Optical and Infrared Polarization of Active Extragalactic Objects

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/l. 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 for this class of object. A combination ofBL 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