CEERS: Diversity of Lyman-Alpha Emitters during the Epoch of Reionization

TL;DR

This study uses JWST/NIRSpec data to analyze the properties of Ly$ ext{α}$ emitters at redshifts around 7.5, revealing diverse ionization states and implications for the size of ionized bubbles during reionization.

Contribution

First detailed spectral analysis of high-redshift Ly$ ext{α}$ emitters with JWST, showing extended Ly$ ext{α}$ emission and diverse ionization scenarios during reionization.

Findings

Ly$ ext{α}$ emission is often extended beyond the NIRSpec slit.

Some galaxies can ionize their own large ionized bubbles.

Detection of Ly$ ext{α}$ indicates diverse reionization environments.

Abstract

We analyze rest-frame ultraviolet to optical spectra of three $z\simeq7.47$ - $7.75$ galaxies whose Ly$\alpha$-emission lines were previously detected with Keck/MOSFIRE observations, using the JWST/NIRSpec observations from the Cosmic Evolution Early Release Science (CEERS) survey. From NIRSpec data, we confirm the systemic redshifts of these Ly$\alpha$ emitters, and emission-line ratio diagnostics indicate these galaxies were highly ionized and metal poor. We investigate Ly$\alpha$ line properties, including the line flux, velocity offset, and spatial extension. For the one galaxy where we have both NIRSpec and MOSFIRE measurements, we find a significant offset in their flux measurements ($\sim5\times$ greater in MOSFIRE) and a marginal difference in the velocity shifts. The simplest interpretation is that the Ly$\alpha$ emission is extended and not entirely encompassed by the NIRSpec slit. The cross-dispersion profiles in NIRSpec reveal that Ly$\alpha$ in one galaxy is significantly more extended than the non-resonant emission lines. We also compute the expected sizes of ionized bubbles that can be generated by the Ly$\alpha$ sources, discussing viable scenarios for the creation of sizable ionized bubbles ($>$1 physical Mpc). The source with the highest-ionization condition is possibly capable of ionizing its own bubble, while the other two do not appear to be capable of ionizing such a large region, requiring additional sources of ionizing photons. Therefore, the fact that we detect Ly$\alpha$ from these galaxies suggests diverse scenarios on escape of Ly$\alpha$ during the epoch of reionization. High spectral resolution spectra with JWST/NIRSpec will be extremely useful for constraining the physics of patchy reionization.