Lyα Emission Line Profiles of Extreme [O III]- emitting Galaxies at z ≳ 2: Implications for Lyα Visibility in the Reionization Era

JWST observations have recently begun delivering the first samples of Ly alpha velocity profile measurements at z > 6, opening a new window into the reionization process. Interpretation of z greater than or similar to 6 line profiles is currently stunted by limitations in our knowledge of the intrinsic Ly alpha profile (before encountering the intergalactic medium (IGM)) of the galaxies that are common at z greater than or similar to 6. To overcome this shortcoming, we have obtained resolved (R similar to 3900) 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 [O III]+H beta equivalent width (EW). Our spectra reveal a new population of z similar or equal to 2-3 galaxies with large [O III]+H beta EWs (>1200 & Aring;) and a large fraction of Ly alpha flux emerging near the systemic redshift (peak velocity similar or equal to 0 km s(-1)). These spectra indicate that low-density neutral hydrogen channels are able to form in a subset of low-mass galaxies (less than or similar to 1 x 10(8)M(circle dot)) that experience a burst of star formation (sSFR > 100 Gyr(-1)). Other extreme [O III] emitters show weaker Ly alpha that is shifted to higher velocities (similar or equal to 240 km s(-1)) with little emission near the 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 greater than or similar to 5. 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 similar or equal to 5. We compare these expectations against a new database of z greater than or similar to 6.5 galaxies with robust velocity profiles measured with JWST/NIRSpec.