Multi-Wavelength Observations and Modeling of Loop I

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 approximate to-45 degrees to approximate to+45 degrees in longitudes and up to approximate to 80 degrees 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.