A multi-wavelength analysis of M 81: 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 thethe most spectacular features 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 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.