H I Gas Cycles and Lyman Continuum Optical Depth in Low-Redshift Starbursts
Neutral gas both fuels star formation and determines the propagation of ionizing photons. In this work, we reveal the interactions between HI, star formation, and radiative feedback in two samples of low-redshift starbursts. Using the ALFALFA-Halpha sample, we present the first comparison of starbursts and non-starbursts within a statistical, HI-selected sample. The moderate HI gas fractions of the starbursts relative to non-starbursts indicate efficient HI to H2 conversion and show that the HI supply is largely unaffected by ionizing radiation. Mergers may trigger the more massive starbursts, while the absence of obvious kinematical disturbances in dwarf starbursts may indicate periodic starburst activity, triggered by cycles of gas expulsion and re-accretion. While the ALFALFA-Halpha galaxies demonstrate that starbursts may maintain large \hi~reservoirs, the Green Peas (GPs) illustrate the effects of extreme radiative feedback on neutral gas. To investigate whether the enormous [O III]/[O II] ratios in the most extreme GPs indicate LyC escape, we use photoionization modeling to constrain their ionizing sources and optical depths. Radiation from Wolf-Rayet stars or unusually hot O stars reproduces the observed [O III]/[O II] ratios, but no clear signatures of these stars are present. The GP spectra do suggest the presence of shocks, however, and accounting for shock emission necessitates a low optical depth. We therefore suggest that the GPs may be a new class of low-redshift LyC Emitters (LCEs), and we evaluate this scenario using Hubble Space Telescope COS spectra of four GPs. With these spectra, we develop a simple physical picture of the neutral gas optical depth and geometry that explains the previously enigmatic link between Ly-alpha, Si II, and Si II* lines observed in high-redshift Ly-alpha Emitters. Two GPs are likely optically thin along the line of sight, and their strong, narrow Ly-alpha emission, weak C II absorption, and clear C II* emission may be characteristic of LCE spectra. We also note a striking similarity of the Ly-alpha emission in these GPs to Balmer line emission from stellar ejecta, which further supports a low column density scenario. From our analysis of the ALFALFA-Halpha and GP samples, we suggest that particular burst ages, low halo masses, and extreme, concentrated starbursts may facilitate LyC escape.