Large-Scale Features of the Radio Sky and a Model for Loop I

Abstract

The large-scale radio/microwave sky has been mapped over a range of frequencies from tens of MHz to tens of GHz, in intensity and polarization. The emission is primarily synchrotron radiation from cosmic ray electrons spiralling in the Galactic magnetic field, in addition to free–free radiation from warm ionized gas. Away from the Galactic plane, the radio sky is dominated by very large (tens of degrees) loops, arcs, spurs and filaments, including the well-known North Polar Spur (NPS), which forms part of Loop I with a diameter of ∼ 120 ∘ . In polarization data, such features are often more discernible due to their high polarization fractions suggesting ordered magnetic fields, while the polarization angles suggest fields that are parallel to the filament. The exact nature of these features are poorly understood. We give a brief review of these features, focussing on the NPS/Loop I, whose polarization directions can be explained using a simple expanding shell model, placing the centre of the shell at a distance of ∼100–200 pc. However, there is significant evidence for a larger distance in the range ∼500–1000 pc, while larger distances including the Galactic Centre are unlikely. We also briefly discuss other large-scale curiosities in the radio sky such as the microwave haze and anti-correlation of H α filaments and synchrotron polarized intensity.