The origin and physical mechanism of the ensemble Baldwin effect
  We have conducted a systematic investigation of the origin and underlying
physics of the line--line and line--continuum correlations of AGNs,
particularly the Baldwin effect. Based on the homogeneous sample of Seyfert 1s
and QSOs in the SDSS DR4, we find the origin of all the emission-line
regularities is Eddington ratio (L/Ledd). The essential physics is that L/Ledd
regulates the distributions of the properties (particularly column density) of
the clouds bound in the line-emitting region.
