We have observed the continuum emission of the interacting galaxy NGC 4631 at \lambda \lambda 870 \mu m and 1.23 mm using the Heinrich-Hertz-Telescope on Mt .
Graham and the IRAM 30-m telescope on Pico Veleta .
We have obtained fully sampled maps which cover the optical emission out to a radius of about 7 ^ { \prime } at both wavelengths .
For a detailed analysis , we carefully subtracted the line contributions and synchrotron and free-free emission from the data , which added up to 6 % at 1.23 mm and 10 % at 0.87 mm .
We combined the flux densities with FIR data to obtain dust spectra and calculate dust temperatures , absorption cross sections , and masses .
Assuming a “ standard ” dust model , which consists of two populations of big grains at moderate and warm temperatures , we obtained temperatures of 18 K and 50 K for the both components .
However , such a model suffers from an excess of the radiation at \lambda 1.23 mm , and the dust absorption cross section seems to be enhanced by a factor 3 compared to previous results and theoretical expectations .
At large galactocentric radii , where the galaxy shows disturbances as a result of gravitational interaction , this effect seems to be even stronger .
Some possibilities to resolve these problems are discussed .
The data could be explained by a very cold dust component at a temperature of 4 – 6 K , an increased abundance of very small grains , or a component of grains with unusual optical properties .
We favour the latter possibility , since the first two lead to inconsistencies .