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

Mechanisms regulating the escape of Lyα photons and ionizing radiation remain poorly understood. To study these processes we analyze VLA 21cm observations of one Green Pea (GP), J160810+352809 (hereafter J1608), and HST COS spectra of 17 GP galaxies at z < 0.2. All are highly ionized: J1608 has the highest [O III] λ5007/[O II] λ3727 for star-forming galaxies in SDSS, and the 17 GPs have [O III]/[O II] ≥ 6.6. We set an upper limit on J1608's HI mass of log M HI /M = 8.4, near or below average compared to similar mass dwarf galaxies. In the COS sample, eight GPs show Lyα absorption components, six of which also have Lyα emission. The HI column densities derived from Lyα absorption are high, log N HI /cm −2 = 19−21, well above the LyC optically thick limit. Using low-ionization absorption lines, we measure covering fractions (f cov ) of 0.1−1, and find that f cov strongly anti-correlates with Lyα escape fraction. Low covering fractions may facilitate Lyα and LyC escape through dense neutral regions. GPs with f cov ∼ 1 all have low neutral gas velocities, while GPs with lower f cov = 0.2 − 0.6 have a larger range of velocities. Conventional mechanical feedback may help establish low f cov in some cases, whereas other processes may be important for GPs with low velocities. Finally, we compare f cov with proposed indicators of LyC escape. Ionizing photon escape likely depends on a combination of neutral gas geometry and kinematics, complicating the use of emission-line diagnostics for identifying LyC emitters.

By clearing neutral gas away from a young starburst, superwinds may regulate the escape of Lyman continuum (LyC) photons from star-forming galaxies. However, models predict that superwinds may not launch in the most extreme, compact starbursts. We explore the role of outflows in generating low optical depths in the Green Peas (GPs), the only known star-forming population with several confirmed and candidate LyC-leaking galaxies. With Hubble Space Telescope UV spectra of 25 low-redshift GPs, including new observations of 13 of the most highly ionized GPs, we compare the kinematics of UV absorption lines with indirect H i optical depth diagnostics: Lyα escape fraction, Lyα peak separation, or low-ionization absorption line equivalent width. The data suggest that high ionization kinematics tracing superwind activity may correlate with low optical depth in some objects. However, the most extreme GPs, including many of the best candidate LyC emitters with weak low-ionization absorption and strong, narrow Lyα profiles, show the lowest velocities. These results are consistent with models for suppressed superwinds, which suggests that outflows may not be the only cause of LyC escape from galaxies.

The published article contained minor errors in Table 1 and Section 3.2. As a result of a coding error, redshifts were not correctly treated in calculating optical extinction and rest-frame optical equivalent widths. Since the sample galaxies are generally at z<0. Figures 1 and 2 have changed slightly, but the changes are too subtle to detect by eye. The changes do not affect any of the conclusions in the published article.

How galaxies reionized the universe remains an open question, but we can gain insights from the low-redshift Green Pea galaxies, one of the only known populations of Lyman continuum (LyC) emitters. Using VLA H i 21 cm observations and HST UV spectra of Green Peas, we investigate how neutral gas content and geometry influence LyC and Lyα escape. Our results suggest that LyC Emitters may have high ratios of star formation rate to H i mass. Low gas covering fractions are common among the population, but not all sightlines are optically thin. Based on the observed relationship between high ionization parameters, low metallicities, and narrow Lyα profiles, we propose that weak stellar feedback at low metallicities results in a gas geometry of dense clumps within a low-density medium, which facilitates Lyα and LyC escape. We address the implications of these results for identifying LyC emitters at high redshift with JWST and ALMA.