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We present observations with the Cosmic Origins Spectrograph onboard theHubble Space Telescope of nine most metal-deficient compact star-forminggalaxies with oxygen abundances 12+log(O/H)=6.97-7.23, redshiftsz=0.02811-0.13320, and stellar masses M*<10^7Msun. We aim to study theproperties of Ly-alpha emission in these extremely metal-deficient objects. Wefind that all nine galaxies are Ly-alpha emitters (LAEs). We examine variousrelations between the Ly-alpha escape fraction fesc(Ly-alpha) and othercharacteristics - such as absolute UV magnitude, oxygen abundance, O32 ratio,stellar mass, Lyman-alpha luminosity and equivalent width EW(Ly-alpha),ionizing photon production efficiency and velocity separation Vsep between thetwo peaks of the Ly-alpha profile - of a large sample of LAEs, including ourlowest-metallicity galaxies and other objects from the literature. We find arelatively tight correlation between fesc(Ly-alpha) and two characteristics,EW(Ly-alpha) and Vsep, whereas no correlation is found between fesc(Ly-alpha)and the oxygen abundance. We also find a relatively tight relation between theLy-alpha and LyC escape fractions. We propose to use the latter relation toestimate indirectly the escaping ionizing radiation in LAEs, when directmeasurements of LyC emission are not possible. We show that the globalproperties of low-z LAEs are very similar to those of z>6 galaxies. They arethus ideal local proxies for studying physical processes during the epoch ofreionization of the Universe.

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