Diverse properties of Ly-alpha emission in low-redshift compact
  star-forming galaxies with extremely high [OIII]/[OII] ratios
  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=[OIII]5007/[OII]3727~22-39, aiming
to study the properties of Ly-alpha emission in such conditions. We find a
diversity in Ly-alpha properties. In five galaxies Ly-alpha emission line is
strong, with equivalent width (EW) in the range 45-190A. In the remaining
galaxies, weak Ly-alpha emission with EW(Ly-alpha)~2-7A is superposed on a
broad Ly-alpha absorption line, indicating a high neutral hydrogen column
density N(HI)~(1-3)x10^21 cm^-2. We examine the relation between the Ly-alpha
escape fraction fesc(Ly-alpha) 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-alpha) can be an indirect measure of
escaping LyC radiation. The usefulness of O32, of the Ly-alpha equivalent width
EW(Ly-alpha) and of the Ly-alpha peak separation Vsep as indirect indicators of
Ly-alpha leakage is also discussed. It is shown that there is no correlation
between O32 and fesc(Ly-alpha). We find an increase of fesc(Ly-alpha) with
increasing EW(Ly-alpha) for EW(Ly-alpha)<100A, but for higher EW(Ly-alpha)>150A
the fesc(Ly-alpha) is nearly constant attaining the value of ~0.25. We find an
anticorrelation between fesc(Ly-alpha) 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-alpha and ionizing radiation leakage.
