Evolution of Broad-line Emission from Active Galactic Nuclei
  Apart from viewing-dependent obscuration, intrinsic broad-line emission from
active galactic nuclei (AGNs) follows an evolutionary sequence: Type $1 \to
1.2/1.5 \to 1.8/1.9 \to 2$ as the accretion rate onto the central black hole is
decreasing. This spectral evolution is controlled, at least in part, by the
parameter $L_{\rm bol}/M^{2/3}$, where $L_{\rm bol}$ is the AGN bolometric
luminosity and $M$ is the black hole mass. Both this dependence and the
double-peaked profiles that emerge along the sequence arise naturally in the
disk-wind scenario for the AGN broad-line region.
