The “Alignment Effect” and the Evolution of Dust in High‐Redshift Radio Galaxies

Abstract

The alignment of optical emission along the axis of radio emission in high-redshift radio galaxies is now commonly observed. In many cases this aligned component can be shown to be significantly polarized, a result that has led to models for the emission that employ scattering of light emitted from the active galactic nucleus. Both electron scattering and dust scattering have been proposed, and both mechanisms have positive and negative attributes. One aspect of dust scattering that has not been explored previously is the response of the dust grains to the passage of the strong shock associated with the radio source. The scattering medium must be distributed over many tens of kiloparsecs, and it must survive for a time comparable to the age of the radio source. Analogies with conditions in our own interstellar medium suggest that the dust is contained in gas clouds of high density contrast and small filling factors embedded in a more diffuse medium. The survival of dust grains in such an environment after the passage of a high-speed (109-1010 cm s-1) shock associated with the radio jet is calculated for a wide range of parameters. It is found that for most of the cloud configurations the grains are destroyed as a scattering population by sputtering processes in a time much less than the minimum radio source lifetime of ~10 million yr. Thus polarization due to scattering by an in situ population of grains may be somewhat problematic. Alternate methods for providing the needed grain population, either by grain replenishment via outward convection from a dust rich galactic interior or by local production from a population of stars formed by passage of the radio jet, are explored.