The efforts described above led to many of the observational and theoretical underpinnings of our present understanding of AGN. The enormous effort devoted to AGN in recent years has led to many further discoveries and posed exciting challenges.
Massive international monitoring campaigns (Peterson 1993) have revealed ionization stratification with respect to radius in the BLR, that the BLR radius increases with luminosity, and that the gas is not predominantly in a state of radial flow inwards or outwards. This suggests the likelihood of orbiting material. Models involving a mix of gas with a wide range of densities and radii may give a natural explanation of AGN line ratios (Baldwin et al. 1995). Chemical abundances in QSOs have been analyzed in the context of galactic chemical evolution (Hamann and Ferland 1993). Recent theoretical work indicates that the observed, centrally peaked line profiles can be obtained from a wind leaving the surface of a Keplerian disk (Murray and Chiang 1997).
Efforts to understand the broad absorption lines (BALs) of QSOs have intensified in recent years. The geometry and acceleration mechanism are still unsettled, although disk winds may be involved here too (Murray et al. 1995). Partial coverage of the continuum source by the absorbing clouds complicates the effort to determine chemical abundances (e.g., Arav 1997).
The black hole model has gained support from indirect evidence for massive
black holes in the center of the Milky Way and numerous nearby galaxies
(see Rees 1997).
This includes the remarkable "H2O megamaser" VLBI
measurements of the Seyfert galaxy NGC 4258
(Miyoshi et al. 1995),
which give strong evidence for a black hole of mass
4 × 107
M
.
X-ray observations suggest reflection of X-rays incident on an
accretion disk
(Pounds et al. 1989),
and extremely broad Fe K
emission lines may give a direct look at material
orbiting close to the black hole
(Tanaka et al. 1995).
These
results reinforce the black hole picture, but much remains to be done to
understand the physical processes at work in AGN. In spite of
much good work, the origin and fueling of the hole, the physics of the
disk, and the jet production mechanism still are not well understood.
The nature of the AGN continuum remains unsettled; for example, the
contribution of the disk to the optical and ultraviolet continuum is still
debated
(Koratkar and Blaes
1999).
The primary X-ray emission mechanism and the precise role of thermal and
nonthermal emission in the infrared remain unclear
(Wilkes 1999).
Blazars have proved to be strong
-ray
sources, with detections up to TeV energies
(Punch et al. 1992).
Radio emission was key to the discovery of quasars, and radio techniques have seen great progress. The Very Large Array in New Mexico has produced strikingly detailed maps of radio sources, and shown the narrow channels of energy from the nucleus to the extended lobes. Maps of "head-tail" sources in clusters of galaxies shows the interplay between the active galaxy and its environment. The Very Long Baseline Array (VLBA) will yield improved measurements of structures on light-year scales in QSOs and provide insights into relativistic motions in AGN. Likewise, new orbiting X-ray observatories promise great advances in sensitivity and spectral resolution.
The Hubble Deep Field and other deep galaxy surveys have led to the measurement of redshifts for galaxies as high as those of QSOs. This is already stimulating increased efforts to understand the interplay between AGN and the formation and evolution of galaxies.
The decline of AGN as an active subject of research is nowhere in sight.