New Insights on Ly-alpha and Lyman Continuum Radiative Transfer in the Greenest Peas

TL;DR

This study investigates the radiative transfer of Ly-alpha and Lyman continuum in Green Pea galaxies, revealing how their ionization, metallicity, and gas geometry influence Ly-alpha escape and potential LyC leakage, informing reionization models.

Contribution

It provides new insights into the connection between galaxy properties, gas geometry, and Ly-alpha/LyC escape in highly ionized Green Pea galaxies, using HST COS spectra.

Findings

High [O III]/[O II] correlates with high Ly-alpha EWs.

Ly-alpha escape is linked to low gas covering fractions and clumpy gas geometry.

Delta_v,LyA traces residual transparency and ionization state.

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-alpha, galaxy properties, and low-ionization UV lines. Galaxies with high [O III]/[O II] have higher H-alpha equivalent widths (EWs), and high intrinsic Ly-alpha production may explain the prevalence of high Ly-alpha EWs among GPs. While Ly-alpha escape fraction is closely linked to low gas covering fractions, implying a clumpy gas geometry, narrow Ly-alpha velocity peak separation (delta_v,LyA) correlates with the ionization state, suggesting a density-bounded geometry. We therefore suggest that delta_v,LyA may trace the residual transparency of low-column-density pathways. Metallicity is associated with both [O III]/[O II] and delta_v,LyA. 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-alpha emission strength and is related to Ly-alpha profile shape. However, as expected for optically thin objects, the GPs with the lowest delta_v,LyA 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.