Astronomical instrumentation

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.

Abstract:

Optical spectroscopy and near-infrared photometry of 19 optically faint galaxies which have been discovered as intense 100-micron sources by IRAS are reported. All have low-excitation emission lines with redshifts up to 0.2, and some exceed 10 to the 12th solar luminosity in their infrared luminosities. No evidence of Seyfert-like spectra is found, but instead starburst activity is suspected to power the infrared emission. Many of the galaxies are clearly disturbed or show evidence of interaction with neighbors; the highest redshift source consists of a trio of emission galaxies. By selecting IRAS galaxies with faint optical counterparts, a subset of infrared-luminous galaxies which do not show the Seyfert characteristics seen in surveys of extragalactic IRAS sources identified with optically brighter objects is isolated. The intrinsic luminosities of some of the sample are higher than those of any previously known active galaxies, and are comparable to those of many quasars.