Kinematics and Optical Depth in the Green Peas: Suppressed Superwinds in Candidate LyC Emitters

TL;DR

This study investigates how outflows and superwinds influence the escape of ionizing radiation in Green Pea galaxies, revealing that suppressed superwinds in the most extreme cases may still permit LyC photon escape, challenging existing models.

Contribution

It provides observational evidence that suppressed superwinds can facilitate LyC escape in Green Pea galaxies, contrasting with prior models linking strong outflows to photon leakage.

Findings

High ionization kinematics may correlate with low optical depth.

Extreme Green Peas show low velocities despite being LyC candidates.

Suppressed superwinds may enable LyC escape, challenging previous assumptions.

Abstract

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 HI optical depth diagnostics: Ly-alpha escape fraction, Ly-alpha 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 Ly-alpha 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.