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 MgII
2796,2803 emission lines (I(2796,2803)/I(Hbeta)=10-38 per cent) and
non-resonant FeII* 2612,2626 emission lines are observed in spectra of five and
three galaxies, respectively. We find high electron densities Ne~400cm-3,
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 logN/O=-1.16,
close to the maximum values found for star-forming (SF) galaxies in the
metallicity range of 12+logO/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(Hbeta)~200-400A, high ionisation
parameter (log(U)=-2.5 to -1.7), high average ionising photon production
efficiency \xi= 10^{25.54} Hz erg-1 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 HeI 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(HI)=10^{17}-5x10^{17} cm-2 that permit leakage of LyC
radiation, likely due to their density-bounded HII regions.
