A Multi-wavelength analysis of M81: insight on the nature of Arp's loop
  Context: The optical ring like structure detected by Arp (1965) around M81
(commonly referenced as "Arp's loop") represents one of the most spectacular
feature observed in nearby galaxies. Arp's loop is commonly interpreted as a
tail resulting from the tidal interaction between M81 and M82. However, since
its discovery the nature of this feature has remained controversial. Aims: Our
primary purpose was to identify the sources of optical and infrared emission
observed in Arp's loop. Methods: The morphology of the Arp's loop has been
investigated with deep wide-field optical images. We also measured its colors
using IRAS and Spitzer-MIPS infrared images and compared them with those of the
disk of M81 and Galactic dust cirrus that fills the area where M81 is located.
Results: Optical images reveal that this peculiar object has a filamentary
structure characterized by many dust features overlapping M81's field. The
ratios of far-infrared fluxes and the estimated dust-to-gas ratios indicate the
infrared emission of Arp's loop is dominated by the contribution of cold dust
that is most likely from Galactic cirrus. Conclusions: The above results
suggest that the light observed at optical wavelengths is a combination of
emission from i) a few recent star forming regions located close to M81, where
both bright UV complexes and peaks in the HI distribution are found, ii) the
extended disk of M81 and iii) scattered light from the same Galactic cirrus
that is responsible for the bulk of the far infrared emission.
