There is now general consensus that the long-standing paradigm for active galactic nuclei (AGNs) is basically correct, i.e., that AGNs are fundamentally powered by gravitational accretion onto supermassive collapsed objects. Details of the inner structure of AGNs, however, remain sketchy, although both emission lines and absorption lines reveal the presence of large-scale gas flows on scales of hundreds to thousands of gravitational radii. The accretion disk produces a time-variable high-energy continuum that ionizes and heats this nuclear gas, and the broad emission-line fluxes respond to the changes in the illuminating flux from the continuum source. The geometry and kinematics of the broad-line region (BLR), and fundamentally its role in the accretion process, are not understood. Immediate prospects for understanding this key element of AGN structure do not seem especially promising with the realization that the angular size of the nuclear regions projects to only microarcsecond scales even in the case of the nearest AGNs. Unfortunately, there is only very limited information about the BLR from the emission-line profiles alone, since many simple kinematic models are highly degenerate. Nevertheless, it has been possible to draw a few basic interferences about the nature of the BLR:
The conclusion that gravity is important leads us directly to an estimate of the black hole mass, which we take to be
where G is the gravitational constant and f is a scaling factor of order unity that depends on the presently unknown geometry and kinematics of the BLR.
In this brief introduction, we already see the two major reasons that understanding the BLR is of critical importance to understanding the entire quasar phenomenon: (1) we need to understand how the accretion/outflow processes work in AGNs and (2) we need to understand the geometry and kinematics of the BLR to assess possible systematic uncertainties in AGN black-hole mass measurements.