  The existence of density-bounded HII regions in spiral galaxies is supported
by means of a spectral indicator based on the intensity of the [OI]6300A
forbidden line. A grid of photoionization models providing spectral information
of density-bounded nebulae is presented in order to test the validity of our
indicator. The indicator is applicable to typical observed HII regions with
solar or higher metallicities, and for the range of ages observed in normal HII
regions. When applying the diagnostic to a large sample of HII regions of
spiral galaxies taken from the literature, we find that a fraction of the
selected HII regions with emission detected in the [OI]6300A line turn out to
be consistent with the predictions of density-bounded photoionization models.
Preliminary estimates of the fraction of Lyman continuum photons escaping
ranges from 20% to 40%. The number of density-bounded HII regions could be even
larger after confirmation of those regions for which the [OI] line was not
detected, provided they are good density-bounded candidates. We stress the
importance of having combined information on both good measurements or reliable
upper limits for the intensity of this line, and good determinations of the
Halpha luminosity for complete samples of HII regions in galaxies in order to
make proper estimations as to whether UV photons escaping from HII regions are
the main source of ionization of the diffuse ionized medium in galaxies.
