91̽»¨

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.

Abstract:

We present 8- to 13-μm spectra of the main-belt asteroids 1, 2, 6, 7, 10, 16, 45, 51, 56, 65, 78, and 451. None of these exhibit pronounced emission features, and all can be fitted reasonably well using a standard thermal model. We also present wide- and narrowband photometry of 19 Fortuna, which has previously been reported to show an emission feature in this region. © 1985.

Abstract:

Spectra of the central core and surrounding coma of Comet IRAS-Araki-Alcock (1983d) were obtained at 8-13 μm on 11 May and 2-4 μm on 12 May 1983. Spatially resolved measurements at 10 μm with a 4-arcsec beam showed that the central core was more than 100 times brighter than the inner coma only 8 arcsec away; for radially outflowing dust, the brightness ratio would be a factor of 8. The observations of the central core are consistent with direct detection of a nucleus having a radius of approximately 5 km. The temperature of the sunlit hemisphere was > 300 K. Spectra of the core are featureless, while spectra of the coma suggest weak silicate emission. The spectra show no evidence for icy grains. The dust producton rate on 11.4 May was ∼ 105 g/sec, assuming that the gas flux from the dust-producing areas on the nucleus was ∼ 10-5 g/cm2/sec. © 1985.

Abstract:

Although most of the luminosity of Seyfert galaxy nuclei is emitted in the IR, the relative amounts of thermal dust and non-thermal emission are uncertain. Seyfert galaxies generally have smooth and featureless IR spectra, in contrast to the prominent dust emission features seen in galaxies whose nuclei resemble giant H II regions - the star burst nuclei1,2. Nevertheless, in type 2 Seyferts, the general form of the energy distribution and lack of marked variability suggest that here too thermal dust emission is dominant 3-5, and in NGC1068 this supposition is 91̽»¨ed by the resolved size of its nucleus6. Dust is observed in absorption at 10 m (refs 7,8) and in emission near 20 m (ref. 9). Polarization has been detected from the UV to 10 m, and attributed to scattering and absorption at short wavelengths10 with a possible non-thermal component11,12, to scattering and non-thermal radiation in the near IR9, and to either absorption or non-thermal emission in the 10 m region13. We present here spectropolarimetric measurements of NGC1068, between 8 and 13 m, which show that the polarized flux at these wavelengths is intrinsic to the emission mechanism and not due to absorption by aligned grains. © 1984 Nature Publishing Group.