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Issue A&A Volume 385, Number 3, April III 2002 Page(s) L23 - L26 Section Letters DOI 10.1051/0004-6361:20020222

A&A 385, L23-L26 (2002)
DOI: 10.1051/0004-6361:20020222

Letter

The relation of PAH strength with cold dust in galaxies

M. Haas¹, U. Klaas¹ and S. Bianchi<sup>1, 2, 3

1</sup>  Max-Planck-Institut für Astronomie (MPIA), Königstuhl 17, 69117 Heidelberg, Germany
²  Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Str. 1, Postfach 1317, 85741 Garching, Germany
³  European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching, Germany

(Received 17 January 2002 / Accepted 11 February 2002 )

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 m continuum emission and the strength of the PAH 7.7 m line. The PAH 7.7 m peak to 850 m 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 m, 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.

Key words: galaxies: NGC 891, NGC 1569, NGC 6946, NGC 7331, Arp 244 -- ISM -- infrared: galaxies

Offprint request: M. Haas, haas@mpia.de

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