-fainter galaxies with bluer UV continuum slopes likely have higher escape fractions of Ly$\alpha$ photons. We also find that galaxies with higher Ly$\alpha$ line emission tend to produce ionizing photons more efficiently. The most Ly$\alpha$-luminous galaxy in the sample has a very high ${\xi }_{\mathrm{ion},0}$ of log${}_{10}$ ${\xi }_{\mathrm{ion},0}$ (Hz erg${}^{-1}$) $>26$. Our results support that Ly$\alpha$ galaxies may have served as an important contributor to the cosmic reionization. Blue and bright Ly$\alpha$ galaxies are excellent targets for $JWST$ follow-up spectroscopic observations.

Accepted by ApJL; an improved version

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