Radio Continuum Emission from Galaxies

Abstract

The continuum radio emission detected from external galaxies is produced either by thermal process or by the synchrotron process with the latter dominating in most cases. For the synchrotron process the volume emissivity is given by

$$ \varepsilon \left( \upsilon \right) = {\rm{n}}{\mkern 1mu} {{\rm{B}}^{\frac{{{\rm{1 + }}\gamma }}{2}}} {\upsilon ^{\frac{{1 - \gamma }}{2}}} = {\rm{n}}{\mkern 1mu} {{\rm{B}}^{{\rm{1 - }}\alpha }}{\upsilon ^\alpha } $$

where the number of relativistic electrons with energy E is N(E) = n E^−γ, B is the magnetic field and v the frequency. Hence an observation of the radio continuum is a measurement of the product of the magnetic field strength and the relativistic particle density. The general questions which arise in interpreting these radio continuum observations are: where do the magnetic fields and relativistic electrons come from and how are they contained?