The relation of PAH strength with cold dust in galaxies

Abstract

Mid-infrared spectra and submillimetre maps are investigated for five galaxies covering a range of star forming activity. We find a good spatial coincidence between the 850 μm continuum emission and the strength of the PAH 7.7 μm line. The PAH 7.7 μm peak to 850 μm flux ratio lies in the range around 2 with a moderate dispersion across the galaxies. Both PAH and cold dust emission correlate also with the emission from very small grains at 14.3 μm, but regions with starbursts show an excess in the very small grain emission. This suggests that the PAH carriers are preferentially related to the regions dominated by cold dust and molecular clouds, where they are excited mainly by the interstellar radiation field. The lack of increased PAH/submm ratio in starburst knots suggests that starbursts play a minor role for powering the PAH emission in galaxies.