2022MNRAS.515.2864I

We present observations with the Cosmic Origins Spectrograph onboard the Hubble Space Telescope of seven compact low-mass star-forming galaxies at redshifts, z, in the range 0.3161-0.4276, with various O₃Mg₂ = [O III] λ5007/Mg II λ2796+2803 and Mg₂ = Mg II λ2796/Mg II λ2803 emission-line ratios. We aim to study the dependence of leaking Lyman continuum (LyC) emission on the characteristics of Mg II emission together with the dependencies on other indirect indicators of escaping ionizing radiation. LyC emission with escape fractions f_esc(LyC) = 3.1-4.6 per cent is detected in four galaxies, whereas only 1σ upper limits of f_esc(LyC) in the remaining three galaxies were derived. A strong narrow Lyα emission line with two peaks separated by $V_{\rm sep}\, \sim$ 298-592 km s^–1 was observed in four galaxies with detected LyC emission and very weak Lyα emission is observed in galaxies with LyC non-detections. Our new data confirm the tight anticorrelation between f_esc(LyC) and V_sep found for previous low-redshift galaxy samples. V_sep remains the best indirect indicator of LyC leakage among all considered indicators. It is found that escaping LyC emission is detected predominantly in galaxies with Mg$_2\, \gtrsim$ 1.3. A tendency of an increase of f_esc(LyC) with increasing of both the O₃Mg₂ and Mg₂ is possibly present. However, there is substantial scatter in these relations not allowing their use for reliable prediction of f_esc(LyC).