Mid-infrared Interferometry of AGNs with VLTI-MIDI

Jaffe and members of the MIDI team accomplished the first successful detection of an active galactic nucleus (AGN -NGC 1068) with an interferometer in the mid-infrared wavelength range. Their scientific aim was to determine the structure of the 'obscuring torus' which was postulated for decades as the cause of the observational differences between AGNs of the Seyfert 1 and Seyfert 2 types. The former show an intense nonthermal central source and broad emission lines. These are lacking in the latter and assumed to be extinguished by a thick dust structure. The MIDI measurements spatially and spectrally resolved emission at 10$\mu$m from a structure about $4\times2$ parsec across in NGC 1068. Jaffe and collaborators found the overall size and temperature to be consistent with existing model predictions for a torus, but the detailed temperature distribution did not fit any existing model. The 10$\mu$m band is dominated by absorption from silicate minerals in the dust. However, the profile from the central regions of the galaxy differed from that normally seen in Galactic stars, suggesting that the higher temperature in the former has changed the minerals in the dust.

図: Observed MIDI spectra from the nucleus of NGC 1068 compared with model predictions. Thin jagged lines show observed flux versus wavelength, hatched areas show r.m.s. scatter, and thick smooth lines show model predictions. The absorption dip near 10 $\mu$m is caused primarily by silicate dust.