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

more »

... d, 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. Galaxies 2018, 6, 62 2 of 18 a relatively nearby interstellar medium (ISM) [2, 4] , and/or with a joint envelope from stellar wind of Sco-Cen OB association. This point of view lies close to the concept of giant HI supershells ubiquitously produced in the ISM disk by OB-associations with total energy release up to 10 SNe [6, 7] , see also theoretical arguments in [8] , and later development in [9] . Sco-Cen is the nearest such region of a recent active star formation [10] , and given its current position falls into the direction of the center of Loop I, it seems likely to be a very natural source of energy for maintaining this structure. Galacto-centric model. The second emerged from 10 GHz observations of an Ω-shaped lobe extending upward at b ≈ 1 • in the very central Galactic region (|l| < 1 • ) [11] . Line of sight spread model. 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] . Later on such kind of structures have been described numerically in 2D MHD as hydraulic jumps combined with shocks from spiral density waves-the resulted plum-like vertical structures extending up to ∼0.5-1 kpc [14, 15] . It is clear that the whole variety of structural features observed toward the Galactic center stems from crowded events along the line of sight. The problem is therefore to disentangle superposing contributions and identify the driving sources. 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, also [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. The following issues are therefore critical for understanding the origin of Loop I: