The Low-redshift Lyman-continuum Survey: [S ii] Deficiency and the Leakage of Ionizing Radiation

The relationship between galaxy characteristics and the reionization of the universe remains elusive, mainly due to the observational difficulty in accessing the Lyman continuum (LyC) at these redshifts. It is thus important to identify low-redshift LyC-leaking galaxies that can be used as laboratories to investigate the physical processes that allow LyC photons to escape. The weakness of the [S ii ] nebular emission lines relative to typical star-forming galaxies has been proposed as a LyC predictor. In this paper, we show that the [S ii ] deficiency is an effective method to select LyC-leaking candidates using data from the Low-redshift LyC Survey, which has detected flux below the Lyman edge in 35 out of 66 star-forming galaxies with the Cosmic Origins Spectrograph on board the Hubble Space Telescope. We show that LyC leakers tend to be more [S ii ] deficient and that the fraction of their detections increases as [S ii ] deficiency becomes more prominent. Correlational studies suggest that [S ii ] deficiency complements other LyC diagnostics (such as strong Ly α emission and high [O iii ]/[O ii ]). Our results verify an additional technique by which reionization-era galaxies could be studied.