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

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.

Herschel★and JCMT observations of the early-type dwarf galaxy NGC 205

“…The detection is below the 3σ level, but it coincides with an optical tidal tail reported in Saviane, Monaco & Hallas (2010). However, we cannot rule out the possibility that the faint blob corresponds to foreground Galactic cirrus emission similar to the emission identified in the surroundings of M81 (Davies et al 2010; Sollima et al 2010) or to emission originating from one or multiple background sources.…”

Tidal streams around galaxies in the SDSS DR7 archive

“…If the remains of the dwarf would not clearly be visible, it would not be sure whether the detected feature would be a result from a tidal disruption of a dwarf galaxy or interaction within the galaxy group or even galactic cirrus, although this can be ruled out most of the time since it has a much more extended and diffuse structure than the found features. Furthermore it would be very unlikely that smaller, sharper parts of the cirrus would happen to be right in front of several small galaxies, distributed of a large portion of the sky, as it may be the case with single, larger galaxies like M 81 (Sollima et al 2010). See Fig.…”

The IAC Stripe 82 Legacy Project: a wide-area survey for faint surface brightness astronomy

“…A closer insight into the properties and characteristics of the Galactic dust emission is urgently needed as its range in surface brightness intersects in large parts with the brightness of the aforementioned science cases, from stellar haloes and streams to the ICL. Indeed, Arp's loop, a suspected stream-like ring around M81 suggesting an interaction between M81 and M82, recently turned out to be likely a superposition of a few recent starforming regions located close to M81, M81's extended disc and Galactic Cirrus emission (Davies et al 2010;Sollima et al 2010), the latter being responsible for the diffuse appearance of the feature.…”