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We propose to infer ionising continuum leaking properties of galaxies bylooking at their Lyman-alpha line profiles. We carry out Lyman-alpha radiationtransfer calculations in two models of HII regions which are porous to ionisingcontinuum escape: 1) the so-called "density bounded" media, in which massivestars produce enough ionising photons to keep the surrounding interstellarmedium transparent to the ionising continuum, i.e almost totally ionised, and2) "riddled ionisation-bounded" media, surrounded by neutral interstellarmedium, but with holes, i.e. with a covering factor lower than unity. TheLyman-alpha spectra emergent from these configurations have distinctivefeatures: 1) a "classical" asymmetric redshifted profile in the first case, butwith a small shift of the maximum of the profile compare to the systemicredshift (Vpeak < 150 km/s); 2) a main peak at the systemic redshift in thesecond case (Vpeak = 0 km/s), with, as a consequence, a non-zero Lyman-alphaflux bluewards the systemic redshift. Assuming that in a galaxy leakingionising photons, the Lyman-alpha component emerging from the leaking starcluster(s) dominates the total Lyman-alpha spectrum, the Lyman-alpha shape maybe used as a pre-selection tool to detect Lyman continuum (LyC) leakinggalaxies, in objects with well determined systemic redshift, and high spectralresolution Lyman-alpha spectra (R >= 4000). The examination of a sample of 10local starbursts with high resolution HST-COS Lyman-alpha spectra and known inthe literature as LyC leakers or leaking candidates, corroborates ourpredictions. Observations of Lyman-alpha profiles at high resolution shouldshow definite signatures revealing the escape of Lyman continuum photons fromstar-forming galaxies.

Using Lyman-alpha to detect galaxies that leak Lyman continuum