On the nature of dust clouds in the region towards M 81 and NGC 3077

Abstract

Aims: There is some controversy on the nature of dust clouds found in direction of the interacting galaxy triplett M 81, M 82, and NGC 3077. Are they associated with the tidal arms seen in HI around those galaxies or are they simply Galactic foreground clouds?
Methods: Data from the SPIRE instrument onboard Herschel and MIPS onboard of Spitzer are used to derive physical parameters for the dust clouds. These observions are compared to CO clouds previously mapped with the IRAM and the FCRAO radio telescopes.
Results: SPIRE and MIPS maps show several dust clouds north of M 81 and south of NGC 3077. Modelling of the dust emission provides total hydrogen column densities between 1.5 and 5.0 × 10²⁰ cm^-2. Dust temperatures are between 13 to 17 K. No significant difference in the dust emission can be found between individual clouds. It is shown that CO line emission provides the best clues on the origin of those clouds. Most of the clouds seen towards M 81 are associated with small-area molecular structures (SAMS), i.e. tiny CO clouds of Galactic origin. The clouds seen towards NGC 3077 are partly associated with the tidal arms and are partly in the Galactic foreground associated with SAMS.