Diverse properties of Ly α emission in low-redshift compact star-forming galaxies with extremely high [O iii]/[O ii] ratios

Abstract

ABSTRACT We present observations with the Cosmic Origins Spectrograph onboard the Hubble Space Telescope of eight compact star-forming galaxies at redshifts z = 0.02811–0.06540, with low oxygen abundances 12 + log(O/H) = 7.43–7.82 and extremely high emission-line flux ratios O32 = [O iii] λ5007/[O ii] λ3727 ∼ 22–39, aiming to study the properties of Ly α emission in such conditions. We find a diversity in Ly α properties. In five galaxies Ly α emission line is strong, with equivalent width (EW) in the range 45–190 Å. In the remaining galaxies, weak Ly α emission with EW(Ly α) ∼ 2–7 Å is superposed on a broad Ly α absorption line, indicating a high neutral hydrogen column density N(H i) ∼ (1 – 3)× 1021 cm−2. We examine the relation between the Ly α escape fraction fesc(Ly α) and the Lyman continuum escape fraction fesc(LyC), using direct measures of the latter in eleven low-redshift LyC leakers, to verify whether fesc(Ly α) can be an indirect measure of escaping LyC radiation. The usefulness of O32, of the Ly α equivalent width EW(Ly α), and of the Ly α peak separation Vsep as indirect indicators of Ly α leakage is also discussed. It is shown that there is no correlation between O32 and fesc(Ly α). We find an increase of fesc(Ly α) with increasing EW(Ly α) for EW(Ly α) <100 Å, but for higher EW(Ly$\alpha)\gtrsim$150 Å the fesc(Ly α) is nearly constant attaining the value of ∼0.25. We find an anticorrelation between fesc(Ly α) and Vsep, though not as tight as the one found earlier between fesc(LyC) and Vsep. This finding makes Vsep a promising indirect indicator of both the Ly α and ionizing radiation leakage.