Lyman alpha and Lyman continuum emission of MgII-selected star-forming galaxies

TL;DR

This study investigates how MgII emission line ratios and Ly-alpha profiles relate to the escape of ionizing Lyman continuum radiation in low-mass star-forming galaxies at intermediate redshifts, confirming Vsep as a key indicator.

Contribution

It provides new observational evidence linking LyC escape fractions to MgII emission ratios and Ly-alpha peak separation, emphasizing Vsep as a reliable indirect indicator.

Findings

LyC escape fractions of 3.1-4.6% detected in four galaxies.

Strong narrow Ly-alpha emission with two peaks correlates with LyC leakage.

Vsep is confirmed as the best indirect indicator of LyC escape.

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=[OIII]5007/MgII 2796+2803 and Mg2=MgII 2796/MgII 2803 emission-line ratios. We aim to study the dependence of leaking Lyman continuum (LyC) emission on the characteristics of MgII emission together with the dependences 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 1sigma upper limits of fesc(LyC) in the remaining three galaxies were derived. A strong narrow Ly-alpha emission line with two peaks separated by Vsep~298-592 km/s was observed in four galaxies with detected LyC emission and very weak Ly-alpha emission is observed in galaxies with LyC non-detections. Our new data confirm the tight anti-correlation 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 Mg2>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).