Detection of high Lyman continuum leakage from four low-redshift compact star-forming galaxies

Following our first detection reported in Izotov et al., we present the detection of Lyman continuum (LyC) radiation of four other compact star-forming galaxies observed with the Cosmic Origins Spectrograph (COS) onboard the <it>Hubble Space Telescope</it>. These galaxies, at redshifts of <it>z</it> ∼ 0.3, are characterized by high emission-line flux ratios [O <scp>iii</scp>] λ5007/[O <scp>ii</scp>] λ3727 ≳ 5. The escape fractions of the LyC radiation <it>f</it><inf>esc</inf>(LyC) in these galaxies are in the range of ∼6–13 per cent, the highest values found so far in low-redshift star-forming galaxies. Narrow double-peaked Ly α emission lines are detected in the spectra of all four galaxies, compatible with predictions for LyC leakers. We find escape fractions of Ly α, <it>f</it><inf>esc</inf>(Ly α) ∼ 20–40 per cent, among the highest known for Ly α emitting galaxies. Surface brightness profiles produced from the COS acquisition images reveal bright star-forming regions in the centre and exponential discs in the outskirts with disc scalelengths α in the range ∼0.6–1.4 kpc. Our galaxies are characterized by low metallicity, ∼1/8–1/5 solar, low stellar mass ∼(0.2–4) × 109 M<inf>⊙</inf>, high star formation rates, SFR ∼ 14–36 M<inf>⊙</inf> yr−1, and high SFR densities, Σ ∼ 2–35 M<inf>⊙</inf> yr−1 kpc−2. These properties are comparable to those of high-redshift star-forming galaxies. Finally, our observations, combined with our first detection reported in Izotov et al., reveal that a selection for compact star-forming galaxies showing high [O <scp>iii</scp>] λ5007/[O <scp>ii</scp>] λ3727 ratios appears to pick up very efficiently sources with escaping LyC radiation: all five of our selected galaxies are LyC leakers.