The radio emission from the Galaxy at 22 MHz

Abstract

We present maps of the 22 MHz radio emission between declinations -28° and +80°, covering ~73% of the sky, derived from observations with the 22 MHz radiotelescope at the Dominion Radio Astrophysical Observatory (DRAO). The resolution of the telescope (EW × NS) is 1.1° × 1.7° secant(zenith angle). The maps show the large scale features of the emission from the Galaxy including the thick non-thermal disk, the North Polar Spur (NPS) and absorption due to discrete Hii regions and to an extended band of thermal electrons within 40° of the Galactic centre. We give the flux densities of nine extended supernova remnants shown on the maps. A comparison of the maps with the 408-MHz survey of Haslam et al. (1982) shows a remarkable uniformity of spectral index ($T\propto \nu^{-\beta}$) of most of the Galactic emission, with β in the range 2.40 to 2.55. Emission from the outer rim of the NPS shows a slightly greater spectral index than the distributed emission on either side of the feature. The mean local synchrotron emissivity at 22 MHz deduced from the emission toward nearby extended opaque Hii regions is ~1.5 10-40 Wm-3 Hz-1 sr-1, somewhat greater than previous estimates.