  Mid infrared spectra provide a powerful probe of the conditions in dusty
galactic nuclei. They variously contain emission features associated with star
forming regions and absorptions by circumnuclear silicate dust plus ices in
cold molecular cloud material. Here we report the detection of 6-8um water ice
absorption in 18 galaxies observed by ISO. While the mid-IR spectra of some of
these galaxies show a strong resemblance to the heavily absorbed spectrum of
NGC 4418, other galaxies in this sample also show weak to strong PAH emission.
The 18 ice galaxies are part of a sample of 103 galaxies with good S/N mid-IR
ISO spectra. Based on our sample we find that ice is present in most of the
ULIRGs, whereas it is weak or absent in the large majority of Seyferts and
starburst galaxies. This result is consistent with the presence of larger
quantities of molecular material in ULIRGs as opposed to other galaxy types.
Like NGC 4418, several of our ice galaxy spectra show a maximum near 8um that
is not or only partly due to PAH emission. While this affects only a small part
of the galaxy population studied by ISO, it stresses the need for high S/N data
and refined diagnostic methods, to properly discriminate spectra dominated by
PAH emission and spectra dominated by heavy obscuration. The spectral variation
from PAH emission to absorbed continuum emission near 8um shows strong
similarities with Galactic star forming clouds. This leads us to believe that
our classification of ice galaxy spectra might reflect an evolutionary sequence
from strongly obscured beginnings of star formation (and AGN activity) to a
less enshrouded stage of advanced star formation (and AGN activity), as the
PAHs get stronger and the broad 8um feature weakens.
