Spectroscopy of galaxies in the mid-infrared

The mid-infrared spectral properties of galaxies are described. Most of the galaxies that are bright in the mid-infrared are dominated by emission from dust, although non-thermal emission may be important in the most luminous active nuclei. There are clear differences between the spectra of galaxies with powerful HII regions in their nuclei and those containing AGN. In particular, the family of unidentified emission bands between 3 and 13 μm dominates the spectra in the former objects, but are rarely evident in spectra of AGN. In many AGN, evidence of silicate dust is provided by the presence of the 9.7 μm silicate absorption band; the strong spectral structure in HII region galaxies renders measurement of the silicate band in those objects difficult. Overall, dust properties in galaxies and in the Milky Way are broadly similar, but there are significant differences in the mix of grain components between dust around AGN and other galaxy nuclei. Fine-structure ionic lines, particularly of [NeII], are frequently detected in HII region galaxy nuclei; they have not yet been detected in active nuclei. The relative strengths of the emission lines can yield useful constraints on the excitation of the ionized gas, and the effective temperature of the exciting stars