Lyα emission in low-redshift most metal-deficient compact star-forming galaxies

We present observations with the Cosmic Origins Spectrograph onboard the Hubble Space Telescope of nine most metal-deficient compact star-forming galaxies with oxygen abundances 12 + log(O/H) = 6.97 – 7.23, redshifts z = 0.02811 – 0.13320, and stellar masses M⋆ ≤ 107 M⊙. We aim to study the properties of Lyα emission in these extremely metal-deficient objects. We find that all nine galaxies are Lyα emitters (LAEs). We examine various relations between the Lyα escape fraction fesc(Lyα) and other characteristics – such as absolute UV magnitude, oxygen abundance, O32 ratio, stellar mass, Lyman-alpha luminosity and equivalent width EW(Lyα), ionizing photon production efficiency ξion and velocity separation Vsep between the two peaks of the Lyα profile – of a large sample of LAEs, including our lowest-metallicity galaxies and other objects from the literature. We find a relatively tight correlation between fesc(Lyα) and two characteristics, EW(Lyα) and Vsep, whereas no correlation is found between fesc(Lyα) and the oxygen abundance. We also find a relatively tight relation between the Lyα and LyC escape fractions. We propose to use the latter relation to estimate indirectly the escaping ionizing radiation in LAEs, when direct measurements of LyC emission are not possible. We show that the global properties of low-z LAEs are very similar to those of z > 6 galaxies. They are thus ideal local proxies for studying physical processes during the epoch of reionization of the Universe.