2020MNRAS.497.4293G

Using new Very Large Telescope (VLT)/XShooter spectral observations we analyse the physical properties of five z ∼ 0.3-0.4 confirmed Lyman continuum (LyC) leakers. Strong resonant Mg II λλ2796, 2803 Å emission lines (I(λλ2796, 2803)/I(Hβ) ≃ 10-38 per cent) and non-resonant Fe II* λλ2612, 2626 Å emission lines are observed in spectra of five and three galaxies, respectively. We find high electron densities N_e ∼ 400 cm^–3, significantly higher than in typical low-z, but comparable to those measured in z ∼ 2-3 star-forming galaxies (SFGs). The galaxies have a mean value of log N/O = -1.16, close to the maximum values found for SFGs in the metallicity range of 12 + log O/H ≃ 7.7-8.1. All 11 low-z LyC emitting galaxies found by Izotov et al., including the ones considered in this study, are characterized by high equivalent width (EW) (Hβ) ∼ 200-400 Å, high ionization parameter (log(U) = -2.5 to -1.7), high average ionizing photon production efficiency ξ = 10^25.54 Hz erg^–1, and hard ionizing radiation. On the Baldwin-Phillips-Terlevich (BPT) diagram we find the same offset of our leakers from low-z main-sequence SFGs as that for local analogues of Lyman-break galaxies (LBGs) and extreme SFGs at z ∼ 2-3. We confirm the effectiveness of the He I emission lines diagnostics proposed by Izotov et al. in searching for LyC leaker candidates and find that their intensity ratios correspond to those in a median with low neutral hydrogen column density N(H I) = 10¹⁷-5 x 10¹⁷ cm^–2 that permit leakage of LyC radiation, likely due to their density-bounded H II regions.