What broad emission lines tell us about how active galactic nuclei work
  I review progress made in understanding the nature of the broad-line region
(BLR) of active galactic nuclei (AGNs) and the role BLRs play in the AGN
phenomenon. The high equivalent widths of the lines imply a high BLR covering
factor, and the absence of clear evidence for absorption by the BLR means that
the BLR has a flattened distribution and that we always view it near pole-on.
The BLR gas is strongly self-shielding near the equatorial plane.
Velocity-resolved reverberation mapping has long strongly excluded significant
outflow of the BLR and shows instead that the predominant motions are Keplerian
with large turbulence and a significant net inflow. The rotation and turbulence
are consistent with the inferred geometry. The blueshifting of high-ionization
lines is a consequence of scattering off inflowing material rather than the
result of an outflowing wind. The rate of inflow of the BLR is sufficient to
provide the accretion rate needed to power the AGN. Because the motions of the
BLR are gravitationally dominated, and the BLR structure is very similar in
most AGNs, consistent black hole masses can be determined. The good correlation
between these estimates and masses predicted from the bulge luminosities of
host galaxies provides strong support for the similarity of AGN continuum
shapes and the correctness of the BLR picture presented. It is concluded that
although many mysteries remain about the details of how AGNs work, a general
overall picture of the torus and BLR is becoming clear.
