HI Gas Cycles and Lyman Continuum Optical Depth in Low-Redshift Starbursts

Abstract

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-H-alpha sample, we present the first comparison of starbursts and non-starbursts within a statistically uniform, 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-H-alpha galaxies demonstrate that starbursts may maintain large HI reservoirs, the more powerful starbursts in the Green Pea (GP) galaxies illustrate the effects of extreme radiative feedback on HI. To investigate whether the enormous [OIII]/[OII] ratios in some GPs indicate LyC escape, we constrain their ionizing sources and optical depths using photoionization modeling. Radiation from Wolf-Rayet stars or unusually hot O stars reproduces the observed [OIII]/[OII] 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, SiII, and SiII* lines observed in high-redshift galaxies. Two GPs are likely optically thin, and their strong, narrow Ly-alpha emission, weak CII absorption, and clear CII* emission may be characteristic of LCE spectra. We also note a striking similarity of the Ly-alpha emission in these GPs to Balmer emission from stellar ejecta, which further supports a low column density scenario. From our analysis of the ALFALFA-H-alpha and GP samples, we suggest that particular burst ages, low halo masses, and extreme, concentrated starbursts may facilitate LyC escape.