On the Hα faintness of the North Polar Spur

The ratio of Hα intensity to 1.4-GHz radio continuum intensity in the North Polar Spur (NPS) is measured to be ≲ 50, two orders of magnitude smaller than the values of ~10⁴ observed in the typical shell-type old supernova remnants of the Cygnus Loop and S147. The extremely low Hα-to-radio intensity ratio favours the Galactic Centre explosion model for the NPS, which postulates a giant shock wave at a distance of several kiloparsecs in the hot and low-density Galactic halo with a low hydrogen recombination rate, over the local supernova(e) remnant model.