Probing the Structure of Ionised ISM in Lyman-Continuum-Leaking Green Pea Galaxies with MUSE

Abstract: Lyman continuum (LyC) photons are known to be responsible for reionising the universe after the end of the Dark Ages, which marked a period called the Epoch of Reionisation (EoR). While these high-energy photons are thought to predominantly originate from young, hot, massive stars within the earliest galaxies, and contributions from high-energy sources like quasars and AGN, the origins of these photons are yet not well known and highly debated. Detecting LyC photons from the early galaxies near the EoR is not possible as they get completely absorbed by the intergalactic medium (IGM) on their way to us, which has prompted the development of various indirect diagnostics to study the amount of LyC photons contributed by such galaxies by studying their analogues at low redshifts. In this study, we probe the ionised interstellar medium (ISM) of seven Green Pea galaxies through spatially resolved[O III] λ5007/[O II] λ3727 (O32) and [O III] λ5007/Hα λ6562 (O3Hα) emission-line ratio maps, using data from the Multi Unit Spectroscopic Explorer (MUSE) onboard the Very large telescope (VLT). Out of the two ratios, the former has proven to be a successful diagnostic in predicting Lyman continuum emitters (LCEs). Along with the line ratio maps, the surface brightness profiles of the galaxies are also studied to examine the spatial distribution of the emission lines and the regions from which they originate. The resulting maps indicate whether the ISM of the galaxies is ionization-bounded or density-bounded. Our analysis reveals that a subset of the galaxies with ionization-bounded ISM exhibits pronounced ionisation channels in the outer regions. These channels are potential pathways through which Lyman continuum photons may escape. For density-bounded ISM, the ionised ISM extends well beyond the stellar regions into the halos of the galaxies, highlighting their potential contribution to the ionising photon budget during the EoR. The findings emphasise the importance of spatially resolved ISM studies in understanding the mechanisms facilitating the escape of LyC photons.