Synchrotron radiation and the structure of the galaxy

Abstract

The structure of the Galaxy is still largely unknown. Our only direct source of information is the radiation it emits; electromagnetic radiation at metre wavelengths being particularly important as it remains largely unabsorbed on passage through the Galaxy, allowing an unhindered view of its large-scale structure. Analysis of its distribution over the sky can provide insight into this structure. An attempt was made to investigate the properties of our Galaxy by modelling the radio synchrotron emission at 408MHz and comparing the predicted distribution with a newly available, high resolution survey of the whole sky at this frequency. A model previously employed with some success at 150MHz; was used as a basis for this work. A suite of computer programs was developed to perform the necessary calculations, and to handle and present the results and the survey data. Refinements were made to the basic model to try to reproduce the observed distribution more accurately. A model of the Galaxy emerges featuring a central bar, each end joined by an elliptical arm section. These divide to form four spiral arras which sweep out to a radius of 15kpc. In addition, a number of short 'feather' arms, branching off from the main arms, are required. The local Orion arm is one such 'feather', branching from the Perseus arm, passing 0.5kpc from the sun in the anticentre direction and ending a few kiloparsecs further on. A weak halo of emission extending several kiloparsecs out from the plane is also required. Independent confirmation of this is found from consideration of cosmic ray diffusion through the Galaxy. Finally, comparison was made between the 408MHz survey and the COS-B gamma ray distribution. The similarity between them in the plane provides information about the relationship between various components of the galactic disc.