A multi-wavelength analysis of M 81: insight on the nature of Arp's loop

A. Sollima¹, A. Gil de Paz², D. Martinez-Delgado^1,3, R. J. Gabany⁴, J. J. Gallego-Laborda⁵ and T. Hallas⁶

¹ Instituto de Astrofisica de Canarias, La Laguna, E38205, Spain e-mail: asollima@iac.es (AS)
² Departamento de Astrofisica, Universidad Complutense de Madrid, E28040, Spain
³ Max-Planck-Institut fur Astronomie, Heidelberg, Germany
⁴ Blackbird Observatory, New Mexico, USA
⁵ Fosca Nit Observatory, Montsec Astronomical Park, Ager, Spain
⁶ Hallas Observatory Annex, Foresthill, USA

Received: 18 January 2010
Accepted: 6 April 2010

Abstract

Context. The optical ring-like structure detected by Arp (1965) around M 81 (commonly referenced as “Arp's loop”) represents one of the most spectacular features observed in nearby galaxies. Arp's loop is commonly interpreted as a tail resulting from the tidal interaction between M 81 and M 82. 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 M 81 and Galactic dust cirrus that fills the area where M 81 is located.

Results. Optical images reveal that this peculiar object has a filamentary structure characterized by many dust features overlapping M 81'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 M 81, where both bright UV complexes and peaks in the HI distribution are found, ii) the extended disk of M 81 and iii) scattered light from the same Galactic cirrus that is responsible for the bulk of the far-infrared emission.