Comptes Rendus. Physique (Mar 2022)
North Polar Spur/Loop I: gigantic outskirt of the Northern Fermi bubble or nearby hot gas cavity blown by supernovae?
Abstract
Radio continuum, microwave and gamma-ray images of the Milky Way reveal a conspicuous, loop-like structure that fills almost half of the northern Galactic hemisphere, called Loop I. The interior of Loop I is the most conspicuous region shining in soft X-rays, whose eastern base is a remarkably bright, elongated structure seeming to emerge from the Galactic plane, dubbed the North Polar Spur (NPS). After 40 years of debates, two very different, contradictory views of Loop I/NPS are still defended: on the one hand, the NPS is a gigantic volume of expanding hot gas that envelops and extends the northern “Fermi Bubble” (FB) known to be blown by the Galactic center, and Loop I marks the shock front; on the other hand, the NPS is totally independent of the northern FB, it is a nearby, ordinary cavity of hot gas blown by supernovae, Loop I is its shock front and both are coincidentally located in the direction of the FB. To an observer at the Sun, both can produce the same perspective view, although the former has a size comparable to the Milky Way itself, and the latter a diameter of a few hundreds parsecs. The energy involved varies by 3-4 orders of magnitude, and the solution has important consequences on the structure and history of our Galactic neighborhood, on the age of the North and South FBs and the activity at the Galactic center. Moreover, whatever are the actual shape and distance of Loop I/NPS, accurate modeling of the polarized emission associated with Loop I is important for CMB foreground removal. After a short review, I discuss recent results which have a connection with Loop I/NPS. Some of them have been used as arguments in the two opposite ways, while for others the connections with LoopI/NPS have been overlooked. They involve very different spacecraft, from a 12 Kg Cubesat (HaloSat) to major space-borne observatories (HST, Gaia, and Spektr-RG). I make use of updated 3D maps of dust and a recent massive star catalog. I distinguish arguments based on geometric similarity or dissimilarity from those derived from measurements and physical models. Considering all past and recent constraints, it is clear that there is no entirely local or entirely distant scenario that is free from contradictions with some of the data analyses or from improbable coincidences. I discuss a speculative scenario, partially inspired by recent BF and Milky Way gas models, combining near and far aspects and seeming to be able to meet the various constraints. However, new data and models are needed to bring the controversy to a close and we can still expect new episodes of this long story.
Keywords
