Astronomical instrumentation

Abstract:

The results of 8-13 micron spectropolarimetric observations of Galactic Center sources are presented. All the sources show interstellar polarization due to absorption by aligned silicate grains, and these grains appear to be spatially separated along the line-of-sight from those which produce the polarization at shorter wavelengths. The line of sources IRS 1, 10, 5, and 8 comprising the northern arc all show strong intrinsic polarization due to thermal emission from aligned grains of amorphous silicate-like material. This polarization is very uniform among these sources with position angle closely normal to the line of the arc. It is shown that the data are inconsistent with grain alignment due to streaming, but favor alignment by a strong magnetic field greater than 10 mG directed along and linking the northern arc of sources. A field of this magnitude will have significant influence on the structure and evolution of sources in the Galactic Center. Some possible implications are discussed.

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.