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Optical Observations of Nuclei of Galaxies

Published online by Cambridge University Press:  14 August 2015

Marie-Helene Ulrich
Marie-Helene Ulrich*
Affiliation:
Dept. of Astronomy, University of Texas at Austin, Austin, Tex. 78712, U.S.A. and Observatoire de Paris, 92190 Meudon, France

Abstract

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Several problems concerning the nuclei of galaxies are particularly interesting at present, either because new data have become available from advances in observational techniques, or else because recent observations suggest that earlier conclusions should be revised. These problems are discussed in this article.

Spectrophotometry in the near infrared shows that the late-type stars in the nucleus of M31 are mostly late giants; the M/L ratio in the nucleus is approximately 15, a value in agreement with that calculated independently from the velocity dispersion of the stars.

Infrared emission at 10µ has now been detected in 50 galaxies; the Seyfert-type galaxies are the brightest at 10 µ and there is evidence of variations of the 10 µ flux in NGC 4151, which rules out re-radiation by dust as the mechanism of infrared emission. In each of the galaxies NGC 253 and NGC 3034 an infrared source of dimension ~ 200 pc is present in the centre which coincides in position and dimension with the central component of the radio source at 1415 MHz; since both galaxies are rich in dust and in early-type stars it is plausible that, in these cases, re-radiation by dust is the origin of the infrared emission. Most of the other galaxies detected at 10 µ are spirals with intense and narrow emission lines.

Comparison of the optical and radio properties of nuclei of galaxies show that galaxies with compact nuclear radio sources are more likely to have optical spectra with emission lines than are galaxies without central radio sources. This correlation holds for elliptical and spiral galaxies. Its cause is not understood at present.

The relatively accurate profiles now available for the permitted lines of several Seyfert galaxies show that mass motions are the main broadening agent for these lines. Variations of line intensities have been observed in several Seyfert galaxies. The most likely explanation of these variations is that they are due to the variations of the photoionizing flux; in some cases, however, weakening of the permitted lines could be caused by high-velocity mass motions present in the region emitting the broad component of these lines.

From the investigation of the relatively nearby galaxies it is estimated that at least 5 % of galaxies have expansion motions or non-thermal optical fluxes.

Type
Research Article
Information
Symposium - International Astronomical Union , Volume 58: The Formation and Dynamics of Galaxies , 1974 , pp. 279 - 304
Copyright
Copyright © Reidel 1974 

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