Astronomical instrumentation

Abstract:

Spectropolarimetric observations between 8 - 13 μm of BN, IRc 2, 3, 4 and a further region in the Orion infrared complex BNKL are presented. Apart from IRc 2, all show evidence for absorption by aligned grains and have polarization position angles directed at a region close to IRc 2, a property also shared by the self-luminous source IRc 9 at short wavelengths. The evidence strongly suggests that grains are aligned by the intrinsic angular momentum of photons from IRc 2 and it is shown that this is a quantitatively plausible mechanism for the physical conditions in this part of Orion.

Abstract:

The low-resolution spectra of six Seyfert nuclei and the quasar 3C345 are presented, together with four more starburst nuclei spectra observed in a companion study. The spectra were obtained using the UCL LH-cooled grating spectrometer with an array of 25 As:Si photoconductors in a low resolution mode covering a wavelength range of 7.7-13.3 microns. It is shown that the active galaxies are characterized by power law spectra with a tendency for the spectral index to steepen with the progression quasar-Seyfert 1-Seyfert 2 and with silicate absorption in Seyfert 2 galaxies. In contrast, the starburst galaxy spectra are dominated by emission from unidentified dust-associated features and frequently show the fine structure forbidden line of Ne II. The possible origins for the unidentified features in the starburst galaxy spectra are discussed.

Abstract:

Spectra at 8-13 μ m of the nuclei of five nearby bright spiral galaxies are presented. Four of the galaxies, IC 342, NGC 5195, NGC 5236 and NGC 253 display prominent spectral structure arising from gas and dust emission. They resemble the spectra of other galaxy nuclei undergoing vigorous nuclear star formation, and it is argued that the strong emission structure arises from the interaction of many H II regions in the central regions of the galaxies. NGC 4736 shows no evidence for emission in the dust and gas features seen in almost all starburst galaxies observed to date. It has been classed as a LINER, and the 10-μm spectrum resembles those of the Seyfert galaxies. It may be that the low-luminosity mid-infrared emission in this galaxy is more closely related to that seen in Seyfert galaxies rather than those with photoionized H II regions in their nuclei.