Galaxies in the infrared Ralf

The mid infrared spectra of the starbursts show the 9.7μm silicate absorption feature and strong mid infrared emission bands centered at 6.2, 7.7, and 11.3 μm. Illustrative models of the 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 active galaxies and seven inactive spirals are presented. The far infrared and submillimeter spectrum of the active galaxies can be described by a single modified black–body at a color temperature of 31.5±2.8K. This leads to a ratio of infrared luminosity to gas mass, LIR/Mgas, of ∼ 90 L⊙/M⊙. In contrast, the spectral energy distributions of inactive spirals require, apart from warm dust of 31.8 ± 2.8K, an additional very cold component of at most 12.9 ± 1.7K. This implies a LIR/Mgas ratio of ∼ 3 L⊙/M⊙ for the inactive spirals, a factor ∼ 30 lower than for the active galaxies. The detection of such cold dust can be predicted by radiative transfer models.