Physical Conditions in Nuclei of Spiral Galaxies. III. Analysis of Nuclei of Southern Spiral Galaxies
Abstract
Summary. In this study, we make use of the spectroscopic and photometric results presented in the preceding paper (Alloin and Sareyan, 1974), in order to determine the physical parameters of the emissive gas in the nuclei of some southern spiral galaxies. First of all, we discuss in detail the reddening corrections that are usually applied to these objects: in all cases, this correction appears to be quite uncertain, either because it is a mean correction over a given morphological type of galaxy, or because the correction is based on unjustified physical assumptions. Until we make appropriate observations ([S ii] lines of the same multiplet in the visible range and the infra-red), this uncertainty remains unavoidable. We next investigate the use of both photometric and spectroscopic results, which, generally, do not refer to the same observed region in the nucleus. This source of error - although it is negligible in some cases (quasi- stellar nuclei) - must be kept in mind and assessed whenever possible. It could be reduced by measurements of photometric profiles over the nuclei at different wavelengths. Using the above, we have analyzed the spectroscopic and photometric data. From the emission line intensities, we have deduced physical parameters for the emissive gas in the nuclei of the galaxies NGC 1566 and 7469; NGC 7496 and 7552, and for the large H ii regions in the irregular galaxies NGC 6822 and IC 4662. A comparative study of all the observed nuclei has led to the following conclusions: The nuclei of the Seyfert galaxies, NGC 1068, 1566 and 7469, exhibit strong UV and blue continuous radiation. Their "gas component/stellar component" ratio appears slightly larger than in other nuclei. Regarding their continuous spectral distribution and the physical parameters of the emissive gas, the nuclei of the galaxies NGC 1084, 1097, 7496 and 7552 have a behaviour quite comparable to that of nuclei already studied in detail, such as NGC 2903 and 3351: significant amounts of blue and UV radiation, and moderate excitation and ionization conditions in the gaseous component. They could thus be added to the "blue" class of nuclei (Alloin, 1973). The nucleus of the galaxy NGC 253 presents a continuum which is related rather to the "red" class of nuclei; however it is well known to have a strong dust component and therefore, the reddening correction is particularly doubtful in this case. Finally, we find that a radiative ionization mechanism is the most suitable in all cases. In so far as the correlation between the presence of a narrow radio-component and peculiar properties in the visible range is concerned, we note that practically all of these nuclei, with just one exception, have such a radio core; they also exhibit emission lines and/or blue and UV radiation. Key words: spiral galaxies - nuclei - reddening - physical parameters
- Publication:
-
Astronomy and Astrophysics
- Pub Date:
- July 1974
- Bibcode:
- 1974A&A....33..337A