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Issue A&A Volume 414, Number 2, February I 2004 Page(s) 475 - 486 Section Galactic structure and dynamics DOI 10.1051/0004-6361:20031636

A&A 414, 475-486 (2004)
DOI: 10.1051/0004-6361:20031636

Cold dust in a selected sample of nearby galaxies

I. The interacting galaxy NGC 4631

M. Dumke^{1, 2}, M. Krause¹ and R. Wielebinski<sup>1

1</sup>  Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
²  SMTO, Steward Observatory, The University of Arizona, 933 N. Cherry Avenue, Tucson, Arizona 85721, USA

(Received 14 April 2003 / Accepted 17 October 2003 )

Abstract
We have observed the continuum emission of the interacting galaxy NGC 4631 at 870 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' 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 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

Key words: galaxies: ISM -- radio continuum: galaxies -- galaxies: individual: NGC 4631

Offprint request: M. Dumke, mdumke@mpifr-bonn.mpg.de

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