We have selected a sample of star-forming galaxies and giant HII
regions in which star formation processes are vigorous. This sample
has been well studied at all wavelengths allowing a good knowledge of
the star-formation rates hence the ionizing radiation field produced
by newly born massive stars. The metallicity of the interstellar gas is
also well known and ranges from 1/30Z(solar) to Z(solar).
In addition, the sample has been thoroughly studied with IUE.
UV-optical spectra have been analysed and provide the shape of the
extinction curve needed to fit the data assuming a burst of star
formation.  This revealed that the extinction curve is markedly
different from the Galactic one and closer to the LMC or SMC ones, ie
very small or no bump at 2175 A, and nearly linear UV rise.
The aim of our observations is to try and understand the reason of
this marked difference: is the high UV field destroying some of the
dust components? is it affecting the size distribution? Is the lower
metallicity affecting the dust content of these regions on the large
scale? These questions can be answered by mapping the PAH lines, as
their intensities are linearly linked to both their abundance and to
the intensity of the UV field.
Indeed, since the UV source distribution is well known, the spatial
distribution of PAH emission will tell us whether the dust is affected
more by destruction processes, or if the formation rates are lower due
to a lower metallicity.
Furthermore, these maps will allow a clear statement of which part of
the extinction curve is indeed associated with PAH molecules, as this
is still a matter of controversy.