Normal galaxies
In integrated galaxy spectra, the transition from star dominated emission to interstellar dust re-radiation occurs in the mid-IR.
- ISO studies of mid-IR colours in Virgo and Coma galaxies (Boselli et al 1997, 1998) ? PAH bands are ubiquitous in the interstellar dust, but are destroyed by strong UV fields.
- Small grain emission related to UV radiation field
- Large grain emission proportional to the interstellar radiation field