Radio Frequency Radiation from External Galaxies

Abstract

The study of the radio emission of external galaxies may be said to have begun in 4946 with the observation by Hey, Parsons and Phillips ¹ of fluctuations in the emission from a region in the constellation of Cygnus. They deduced that the fluctuating component “could only originate from a small number of discrete sources”. This was confirmed when Bolton and Stanley ² showed that the fluctuations were due to an intense emitting region less than eight minutes of arc in extent; but five years had to elapse before the extra-galactic origin of this radiation was established. Meanwhile the existence of a whole class of such “discrete sources” had been discovered, and two tentatively identified with external galaxies by Bolton, Stanley and Slee ³ thus suggesting that some types of galaxies might be strong radio-emitters: these tentative identifications were later confirmed and many others made. At the time of writing the number of galaxies from which radio frequency radiation has been detected is about two dozen; a large increase might be expected in the near future.