The distribution of cosmic ray electrons and magnetic field in the galaxy

Abstract

An attempt is made to model the distribution of cosmic ray electrons and magnetic field in the galaxy such that agreement is reached with the observed continuum radiation. A new composite map of galactic spiral structure has been obtained by combining the model of Georgelin, based on observations of H II regions, with the map of neutral hydrogen outside the solar circle of Verschuur. Using models which relate the galactic magnetic field and relativistic electron distribution to this structure, profiles of the synchrotron radio emission along the galactic plane are predicted for 30 < l(^π) < 330. These are compared with the observed profile at 150 MHZ corrected for thermal emission, local sources and extra galactic background. The way in which the spiral arm compression, produced by density waves, falls-off with height above the plane is obtained using magnetohydrodynamic shock theory. The fall-off in emissivity with height above the plane is fitted to the observations of Landecker and Wielebinski. Detailed comparison with observation gives preference for a uniform distribution of electrons in the disc of the galaxy. There is evidence that the observed magnitude of the galactic magnetic field in the neighbourhood of the sun is more typical of an interim than an arm region. A magnetic field which, in the galaxy as a whole, can be decomposed into random and regular components of approximately equal magnitude, is indicated. The observed latitude distribution of brightness temperature at 150 MHZ may be understood if the emissivity distribution with height above the plane has an equivalent half width of 2.75 kiloparsecs, half width at half maximum of 2 kiloparses and extent of about 10 kiloparsecs at the sun. This thick disc or halodecreases in extent towards the galactic centre and increases in extent towards the edge of the galaxy. The two closed galaxy models proposed by Peters, Rasmussen and Westergaard are both discussed in connection with the cosmic ray electron component. Both models are found to be inconsistent with, in one case, the observed positron flux and, in the other, with the observed continuum emission.