Properties of five z∼0.3-0.4 confirmed LyC leakers: VLT/XShooter observations

Using new VLT/XShooter spectral observations we analyse the physical properties of five z ∼ 0.3 - 0.4 confirmed LyC leakers. Strong resonant Mg II λλ2796,2803Å emission lines (I(λλ2796,2803)/I(Hβ) ≃ 10-38 per cent) and non-resonant Fe II* λλ2612,2626Å emission lines are observed in spectra of five and three galaxies, respectively. We find high electron densities N<SUB>e</SUB> ∼ 400 cm<SUP>−3</SUP>, significantly higher than in typical low-z, but comparable to those measured in z ∼ 2 − 3 star-forming galaxies. The galaxies have a mean value of log N/O = -1.16, close to the maximum values found for star-forming (SF) galaxies 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. (2016, 2018), including the ones considered in the present study, are characterised by high EW(Hβ) ∼200-400Å, high ionisation parameter (log(U) = -2.5 to -1.7), high average ionising photon production efficiency ξ = 10<SUP>25.54</SUP> Hz erg<SUP>−1</SUP> and hard ionising radiation. On the BPT diagram we find the same offset of our leakers from low-z main-sequence SFGs as that for local analogues of LBGs and extreme SF galaxies at z ∼ 2 - 3. We confirm the effectiveness of the He I emission lines diagnostics proposed by Izotov et al. (2017) 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) = 10<SUP>17</SUP> - 5× 10<SUP>17</SUP> cm<SUP>−2</SUP> that permit leakage of LyC radiation, likely due to their density-bounded H II regions.