Prof Patrick Roche

Abstract:

The authors present observations of the nucleus of NGC 4418 which show it to be perhaps the first true infrared galaxy. NGC 4418 was found to be very bright in the infrared by IRAS, and ground-based observations have been employed to investigate the nature of the infrared emission. Optical spectra show little evidence of the luminous infrared core of the galaxy, revealing only stellar absorption features and very weak emission lines. The infrared spectra have a very deep minimum at 9.7 μm, implying a visual extinction of very large50 mag and indicating that the source producing the infrared luminosity is completely obscured in the visible. From the relatively warm IRAS colours, and the non-detection of dust or ionic line emission structure in the 8 - 13 μm spectrum, it is likely that NGC 4418 harbours a very heavily obscured active nucleus.

Abstract:

The depth of the 9.7-microns silicate absorption feature towards several of the mid-infrared sources within 2 pc of the center of the Galaxy has been measured. The ratio of the depth of the silicate absorption to the extinction at shorter wavelengths is found to be about twice that determined for sources in the solar neighborhood. The most plausible explanation for this difference is that in the central regions of the Galaxy, there are very few carbon stars, and so the production of carbon-rich dust grains will be much reduced compared with the outer Galactic disc. That the A(V)/tau(9.7) ratio depends on the ratio of C to M-type stars lends support to those models of interstellar dust in which carbon-rich grain materials are responsible for much of the extinction in the visible and near-infrared, whilst those that produce the 9.7-micron absorption feature are formed in oxygen-rich environments.