New Insights on Lyα and Lyman Continuum Radiative Transfer in the Greenest Peas*

Abstract

As some of the only Lyman continuum (LyC) emitters at z  ∼ 0, Green Pea (GP) galaxies are possible analogs of the sources that reionized the universe. We present HST COS spectra of 13 of the most highly ionized GPs, with [O iii ]/[O ii ] = 6–35, and investigate correlations between Ly α , galaxy properties, and low-ionization UV lines. Galaxies with high [O iii ]/[O ii ] have higher H α equivalent widths (EWs), and high intrinsic Ly α  production may explain the prevalence of high Ly α  EWs among GPs. While the Ly α  escape fraction is closely linked to low gas covering fractions, implying a clumpy gas geometry, narrow Ly α  velocity peak separation ( ) correlates with the ionization state, suggesting a density-bounded geometry. We therefore suggest that  may trace the residual transparency of low-column-density pathways. Metallicity is associated with both [O iii ]/[O ii ] and . This trend may result from catastrophic cooling around low-metallicity star clusters, which generates a compact geometry of dense clouds within a low-density inter-clump medium. We find that the relative strength of low-ionization UV emission to absorption correlates with Ly α  emission strength and is related to Ly α  profile shape. However, as expected for optically thin objects, the GPs with the lowest  show both weak low-ionization emission and weak absorption. The strengths of the low-ionization absorption and emission lines in a stacked spectrum do not correspond to any individual spectrum. Galaxies with high [O iii ]/[O ii ] contain a high fraction of LyC emitter candidates, but [O iii ]/[O ii ] alone is an insufficient diagnostic of LyC escape.