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

VII. CONCLUSIONS

The use of optical and infrared polarimetry to study active extragalactic objects is a powerful method to obtain information about the innermost core of the central engines. As Rees (1978a) has stressed, the spatial scale of 10¹⁵ cm indicated by the variability of the polarized optical emission is a thousand times smaller than the compact radio structures of ~ 10¹⁸ cm studied by VLBI. Until recently there were few detailed predictions that could drive specific polarimetric tests of quasar models. The development of black hole accretion theories and relativistic jet models now suggests a wide range of studies.

One general study will be a quantitative analysis of the statistics of occurrence of polarization and large optical variability in objects ranging from the ellipticals with weak compact sources to the bright quasars. Correlating the polarization with on the one hand properties known to be isotropic such as galaxy luminosity, emission line strengths, and symmetric extended radio emission, and on the other with compact flat-spectra, low-frequency variability, and so on, should give a much better understanding of the role of relativistic beaming. Another area that should also be rewarding will be synoptic studies of the rapid variations in polarization and flux, ideally measurements correlated in the X-ray, infrared, and optical spectra. Because the time scales are around a day, coordinated measurements from satellites or several observatories around the world are needed. With modern instrumentation, modest telescopes of ~ 1 m aperture are adequate for this work. These data should shed much light on the primary mechanisms for energy release deep in the central powerhouse.

Additional studies of weakly polarized or dusty nuclei would be extremely valuable. For the majority of QSOs, high quality, multicolor polarimetry and spectropolarimetry should discriminate between scattering and nonthermal origins of the observed polarization. In the Seyfert nuclei, imaging polarimetry from space and spectropolarimetry similar in quality to the beautiful spectrum of NGC 4151 obtained by Schmidt & Miller (1979) could establish the spatial and kinematic relationship of the dust clouds within the narrow and broad line emission regions. The general polarization surveys should be extended into the ultraviolet to study the characteristics of nuclear dust and to smaller apertures to search for dust or nonthermal sources in active and normal galaxies.

ACKNOWLEDGMENTS

We are grateful to many colleagues for letting us have preprints in advance of publication, and especially to Richard Moore for preparing the two tables. This review has benefited also from the help of Drs. Jones, Miley, Moore, Puschell, Rieke, Rudnick, Scheuer, Stein, Tapia, and Wardle, who pointed out errors and suggested improvements in the first draft. This work is supported by the NSF under grant AST-78-22714.