Galaxies in the infrared
  The mid infrared spectra of the starbursts show the 9.7$\mu m$ silicate absorption feature and strong mid infrared emission bands centered at 6.2, 7.7, and 11.3 $\mu$m. Illustrative models of the {\it active} galaxies are presented: As the starbursts are most likely confined to the central region of the galaxy the radiative transfer in the nucleus is solved under the assumption of spherical symmetry. The distribution of stars and dust are adjusted until the complete infrared spectrum of the galaxies are modeled. The dust is described as a mixture of large grains, very small grains and PAH, which are undergoing temperature fluctuations. Although the galactic nuclei are deeply hidden in the dust its global structure can be estimated by the simple calculations presented. ISOPHOT samples of eleven {\it active} galaxies and seven {\it inactive} spirals are presented. The far infrared and submillimeter spectrum of the {\it active} galaxies can be described by a single modified black--body at a color temperature of $31.5 \pm 2.8$ K. This leads to a ratio of infrared luminosity to gas mass, $L_{\rm IR}/M_{\rm gas}$, of $\sim 90$ L$_\odot$/ M$_\odot$. In contrast, the spectral energy distributions of {\it inactive} spirals require, apart from warm dust of $31.8 \pm 2.8$ K, an additional very cold component of at most $12.9\pm 1.7$ K. This implies a $L_{\rm IR}/M_{\rm gas}$ ratio of $\sim 3$ L$_\odot$/ M$_\odot$ for the {\it inactive} spirals, a factor $\sim 30$ lower than for the {\it active} galaxies. The detection of such cold dust can be predicted by radiative transfer models. 
Ralf Siebenmorgen