The giant HII region NGC 588 as a benchmark for 2D photoionisation
  models
  We use optical integral field spectroscopy and 8 and 24 micron mid-IR
observations of the giant HII region NGC 588 in the disc of M33 as input and
constraints for two-dimensional tailor-made photoionisation models. Two
different geometrical approaches are followed for the modelling structure: i)
Each spatial element of the emitting gas is studied individually using models
which assume that the ionisation structure is complete in each element to look
for azimuthal variations across gas and dust. ii) A single model is considered,
and the two-dimensional structure of the gas and the dust are assumed to be due
to the projection of an emitting sphere onto the sky. The models in both
assumptions reproduce the radial profiles of Hbeta surface brightness, the
observed number of ionising photons, and the strong optical emission-line
relative intensities. The first approach produces a constant-density
matter-bounded thin shell of variable thickness and dust-to-gas ratio, while
the second gives place to a radiation-bounded thick shell sphere of decreasing
particle density. However, the radial profile of the 8/24 microns IR ratio,
depending on the gas and dust geometry, only fits well when the thick-shell
model is used. The resulting dust-to-gas mass ratio, which was obtained
empirically from the derived dust mass using data from Spitzer, also has a
better fit using the thick-shell solution. In both approaches, models support
the chemical homogeneity, and the ionisation-parameter radial decrease, These
results must be taken with care in view of the very low extinction values that
are derived from the IR, as compared to that derived from the Balmer decrement.
Besides, the IR can be possibly contaminated with the emission from a cloud of
diffuse gas and dust above the plane of the galaxy detected at 250 micron
Herschel image.
