Lyman continuum leaker candidates among highly ionised, low-redshift dwarf galaxies selected from HeII

Abstract

Contemporary research suggests that the reionisation of the intergalactic medium (IGM) in the early Universe was predominantly realised by star-forming (proto-)galaxies (SFGs). Due to observational constraints, our knowledge on the origins of sufficient amounts of ionising Lyman continuum (LyC) photons and the mechanisms facilitating their transport into the IGM remains sparse. Recent efforts have thus focussed on the study of local analogues to these high-redshift objects. We used archival spectroscopic SDSS DR12 data to select a sample of low-z He II 4686 emitters and restricted it to a set of SFGs with an emission line diagnostic sensitive to the presence of an AGN, which serves as our only selection criterion. Our final sample consists of eighteen low-mass, low-metallicity dwarf galaxies which appear to be predominantly ionised by stellar sources. We find large O32 ratios and [S II] deficiencies, which provide strong indications for these galaxies to be LyC Emitters (LCEs). At least 40% of these objects are candidates for featuring cosmologically significant LyC escape fractions >10%. Their SFHs exhibit strong similarities and almost all galaxies appear to contain an old (>1 Gyr) stellar component, while also harbouring a young, two-stage (~10 Myr and <1 Myr) starburst, which we speculate might be related to LyC escape. The properties of the compact emission line galaxies presented here align well with those observed in many local LCEs. In fact, our sample may prove as an extension to the rather small catalogue of local LCEs, as the extreme interstellar medium (ISM) conditions we find are assumed to facilitate LyC leakage. Notably, all of our eighteen candidates are significantly closer (z<0.1) than most established LCEs. If the inferred LyC photon loss is genuine, this demonstrates that selecting SFGs from He II 4686 is a powerful selection criterion in the search for LCEs.