Lyman Continuum Leakage from Massive Leaky Starbursts: A Different Class of Emitters?

Abstract

The origin of Lyman continuum (LyC) photons responsible for reionizing the Universe remains largely unknown, with the fraction of escaping LyC photons from galaxies at z  ∼ 6 to 12 still uncertain. Direct detection of LyC photons from this epoch is challenging due to intergalactic medium absorption, making lower-redshift analogs valuable for studying LyC leakage. In this study, we present Hubble Space Telescope Cosmic Origins Spectrograph observations of five low-redshift ( z ∼ 0.3) massive starburst galaxies, selected for high stellar mass and weak [S II ] nebular emission, an indirect tracer of LyC escape. LyC leakage is detected in three of the five galaxies, highlighting weak [S II ] as a reliable tracer—a finding supported by recent JWST discoveries of z > 5 galaxies with similarly weak [S II ] emission. The dust-corrected LyC escape fractions ( f esc,H I ), representing LyC photons that would escape without dust, range from 33% to 84%. However, the absolute escape fractions ( f esc,tot ), accounting for both neutral hydrogen absorption and dust attenuation, are substantially lower, between 1% and 3%. This indicates that, although these galaxies are nearly optically thin to H I , their significant dust content restricts LyC escape. These [S II ]-weak, massive leakers differ from typical low-redshift LyC emitters, exhibiting higher metallicity, lower ionization states, greater dust extinction, and higher star formation surface densities. We suggest that feedback-driven winds in these compact starbursts generate ionized channels, allowing LyC escape in line with a “picket-fence” model, indicating a distinct mechanism for LyC leakage.