Galactic Centre hypershell model for the North Polar Spurs

Abstract

The bipolar-hypershell (BHS) model for the North Polar Spurs (NPS-E, -W, and Loop I) and counter southern spurs (SPS-E and -W) is revisited based on numerical hydrodynamical simulations. Propagations of shock waves produced by energetic explosive events in the Galactic Centre are examined. Distributions of soft X-ray brightness on the sky at 0.25, 0.7, and 1.5 keV in the ±50° × ±50° region around the Galactic Centre are modelled by thermal emission from high-temperature plasma in the shock-compressed shell considering shadowing by the interstellar H I and H₂ gases. The result is compared with the ROSAT wide field X-ray images in R2, 4, and 6 bands. The NPS and southern spurs are well reproduced by the simulation as shadowed dumbbell-shaped shock waves. We discuss the origin and energetics of the event in relation to the starburst and/or active galactic nucleus activities in the Galactic Centre.