Abstract
The article aims to overview the origin and current dynamical state of a giant structure on the northern galactic sky—the radio Loop I extending from ≈ − 45 ∘ to ≈ + 45 ∘ in longitudes and up to ≈ 80 ∘ in latitudes over the Galactic center (GC). The main issue addressed here is a description of possible sources of mass and energy able to build up the Loop I and associated structures seen in X-ray, 21 cm, far infrared (FIR), and maintain them on long timescales. This region of the sky is highly crowded, such that contaminations from many projected structures can be tangled, and not always current direct observations look sufficient to disentangle them. At such conditions indirect arguments based on analysis of underlying star formation (SF) rate, morphological features in radio, X-ray and FIR may be important for understanding the origin of Loop I. Simple estimates show that the observed rather weak SF rate is able to create and maintain Loop I, and under certain circumstances can provide the observed east-west asymmetry. However, an explanation of an apparent coexistence of morphologically similar HI and FIR filaments close to Loop I is challenging, indicating that most likely they may belong to the foreground. Recently discovered absorptions in diffuse interstellar bands seem to confirm this picture.
Highlights
Two different points of view are established by the end of 1980s, and to a certain extent they still remain in competition: one suggests predominantly local origin of an expanding superbubble driven by stellar activity in nearby OB associations and local spiral arms within the solid angle ∼π pointing towards l ∼ 0◦, the other point of view is that the North Polar Spur (NPS) and related structures originate near the Galactic center (GC)
We focus on the morphological consistency of Loop I as seen in X-ray, synchrotron, HI and far infrared (FIR) emissions
Energetic events in the Galactic center and the very existence of the Fermi-bubbles make it very attractive to think, that all this is produced by a collective SN activity in the center
Summary
A complementary model was drawn by Sofue [12] who associated the spur-like structures with synchrotron ’banks’ elevated up to ∼1 kpc above the local (within ∼3 kpc) spiral arms In this scenario, a slight (20 to 30◦ ) inclination of the spurs in the anticenter direction is explained by the ram pressure of an outward blowing gas or magnetohydrodymic waves from the Galactic center [12,13]. With rather uncertain distances inferred from the galactic rotation curve within |l | ≤ 30◦ , a reliable complementary tool for distance measurements is 3D-reddening measurements [16], (see, [17]) In such circumstances, a reasonable compromise for understanding a qualitative picture can be found in estimates of timescales, gas and dust mass and energy budget, synchrotron emission (magnetic field, spectral index), morphological features of the structures etc. A systematic analysis of the origin and the distance to the NPS and its relation to the Fermi bubbles based on X-ray and gamma-ray data is given in the review by Kataoka et al [18] in the same special issue of the Galaxies journal
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
