Ly$\alpha$ Emission Line Profiles of Extreme [OIII] Emitting Galaxies at $z\gtrsim2$: Implications for Ly$\alpha$ Visibility in the Reionization Era

Abstract

JWST observations have recently begun delivering the first samples of Ly$\alpha$ velocity profile measurements at $z>6$, opening a new window on the reionization process. Interpretation of $z\gtrsim6$ line profiles is currently stunted by limitations in our knowledge of the intrinsic Ly$\alpha$ profile (before encountering the IGM) of the galaxies that are common at $z\gtrsim6$. To overcome this shortcoming, we have obtained resolved ($R\sim3900$) Ly$\alpha$ spectroscopy of $42$ galaxies at $z=2.1-3.4$ with similar properties as are seen at $z>6$. We quantify a variety of Ly$\alpha$ profile statistics as a function of [OIII]+H$\beta$ EW. Our spectra reveal a new population of $z\simeq 2-3$ galaxies with large [OIII]+H$\beta$ EWs ($>1200\ \r{A}$) and a large fraction of Ly$\alpha$ flux emerging near the systemic redshift (peak velocity $\simeq0$ km s$^{-1}$). These spectra indicate that low density neutral hydrogen channels are able to form in a subset of low mass galaxies ($\lesssim1\times10^8\ M_{\odot}$) that experience a burst of star formation (sSFR $>100$ Gyr$^{-1}$). Other extreme [OIII] emitters show weaker Ly$\alpha$ that is shifted to higher velocities ($\simeq240$ km s$^{-1}$) with little emission near line center. We investigate the impact the IGM is likely to have on these intrinsic line profiles in the reionization era, finding that the centrally peaked Ly$\alpha$ emitters should be strongly attenuated at $z\gtrsim5$. We show that these line profiles are particularly sensitive to the impact of resonant scattering from infalling IGM and can be strongly attenuated even when the IGM is highly ionized at $z\simeq 5$. We compare these expectations against a new database of $z\gtrsim6.5$ galaxies with robust velocity profiles measured with JWST/NIRSpec.