On the hardening of the ionizing radiation in HII regions across
  galactic discs through softness parameters
  We carry out a study of the hardness of the radiation of ionizing clusters in
HII regions. Firstly, we explore the applicability of the softness parameter,
originally defined in the optical for pairs of consecutive ionization stages of
the same species. With the advent of the infrared space observatories, this
definition has been extended to the mid-infrared. We show that the softness
parameters, as determined both in the optical and the mid-infrared wavelengths,
are sensitive to the effective temperature using a sample of data in both
spectral regimes. This is confirmed by comparing the data with a grid of
photoionization models, although no complete agreement has been found even for
different stellar model atmospheres. Finally, we show that both softness
parameters are consistent in the search for radial variations of the hardness
of the ionizing radiation of HII regions in the discs of spiral galaxies. We
find a range of trends, from galaxies showing pronounced gradients to those
showing very flat ones. Although the detectability and slope of these gradients
can be altered by the size and luminosity of the studied HII regions, it looks
that their existence is related to the mass and type of the galaxies and,
hence, to the roperties of the entire disc.
