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The possibility that star-forming galaxies may leak ionizing photons is atthe heart of many present-day studies that investigate the reionization of theUniverse. We test this hypothesis on local blue compact dwarf galaxies of veryhigh excitation. We assembled a sample of such galaxies by examining thespectra from Data Releases 7 and 10 of the Sloan Digital Sky Survey. We arguethat reliable conclusions cannot be based on strong lines alone, and adopt astrategy that includes important weak lines such as [OI] and thehigh-excitation HeII and [ArIV] lines. Our analysis is based on purelyobservational diagrams and on a comparison of photoionization models withwell-chosen emission-line ratio diagrams. We show that spectral energydistributions from current stellar population synthesis models cannot accountfor all the observational constraints, which led us to mimick several scenariosthat could explain the data. These include the additional presence of hardX-rays or of shocks. We find that only ionization-bounded models (or modelswith an escape fraction of ionizing photons lower than 10%) are able tosimultaneously explain all the observational constraints.

Excitation properties of galaxies with the highest [OIII]/[OII] ratios: No evidence for massive escape of ionizing photons