Physical Conditions in Nuclei of Spiral Galaxies. I. Study of Galaxies with a Nuclear Radio- component
Abstract
Summary. In this paper, the nuclei of spiral galaxies are investigated. Their optical properties have been examined in relation to the existence of a narrow radio- source in the nucleus. From the rough study of 89 nuclei of galaxies in Section I, it appears that optical emission lines are preferentially found in nuclei of the radio type. No other conclusions could be drawn, the published data being not precise enough. A detailed study of some objects is then presented. We give the results of scanning observations of fourteen nuclei, through a 10" diaphragm, from 3300 to 7200 A. The final continua have been corrected for reddening and still appear to be separated into two groups: according to the relative intensity of the blue and red light; below, we refer to them as "blue" and "red". Spectroscopic material was obtained for five of these objects. A Carnegie image tube was used in the wavelength range 36007500 A. The absorption and emission features were successively used to determine - the stellar population model best fitting the observed absorption lines. We find that the main contribution, in the visible range is due to A type stars in "blue" nuclei and G type stars in the "red" ones. - the physical parameters of the emitting interstellar The most suitable model has the following characteristics - ionization mechanism by radiation - a low electronic density: Ne 10 - an overabundance of nitrogen by a factor 6 to 17 - a lower electron temperature in the "blue" nuclei (5 10 0K) than in the "red" nuclei (10 0K). Other ionization mechanisms (collisional, relativistic electrons, energetic photons and a 151H radiation) have been examined, but one of them is suitable in these objects. In the radiative case, the ultra-violet radiation field necessary to explain the observed amount of ionized hydrogen is studied for each nucleus in terms of the number of hot stars which are needed. In no case do these hot stars contribute significantly to the continuous flux at > 3500 A. The observed continuum is compared, in each case, to the "synthetic" stellar continuum computed from the stellar population model, and to the thermal radiation from the interstellar gas. No other radiation source is needed to fit the observations in the visible range. Key words: nuclei of galaxies - observations - physical parameters - stellar population - continuum gas.
- Publication:
-
Astronomy and Astrophysics
- Pub Date:
- September 1973
- Bibcode:
- 1973A&A....27..433A