As we pointed out in Chapter 3, the main problem with sustaining an active nucleus by gravitational accretion over its lifetime of at least 108 years is funneling enough mass into the nucleus. Removing a sufficient amount of angular momentum from the gas flowing into the nucleus requires breaking the azimuthal symmetry of the galaxy's gravitational potential. A clear way to do that is by gravitational interactions with other systems, as was originally suggested by Toomre and Toomre (1972) and by Gunn (1979). This provides motivation for examining the nearby environment of AGNs to see if indeed there is evidence for interactions with nearby galaxies. The two specific questions that we want to consider are:

1. What kinds of galaxies harbor AGNs? Are there any discernible differences between galaxies with active nuclei and those without them?

2. Does the presence or absence of companion galaxies have anything to do with whether or not a galaxy harbors an AGN?

We will consider these issues separately, although they are clearly related.

Host Galaxies

The study of the ‘host galaxies’, those galaxies that contain active nuclei, is a very difficult undertaking. The major problems were alluded to at the beginning of this book: the light from the AGN itself often dominates the total light from the galaxy, particularly in the case of the highest-luminosity AGNs, which are spatially rare and thus typically found only at great distances. Consequently the work on the lowerluminosity end of the AGN distribution, i.e., Seyfert galaxies, has tended to yield less ambiguous results.