The idea that active galactic nuclei (AGNs) are powered by accretion at
high rates (up to a few solar masses per year) onto supermassive
(106-108
M
) central
black holes has been around for a long time (e.g.,
Zeldovich and
Novikov 1964).
Furthermore, it has long been realized that
collapse into a black hole may be inevitable in the center of many dense
stellar environments, once the central potential gets sufficiently deep
(for example,
Lynden-Bell 1969,
Rees 1978).
Consequently, one might expect supermassive black holes to
reside at the center of most galaxies.
In addition, searches for the host galaxies of quasars (thought to represent accreting supermassive black holes) have been going on for a long time, following the pioneering works of Kristian (1973), Wyckoff et al. (1980), Hutchings and Campbell (1983), and Hutchings, Janson and Neff (1989). The ground-based studies not only detected host galaxies, but also suggested that some of the hosts have experienced tidal interactions (Boroson, Oke and Green(1982), Kukula et al.(1996))
These tentative detections and suggestions have turned into reality with the spectacular, high-resolution images of quasars obtained with HST (e.g., Bahcall, Kirhakos, and Schneider 1994, Bahcall et al. 1997, Kirhakos et al. 1999). The HST images revealed unambiguously that nine radio-loud quasars reside either in bright elliptical hosts or in strongly interacting systems. These findings probably indicate that interactions with neighboring galaxies play an important role in the fueling of the central black hole in active galaxies (see Krolik 1999a).