The Unified Schemes and Radio Polarization Studies

Abstract

In the unified schemes for extragalactic radio sources, the apparent diversity of different types of sources is interpreted to be largely due to similar objects being seen with different angles of inclination to the observer. The orientation dependence arises due to bulk relativistic motion of the non-thermal jets squirting out from the active nucleus, and anisotropic distribution of circumnuclear material, which is possibly in the form of a torus. Since the extended radio emission from the lobes is believed to be largely isotropic, different unified schemes have been suggested for the high-luminosity Fanaroff-Riley or FR class II sources and the lower luminosity class I objects. In the unified scheme for the FRII radio sources, the radio galaxies are believed to be inclined close to the plane of the sky while the quasars are observed with small viewing angles. On the other hand the lower-luminosity FRI sources are believed to be the parent or misaligned population of the BL Lac objects (Barthel 1989; Antonucci 1993; Urry and Padovani 1995). In this paper we discuss some of the polarization studies which might be relevant for testing or constraing the unified schemes.