For active galaxies, ISOPHOT photometry confirms the existence of a single dust component that accounts for the complete FIR and millimeter continuum emission. The mean color temperature is 32 K, the implied ratio is / . Because the dust masses in these active galaxies are derived from the 1.3 mm fluxes in the central area and the CO emission shows some, although weak extension beyond this beam, the total dust masses may be underestimated.
For our sample of inactive spirals, the extrapolation of the ISO data to 1.3 mm gives, for any reasonable dust absorptivity, a flux much smaller than actually observed. Consequently, the flux at 1.3 mm must be due to a component distinctly colder than the dust responsible for the emission below 150 µm. We find with conservatively low estimates for the total 1.3 mm dust emission that this component is at about 13 K, but it may be even colder. On the basis of a larger sample and improved reduction software, we thus corroborate our previous claim (KSZC) on the existence of a very cold dust component in inactive spirals. The ratio becomes 3 / and is therefore much lower than derived for active galaxies.
© European Southern Observatory (ESO) 1999
Online publication: November 3, 1999