The image

The integrated flux nebula (IFN) in this field, which includes Arp's loop, caught me by surprise, not only for its abundance of tangled features, but also for how well it stood out against the noise in those dim regions of the frame. I tend to put the black point of an image just slightly below its histogram floor, since I favour the stronger contrast that creates, but in this case I relaxed the black point much more than is my usual habit, to try to bring out the IFN, while still aiming to have the gorgeous spiral pop up above the background. The image is from just over 16 hours of integration, divided roughly equally between luminance and RGB colour. The data was acquired over the course of seven nights in March and April. This is only the 2nd time that I've posted an image at the full resolution of my CDK24/FLI KL4040 camera combination, which is about 0.47"/pixel, since I think (hope!) that this one stands up to closer inspection! The field of view is only slightly cropped, at  just under 31' on a side.

And an astrophysical rabbit hole!

As @James E. noted in the first comment below, there appears to be an HII emission nebula, with a telltale reddish hue, just above and to the right of the dwarf galaxy Holmberg IX (the dwarf itself is above M81 and looks like a blue star cluster). After fussing over that part of the field for a long time before posting the image, I began to look into what, if anything, is known about that curious little nebular loop. I soon dropped into a deep astrophysical rabbit hole: it turns out that there is alot more than meets the eye, as it were, in that tiny part of the sky, which has been the subject of decades of scientific research. So, two days after I posted the image, I'm back with what I hope might be an interesting back story.

The story begins in the early 1980s, with the discovery of several Xray sources in the vicinity of M81, by the Einstein Xray Observatory (the first full-fledged orbiting Xray telescope). The second-most intense of the sources, after the "active" nucleus of the galaxy, was found to be near a faint object previously seen in the optical, that would later be identified as an HII nebula. The Xray source, labelled X-9, was initially thought to be un-associated with M81, given its relatively large angular separation from the disk, and the fact that its Xray flux is about half that of nucleus, and much larger than any of the other Xray sources in the direction of the disk. 

The nebula was discovered with CCD imaging in 1993, but was initially conjectured to be an irregular galaxy. That was quickly followed by a 1995 study of HII regions near M81, using H-alpha imaging and nebular spectroscopy. That study showed that the nebula centre is nearly coincident with X-9, and that if it is at the distance of M81 (as suggested by several indirect lines-of-argument), then it could be a "supershell" (or "super bubble") remnant of a series of supernova explosions. The same study identified an optical "blue knot" near the centre of the nebula (which can be seen in my image): the author argued that the conjunction of the three objects (X-9, nebula, and "blue knot") is unlikely to be a coincidence, and more likely implies that they are connected, perhaps with the blue knot being the optical counterpart of X-9, and the nebula having been created by X-9. 

The mystery of this unusual conjunction led to further investigation, including two studies of timing variations in the flux of X-9 that suggest it is a compact object, with detailed arguments apparently favouring an intermediate-mass black hole of about 100 solar masses, or a black hole binary! And a 2005 study that produced millimeter-wave maps of the molecular complex in the vicinity of Holmberg IX, and that included new H-alpha observations of the HII nebula, suggested that the supershell might be an outflow from X-9 that has shaped the molecular complex, while the "blue knot" might actually be a jet powered by X-9, all of which are located in the outer disk of M81.

So that's the big-picture story of the science (as best as I can reconstruct it) behind the nebular loop that occupies such a tiny part of my image!