NGC 595 is a giant Hɪɪ region located in
the western part of the spiral galaxy M 33. It is
the second in importance in this galaxy, after NGC
604. At 0.84 Mpc, HST
is able to resolve its stellar content. Malumuth
et al. (1996) obtained HST UV, U, B and V images
of this region and derived an ionizing luminosity
of 5 × 1050
phots-1 and an average
reddening
EB-V
= 0.36±0.28 mag. The stars are mostly concentrated
in the central part of the region, where little
emission of gas is seen (the ionized gas lies more
in a shell around the stars, figure 1a). Wilson
& Scoville (1993) showed the molecular gas to
be situated in the south-east part of the region,
just outside of the bright knot of stars.
Viallefond et al. (1986) found a reddening
gradient in the north-east/south-west direction by
observing the Hi gas, which was confirmed by
Malumuth et al. (1996) with stellar
photometry.
We obtained ISO
images for NGC 595 in the 5.0 to 8.5
μm range. The emission
in this spectral range is dominated by the
so-called PAH bands.
Current interpretation of these has them
originating from stochastically heated molecules.
Two of these bands are located in the range
observed, at 6.2 μm and
7.7 μm. Stochastic
heating implies that the in-band flux is directly
proportional to the number of photons absorbed by
the molecules. For typical HII regions, Cohen et
al. (1989) found 0.58 for the
I6.2/I7.7
in-band ratio. However many processes, ionization,
dehydrogenation, can modify this ratio.
Furthermore, an underlying continuum is present
though its exact origin is unknown.
Clusters in Extragalactic HII Regions and their Modelling