2017MNRAS.471..548I

We present Large Binocular Telescope spectrophotometric observations of five low-redshift (z < 0.070) compact star-forming galaxies (CSFGs) with extremely high emission-line ratios O₃₂ = [O III] λ5007/[O II] λ3727, ranging from 23 to 43. Galaxies with such high O₃₂ are thought to be promising candidates for leaking large amounts of Lyman continuum (LyC) radiation and, at high redshifts, for contributing to the reionization of the Universe. The equivalent widths EW(H β) of the H β emission line in the studied galaxies are very high, ∼350-520 Å, indicating very young ages for the star formation bursts, <3 Myr. All galaxies are characterized by low oxygen abundances 12+logO/H = 7.46-7.79 and low masses M_* ∼ 10⁶-10⁷ M_☉, much lower than the M_* for known low-redshift LyC leaking galaxies, but probably more typical of the hypothetical population of low-luminosity dwarf LyC leakers at high redshifts. A broad H α emission line is detected in the spectra of all CSFGs, possibly related to expansion motions of supernova remnants. Such rapid ionized gas motions would facilitate the escape of the resonant Ly α emission from the galaxy. We show that a high O₃₂ may not be a sufficient condition for LyC leakage and propose new diagnostics based on the He I λ3889/λ6678 and λ7065/λ6678 emission-line flux ratios. Using these diagnostics, we find that three CSFGs in our sample are likely to have density-bounded H II regions and are thus leaking large amounts of LyC radiation. The amount of leaking LyC radiation is probably much lower in the other two CSFGs.