Stellar Absorption Lines and the Nuclear Continuum in Seyfert Galaxies

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Session 108 -- Seyfert Galaxies
Display presentation, Thursday, 12, 1995, 9:20am - 6:30pm

[108.12] Stellar Absorption Lines and the Nuclear Continuum in Seyfert Galaxies

C. H. Nelson (STScI), M. Whittle (U. Va.)

The stellar absorption lines in the nuclear spectra of Seyfert galaxies are diluted by a strong featureless continuum. In the standard model for active galaxies the continuum is produced by non-thermal processes in the nucleus, usually following a power-law of the form $f_{\nu} \sim \nu^{-\alpha}$. However, a young stellar population will also produce a featurless continuum. Because star formation and nuclear activity frequently occur in the same galaxy it is often unclear how much of the nuclear continuum emission arises from a population of young stars and how much is truly non-thermal in origin.

By comparing the strengths of stellar absorption features in different spectral regions we can attempt to distinguish the contributions of various emission components to the overall spectral energy distribution. The Ca II triplet ($\sim \lambda 8600$\AA) and the Mg b lines ($\sim \lambda 5200$\AA) are well suited for such an analysis since they are separated in wavelength and are also strong in normal stellar populations. We present an analysis of the strengths of these features in spectra of Seyfert nuclei.

We find that for most of our sample the line strengths are consistent with dilution of a normal galaxy spectrum by a power law continuum, in accord with the standard model for AGN. However, for a number of Seyferts in our sample, it appears that dilution by a power law continuum cannot completely explain the relative line strengths and an additional source of continuum emission seems necessary to account for the dilution of the Mg lines. A similar result was reported by Terlevich et al. (1990) who attributed excess Ca II triplet line strength to a population of M supergiants in a concentrated nuclear star cluster. Although we agree that a burst of star formation may produce the observed Mg~b dilution, a more detailed investigation involving the off-nuclear spectral characteristics and modeling of the effects of young stellar populations is required.

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