A Spectrophotometric Study of Seyfert Nuclei

Citation

Anderson, Kurt Steven Jarl (1969) A Spectrophotometric Study of Seyfert Nuclei. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/H52P-ZH29. https://resolver.caltech.edu/CaltechETD:etd-05022008-085808

Abstract

Photoelectric and photographic spectrophotometry have been utilized to infer some of the properties of the nuclear regions of eight Seyfert galaxies and two additional objects displaying somewhat more normal spectra. The observations were principally made during 1966 and 1967 and cover much of the region between 3300Å and 10,000Å. For each of the objects, absolute continuous energy distributions are obtained. In addition, the relative and absolute emission-line intensities and profiles were derived from the observational data. It is found that the emission-line profiles, and the profiles of the Balmer line cores can be consistently interpreted in terms of mass-motions and/or turbulence within the emitting regions. The wings of the Balmer lines are difficult to interpret in this fashion and evidence is presented which indicates that the relevant broadening mechanism is electron scattering. This process requires that the emission nuclei possess significant optical depth in the Balmer lines. The anomalously steep Balmer decrements which are observed, as well as the presence of the Balmer wings, may also be consistent with such optical depth. Absorption lines seem to be extremely weak, or absent, in the nuclear spectra and the continuous energy distributions bear little resemblance to those of normal nuclei. The presence of a relatively flat non-thermal component to the observed spectrum is indicated. Approximate forms for these non-thermal spectra are derived from the observational data and auxiliary calculations. It is found that this non-thermal flux constitutes a large part of the blue and ultraviolet fluxes observed in the Seyfert nuclei. It is this component which is responsible for the unusually blue colors of these objects and for the weakness of the absorption lines. This non-thermal component may also be responsible for the observed wide ranges of ionization. Rough calculations indicate that sufficient ionizing flux will result from an extrapolation of the optical non-thermal spectrum to the far ultraviolet.

It is concluded that the Seyfert nuclei are objects characterized by large internal velocity dispersions, a sizeable non-thermal component, and significant optical thickness in the Balmer lines. The levels of ionization are quite mixed in the line-emitting regions and the source of this ionization is either synchrotron photons or the conversion of the kinetic energy of cloud motions into ionization energy by means of cloud interactions. Of the two, the former seems the more likely. Finally, the sources of the excitation are considered, but no clear distinction between the various mechanisms is possible on the basis of the available data.

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