Coexistence of Gravitationally Bound and Radiation Driven CIV Emission
  Line Regions in Active Galactic Nuclei
  There are mutually contradictory views in the literature of the kinematics
and structure of high-ionization line (e.g. CIV) emitting regions in active
galactic nuclei (AGNs). Two kinds of broad emission line region (BELR) models
have been proposed, outflow and gravitationally bound BELR, which are supported
respectively by blueshift of the CIV line and reverberation mapping
observations. To reconcile these two apparently different models, we present a
detailed comparison study between the CIV and MgII lines using a sample of AGNs
selected from the Sloan Digital Sky Survey. We find that the kinematics of the
CIV region is different from that of MgII, which is thought to be controlled by
gravity. A strong correlation is found between the blueshift and asymmetry of
the CIV profile and the Eddington ratio. This provides strong observational
support for the postulation that the outflow is driven by radiation pressure.
In particular, we find robust evidence that the CIV line region is largely
dominated by outflow at high Eddington ratios, while it is primarily
gravitationally bounded at low Eddington ratios. Our results indicate that
these two emitting regions coexist in most of AGNs. The emission strength from
these two gases varies smoothly with Eddington ratio in opposite ways. This
explanation naturally reconciles the apparently contradictory views proposed in
previous studies. Finally, candidate models are discussed which can account for
both, the enhancement of outflow emission and suppression of normal BEL, in AGN
with high Eddington ratios.
