  The possibility that star-forming galaxies may leak ionizing photons is at
the heart of many present-day studies that investigate the reionization of the
Universe. We test this hypothesis on local blue compact dwarf galaxies of very
high excitation. We assembled a sample of such galaxies by examining the
spectra from Data Releases 7 and 10 of the Sloan Digital Sky Survey. We argue
that reliable conclusions cannot be based on strong lines alone, and adopt a
strategy that includes important weak lines such as [OI] and the
high-excitation HeII and [ArIV] lines. Our analysis is based on purely
observational diagrams and on a comparison of photoionization models with
well-chosen emission-line ratio diagrams. We show that spectral energy
distributions from current stellar population synthesis models cannot account
for all the observational constraints, which led us to mimick several scenarios
that could explain the data. These include the additional presence of hard
X-rays or of shocks. We find that only ionization-bounded models (or models
with an escape fraction of ionizing photons lower than 10%) are able to
simultaneously explain all the observational constraints.
