MIDI Sees Active Galactic Nuclei

Abstract

One of the primary motivations for building the VLTI was to observe the inner, dusty regions of Active Galactic Nuclei (AGNs). Although the VLTI is limited to relatively bright objects, there are a dozen or so Seyfert galaxies, and at least one quasar, that are bright enough to see. These are the only extragalactic targets so far observed with optical/infrared interferometers. The VLTI mid-Infrared instrument, MIDI, is a spectroscopic instrument working in the N-band (8–14 μ) which corresponds to the peak emission from blackbodies at ∼ 300 K. With a few exceptions, for example some cases of synchrotron emission, MIDI has mostly been used to observe warm dust at near this temperature. For AGNs the primary questions concerning dust are related to the unified theories that attempt to reduce the bewildering number of AGN types by attributing many of their characteristics to the obscuration of the nuclear regions by a dust structure, perhaps toroidal in form, with a characteristic size of a parsec or so. This scale size is determined by the typical UV luminosity of an AGN core, ∼ 1044 erg s−1, and the need to keep the dust cooler than its sublimation temperature, ∼ 1500 K. For the nearest extragalactic AGNs at distances of ∼ 10 Mpc direct measurement of the dust morphology then requires a resolution of order 10 milliarcsec (mas). Single telescope observations of AGNs have shown mid-IR SEDs that prove the existence of warm dust there, but attempts to constrain the nature of the torus from these SEDs have proved difficult. Thus the primary motivation of the interferometric measurements was to image the dust structures and determine their shape, size, temperature and orientation with respect to the axes of the galaxy, the radio structures and the larger scale ionization structures. We also hoped to learn something about their chemistry from the shape of the silicate emission feature at 10 μ and possibly about their degree of inhomogeneity or “clumpiness”. The crudest taxonomic classification divides AGNs into “Type 1” where the torus is seen face-on, and “Type 2” where it is seen edge-on. In the first case we expect to see the AGN core as a bright unresolved point source surrounded by a near-circular cooler emission disk. In the second case we would only see the cooler disk and we would expect it to be flattened.