Annu. Rev. Astron. Astrophys. 1977. 15: 69-95
Copyright © 1977 by . All rights reserved

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1. INTRODUCTION

This review appears nearly a decade after the Tucson conference "Seyfert Galaxies and Related Objects" held in February, 1968. At that time, realization of the similarities between the QSOs and the nuclei of Seyfert galaxies led 56 astrophysicists to contribute their ideas regarding Seyfert galaxies [Astronomical Journal, Vol. 73 (1968), pp. 836-943]. A number of questions were posed regarding the energetic events in Seyfert galaxies. Most of these remain unanswered. Progress has been made in increasing the sample of Seyfert galaxies, in describing the physical conditions in their nuclei, and in delineating the relation between Seyferts and QSOs. Two basic aspects of the Seyfert galaxy problem have been emphasized. One is the nature of redshifts and whether a study of Seyfert galaxies leads to proof that QSO redshifts, by analogy, are cosmological. The other is the search for an explanation of the extraordinary energy source that powers the nuclei of Seyfert galaxies. There are many reasons to believe that the same mechanism of energy generation operates within most Seyfert nuclei as in the QSOs. To a certain extent, this has been a frustrating research problem because of the lack of progress in deciphering the fundamental mechanisms involved. On the other hand, far more observational research is being directed toward Seyfert galaxies than ever before, so this deserves to be chronicled.

This review makes no attempt to incorporate the numerous observations and theories concerning the general problem of galactic nuclei, even though this is certainly relevant to Seyfert galaxies. Some judgment had to be used as to the point when the review was sufficiently comprehensive to encourage new researchers to consider the problems but not so encyclopedic as to be intimidating. An earlier review in this series provided a comprehensive discussion of galactic nuclei including data on Seyferts known at the time (Burbidge 1970). Somewhat more recent summaries of observational work on Seyfert galaxies are given by Osterbrock (1971), Sargent (1971), and Ulrich (1974). A comprehensive theoretical review is that of Saslaw (1974). Listings of Seyfert galaxies and notes on the morphology and spectra of individual objects are given by Khachikian & Weedman (1974) and Adams (1977). The relation between Seyferts and other emission-line galaxies has recently been discussed by van den Bergh (1975a) and Weedman (1977). Osterbrock (1977a) compares the properties of Seyfert and radio galaxies. The role of Seyfert galaxies in the redshift controversy is discussed by Weedman (1976a). The present review emphasizes the description of the Seyfert galaxy phenomenon as synthesized from recent observational studies; most of this summary is based on work done since 1970. Unfortunately, we have so far been unable to explain much of what we see.

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