Lyman alpha and Lyman continuum emission of Mg ii-selected star-forming galaxies

Abstract

ABSTRACT 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 fesc(LyC) = 3.1–4.6 per cent is detected in four galaxies, whereas only 1σ upper limits of fesc(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 fesc(LyC) and Vsep found for previous low-redshift galaxy samples. Vsep 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 fesc(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 fesc(LyC).