The VLT Interferometer - ESO

22 CHAPTER 2. SCIENTIFIC ASPECTS
radiated per unit surface derived from the resolution, even partial, of the
central engine in the continuum will provide for the first time constraints
on the size of the central engine.
The Broad Line Region (BLR) surrounding the central source might be
easier to resolve, at least in high luminosity objects. Indeed [Davidson 1977J
has pointed out the interesting property that the broad emission lines ratios
are roughly constant whatever the intrinsic luminosity of the objects.
It means that the ionizing flux from the central source has always the same
level in the line forming region; at given apparent luminosity, the angular
diameter of the BLR should be the same, whatever the redshift z as long
as it is smaller than "-' 0.5. For larger z, at given apparent magnitude,
the larger is z, the larger should be the angular diameter. The extension
of the BLR ranges around JL/10 47 in pc. In fact the high resolution
of the VLT Interferometer is still required, to get information about the
fine structure of the BLR which is likely highly anisotropic and probably
comparable to the radio VLBI structure. For instance in 3C273, the BLR
could be fully resolved with a baseline 100 m long.
Finally the Narrow emitting Line Region has dimensions about 10 3 larger
than the BLR. Its structure is already marginally known from speckle interferometry
studies in a few bright and close nuclei (NGC 1068, NGC
4151). [Afanas'jev et al 1988] have observed at the wavelengths of the
narrow Ho: and [OIII] emission lines in NGC 1068. They report that the
unresolved central source is surrounded by an elliptical component whose
eccentricity as well as the position angle varies with the distance to the
centre, fitting the radio continuum map. To get a picture of the whole
nucleus, would be a primary goal of the VLT Interferometer and would
in particular allow to test the unifying model.
Infrared observations are required to address the question of determining
the fraction of the energy coming from the non-stellar central source and
from the star formation. What is the emission process( es) accounting for
the infrared radiation? Is it a non-thermal synchrotron source, expected
to be unresolved? Is it a non-thermal radiation possibly scattered by an
electron cloud, as suggested for NGC 1068 ? Is it the thermal radiation of
the dust heated by the central source? In that case, the infrared source is
expected to be small, but resolved. Is it a thermal radiation from an extended
star formation region? Is it the thermal radiation by dust heated
through shocks during a merger? From speckle interferometry observa-