Constraints on the outer radius of the broad emission line region of
  active galactic nuclei
  Here we present observational evidence that the broad emission line region
(BELR) of active galactic nuclei (AGN) generally has an outer boundary. This
was already clear for sources with an obvious transition between the broad and
narrow components of their emission lines. We show that the narrow component of
the higher-order Paschen lines is absent in all sources, revealing a broad
emission line profile with a broad, flat top. This indicates that the BELR is
kinematically separate from the narrow emission line region. We use the virial
theorem to estimate the BELR outer radius from the flat top width of the
unblended profiles of the strongest Paschen lines, Pa alpha and Pa beta, and
find that it scales with the ionising continuum luminosity roughly as expected
from photoionisation theory. The value of the incident continuum photon flux
resulting from this relationship corresponds to that required for dust
sublimation. A flat-topped broad emission line profile is produced by both a
spherical gas distribution in orbital motion as well as an accretion disc wind
if the ratio between the BELR outer and inner radius is assumed to be less than
~100 - 200. On the other hand, a pure Keplerian disc can be largely excluded,
since for most orientations and radial extents of the disc the emission line
profile is double-horned.
