Abstract

Various observational tests of unified schemes for radio sources are reviewed critically. It is shown that, although some of the results might raise doubts, there exists no definitive evidence against these schemes.

Introduction

There is now considerable evidence to support the idea that the radiation from active galactic nuclei (AGN) is emitted anisotropically. For radio-loud objects much of the evidence has been gathered at radio wavelengths, and includes the measurements of superluminal motions in radio cores (e.g. Zensus 1988), the statistics of core and lobe dominated radio sources (e.g. Orr & Browne 1982), and the polarization asymmetry in the extended radio structures (e.g. Garrington et al. 1988, Laing 1988). For the radio-quiet objects the beautiful emission line images of radiation cones (e.g. Pogge 1989, Tadhunter & Tsvetanov 1989), and the detection of scattered broad lines within the extended regions (e.g. Miller et al. 1991), are also very strong evidence in favour of anisotropy.

These observations are comforting for the proponents of unified schemes who use anisotropy to explain the relationships between certain classes of AGN in terms of orientation effects. However, not all of the evidence is so positive, and in this paper I review some recent results which are less supportive of unified schemes.

Types of unified schemes

There is not just one unified scheme based on anisotropy/orientation effects, but several, and it is important to be clear about what we are discussing.