The angular broadening of compact radio sources observed through ionized gas in a rich cluster of galaxies

Abstract

We report a novel method by which ionized gas in rich clusters of galaxies at high galactic latitude may be detected. The transonic motions of galaxies in clusters stir the intergalactic medium, inducing shear which give rise to magnetohydrodynamic instabilities. One of these, the mirror instability causes small-scale density inhomogeneities to develop, which grow to a level (..delta..rho/rho) of about 3%. These have a typical scale length of 10 times the ion Larmor radius, and would broaden the angular diameters of compact radio sources seen through them to approx.0.l3 (8l.5/v)/sup 2/ arcseconds at an oberving frequency v MHz, for a gas density approx.10/sup -3/ cm/sup -3/, temperature approx.10/sup 8/ K, and ambient magnetic field approx.10/sup -7/ gauss. Scattering in the Galaxy would be less important at high galactic latitudes.