Annu. Rev. Astron. Astrophys. 1980. 18: 321-361 Copyright © 1980 by Annual Reviews. All rights reserved

I. INTRODUCTION

In bringing together the material for this review our attention was drawn repeatedly to the remarkably similar characteristics of virtually all strongly polarized extragalactic objects, which are found as the nuclei of giant elliptical galaxies or as quasi-stellar sources. These are violent variability, a compact, flat-spectrum radio source, and a very smooth continuum extending at least to 10 µ. These properties are common to the polarized sources in BL Lac objects over a wide range of luminosity and to some QSOs and radio galaxies. In view of the similarities, which suggest a common process of energy release close to the central core of these objects, we will treat them all as a single group. In a memorable banquet speech at the Pittsburgh meeting on BL Lac objects (the only words spoken not faithfully reported in the proceedings) Ed Spiegel suggested the name "blazar" for this class of object. A combination of B L Lac object and quasar, with a strong feeling of the characteristic violent optical flaring, blazar seems an excellent name, one which we will adopt throughout the review. As we shall discuss in Sections V and VI, blazars may not be a different type of object from most quasars or active elliptical galaxies. These normal objects may have jets whose emission is beamed by relativistic bulk motion, and show blazar characteristics only when pointed at us. In reviewing the observational data we will be especially conscious of properties that could help distinguish an isotropic source from a beamed one.

The blazars form only a small portion of active extragalactic objects. Optical emission from most QSOs and Seyfert nuclei shows only very small polarization and little optical variability, and is perhaps the result of thermal or scattering processes. For these objects we will try to assess the information about source geometry and emission that can be derived from polarization data, particularly from the spectrum of polarization.

Polarization of active extragalactic objects has not been comprehensively reviewed in the past except by Visvanathan (1974) and Hagen-Thorn (1974). Both authors include rather complete discussions of the polarization by scattering in normal galaxies and the dusty halo of M82. In the interest of brevity we will not discuss these types of extended objects, except to point the reader to recent discussions of M82 by Bingham et al. (1976) and by Schmidt, Angel & Cromwell (1976). In this review we consider first the compact extragalactic sources for which strong polarization has been measured. The number of these is small enough, about 60, that we are able to at least mention them all. In Section II we first list the blazars, which form the large majority of the strongly polarized objects, and summaries of the observational data of some individual objects are given. The optical-infrared polarimetric properties and correlations of polarization with other properties of blazars are discussed in Section III. The remaining compact objects that show high polarization are PHL 5200 and knots in the jet of M87, whose properties are reviewed in Section II, and several Seyfert nuclei. The polarization properties of Seyfert nuclei, nearly all of which seem to be polarized by dust, and the majority of QSOs, which have only weak polarizations of unknown origin, are reviewed in Section IV. Theoretical models for the origin of optical-infrared polarization are reviewed in Section V, while in Section VI we consider how the observations of polarization can be accommodated by relativistically beamed jets. We finish in Section VII with some directions for future work.