Cold dust in a selected sample of nearby galaxies. I. The interacting galaxy NGC4631

TL;DR

This study maps and analyzes the cold dust emission in the interacting galaxy NGC4631, revealing excess millimeter emission and suggesting the presence of grains with unusual properties or very cold dust components.

Contribution

First detailed millimeter-wave dust emission maps of NGC4631 with analysis of dust properties and identification of excess emission indicating unusual dust characteristics.

Findings

Detected excess emission at 1.23mm compared to standard models.

Found dust absorption cross section enhanced by a factor of 3.

Suggested the presence of grains with unusual optical properties or very cold dust.

Abstract

We have observed the continuum emission of the interacting galaxy NGC4631 at 0.87 and 1.23mm 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 1.23mm and 10% at 0.87mm. 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 18K and 50K for the both components. However, such a model suffers from an excess of the radiation at 1.23mm, 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 4-6K, 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.