CLASSY VII Lyα Profiles: The Structure and Kinematics of Neutral Gas and Implications for LyC Escape in Reionization-era Analogs

Ly α line profiles are a powerful probe of interstellar medium (ISM) structure, outflow speed, and Lyman-continuum escape fraction. In this paper, we present the Ly α line profiles of the Cosmic Origins Spectrograph (COS) Legacy Archive Spectroscopic SurveY, a sample rich in spectroscopic analogs of reionization-era galaxies. A large fraction of the spectra show a complex profile, consisting of a double-peaked Ly α emission profile in the bottom of a damped, Ly α absorption trough. Such profiles reveal an inhomogeneous ISM. We successfully fit the damped Ly α absorption and the Ly α emission profiles separately, but with complementary covering factors, a surprising result because this approach requires no Ly α exchange between high- N _H _i and low- N _H _i paths. The combined distribution of column densities is qualitatively similar to the bimodal distributions observed in numerical simulations. We find an inverse relation between Ly α peak separation and the [O iii ]/[O ii ] flux ratio, confirming that the covering fraction of Lyman-continuum-thin sightlines increases as the Ly α peak separation decreases. We combine measurements of Ly α peak separation and Ly α red peak asymmetry in a diagnostic diagram, which identifies six Lyman-continuum leakers in the COS Legacy Archive Spectrocopy SurveY (CLASSY) sample. We find a strong correlation between the Ly α trough velocity and the outflow velocity measured from interstellar absorption lines. We argue that greater vignetting of the blueshifted Ly α peak, relative to the redshifted peak, is the source of the well-known discrepancy between shell-model parameters and directly measured outflow properties. The CLASSY sample illustrates how scattering of Ly α photons outside the spectroscopic aperture reshapes Ly α profiles because the distances to these compact starbursts span a large range.