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 Hubble Space Telescope. These galaxies, at redshifts of z ∼ 0.3, are characterized by high emission-line flux ratios [O III] λ5007/[O II] λ3727 ≳ 5. The escape fractions of the LyC radiation f_esc(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, f_esc(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_⊙, high star formation rates, SFR ∼ 14-36 M_⊙ yr^-1, and high SFR densities, Σ ∼ 2-35 M_⊙ 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 III] λ5007/[O II] λ3727 ratios appears to pick up very efficiently sources with escaping LyC radiation: all five of our selected galaxies are LyC leakers.