Although our understanding of the AGN phenomenon has increased dramatically, major fundamental issues remain unsettled. These include the precise mechanisms involved in feeding the central black hole, the relationship between AGN activity and ultraluminous starbursts, and the reason why some AGN in elliptical galaxies are radio loud (but not those in spirals). The relationship between members of the AGN zoo, ranging from the brightest quasars to barely-active Seyfert galaxies, is not understood.
There has been much debate on the relation between radio-loud quasars and radio galaxies (for a review see Antonuci, 1993). It could be that these objects are intrinsically similar but with differences that can be related either to their evolution, their environment or their orientation to our line of sight. The third possibility is currently the most favoured. In this scenario, the bright nucleus is surrounded by an obscuring torus of dust and gas, resulting in twin beams of continuum emission. If our line of sight falls along one of these beams then we see a quasar, otherwise the object appears as a radio galaxy.
A similar explanation can be applied to Seyferts, in which double cone-shaped emission has been observed directly. In Seyfert 1 galaxies, the Broad Line Region (BLR) can be seen in emission lines, but in Seyfert 2s our line of sight to the nucleus is obscured by the torus, making the BLR undetectable except in scattered light. One way to investigate the unified model directly is to image the very central part of the active nucleus.
Infrared imaging of the central regions is clearly important for our understanding of AGN. Dust near the nucleus will be heated by the UV flux from the central engine and there is good evidence that most, if not all, the infrared emission from AGN comes from heated dust (see Barvainis, 1992). The size of the emitting region is quite small. For example it is 5h-175 pc or 0.16" +/- 0.04 (1sigma ) in NGC 4151 (Neugebauer et al. 1990) at 10 microns.
About 20-30 nearby Seyfert galaxies are bright enough to be used as references for fringe tracking. For these, the central parsec will be probed in the optical and infrared. It is also useful to probe larger scales in more distant objects, to trace any cosmological evolution. Figure 5 shows simulated images of a typical AGN observed at different angular resolutions and Figure 6 shows how the angular sizes of the relevant regions scale with redshift.
If fringe tracking cannot be done on the object itself, a nearby reference star can be used. It is thus important to search for new objects (radio selected or by-products of planned surveys) located near bright stars. The overall impact of VLTI in extra-galactic astronomy will depend on the sky coverage. Calculations for the adaptive optics system on a unit telescope in the visible predict a sky coverage of about 1% (Theodore et al., 1994). This increases by a factor of 4-5 if the correction is done in the near infrared and becomes even greater in the mid-infrared. A catalogue of bright stars in the near infrared is needed to search efficiently for observable objects.
Figure 6: Angular extension as a function of redshift for regions with linear sizes of 1 pc (open squares), 10 pc (open triangles), 100 pc (filled squares) and 1 kpc (open circles). The calculations assumed H0=75 km/s/Mpc and qo=0.5. The difference in look-back time between adjacent symbols is constant and equal to about 0.5Gyr.