  Using broadband photometry from the Hubble Space Telescope in combination
with Very Large Telescope narrowband Halpha observations of the nearby spiral
galaxy NGC 300, we explore a method for estimating the escape fractions of
hydrogen-ionizing photons from HII regions within this galaxy. Our goal in this
concept study is to evaluate the spectral types of the most massive stars using
the broadband data and estimating their ionizing photon output with the help of
stellar atmosphere models. A comparison with the Halpha flux that gives the
amount of ionized gas in the HII region provides a measure of the escape
fraction of ionizing photons from that region. We performed some tests with a
number of synthetic young clusters with varying parameters to assess the
reliability of the method. However, we found that the derived stellar spectral
types and consequently the expected ionizing photon luminosity of a region is
highly uncertain. The tests also show that on one hand we tended to
overestimate the integrated photon output of a region for young ages and low
numbers of stars, and on the other hand we mostly underestimated the combined
ionizing luminosity for a large stellar number and older cluster ages. We
conclude that the proposed method of using stellar broadband photometry to
infer the leakage of ionizing photons from HII regions is highly uncertain and
dominated by the errors of the resulting stellar spectral types. Therefore this
method is not suitable. Stellar spectra are needed to reliably determine the
stellar types and escape fractions. Studies to this end have been carried out
for the Magellanic Clouds.
