Here I call attention to some current research problems that need further work. Their eventual solution should improve our understanding of quasars and AGNs in important ways.
The Disagreement between Observations and Predictions for Accretion Disks. Koratkar (1997) points out that observations do not confirm most predictions of accretion disk models. For example, the Zheng et al. (1997) composite spectrum for ultraviolet wavelengths does not match predictions, and soft X-ray fluxes are observed to be too flat. Fewer Lyman edges are observed than predicted. Polarization is not seen either, which seems to rule out scattering as a way of smoothing the Lyman edges. An additional theoretical question is how the radiation from the accretion disk couples with that of the hot (X-ray) corona. It is important to resolve these issues if we are to have confidence in this basic part of our concept for quasars and AGNs.
What powers Ultra-luminous IRAS galaxies? Observations by Genzel et al. (1998) indicate that massive stars predominate in 70-80% of the cases, with AGNs dominating in the others. At least half of the systems probably have both an AGN and a circum-nuclear ring of starburst activity. They see no clear trend for the AGN component to dominate in the most compact and presumably most advanced mergers.
Do all galaxies have massive black holes? van der Marel (1997) notes that available data appear consistent with most galaxies having black holes, whose mass roughly correlates with the luminosity of the spheroid (cf. Magorrian et al. 1998). The black holes could have formed in or prior to a quasar phase and grown via mass accretion. Some of the implications of this work are discussed below under the theory section.
Is the broad Fe K line produced directly near a black hole? How well do we understand the origin of X-ray emission in general? Observations of the broad Fe K line in AGNs are widely interpreted as arising in the inner part of accretion disks around black holes and therefore providing both confirmation of the presence of black holes as well as direct information about conditions in the disks. However, Weaver and Yaqoob (1998) have raised questions about whether the emission in fact does occur so close to the centers of AGNs. More generally, intensive monitoring of NGC 7469 in X-rays and the ultraviolet by Nandra et al. (1998) provides strong constraints on quasar models. The data are not consistent with the UV emission being reprocessed by gas absorbing X-rays nor with the X-rays arising from Compton upscattering of the UV radiation.
These are just some examples of research problems in need of solution for us both to have confidence in our general picture of quasars and AGNs being powered by accretion onto massive black holes and to develop a quantitative understanding that explains the major observed features of these objects.