Astronomical instrumentation

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.