Radio Observations of Ionized Hydrogen Nebulæ and Other Discrete Sources at a Wave-length of 9.4 cm.

Abstract

IN recent attempts^1–3 to account for the general radio emission from the Galaxy as the integrated radiation of discrete galactic radio sources, it was found necessary to introduce the effect of ionized interstellar hydrogen, H II regions, to fit the observed radio isophotes at various frequencies. Scheuer and Ryle⁴ detected, at wave-lengths of 1.4 and 3.7 m., a bright band of emission confined within 2° of the galactic equator superimposed on the general radiation. They identified the bright band with thermal emission from a distribution of H II regions. H II regions are strongly concentrated in the galactic plane. The large source, Cygnus X, has been tentatively described as thermal emission⁵, perhaps from several bright H II regions surrounding γ Cygni⁵, or from the integrated radiation of all the H II regions in the spiral arm in the direction of Cygnus⁸. However, Baldwin⁶ attempted unsuccessfully to detect, at a wave-length of 1.4 m., a very bright individual H II region, the Orion nebula. Observations of discrete sources have been almost entirely confined to metre wave-lengths, where spectra of more than twenty sources have been obtained⁷. Piddington and Minnett^5,8 observed five discrete sources at a wavelength of 25 cm. and the emission at 25 and 10 cm. from a region near the galactic centre. This was an appreciable extension of the short-wave limit of source spectra.