Observations and interpretations of radial gradients of absorption features in galaxies

Abstract

Observations are presented of equivalent widths for strong spectral features with λ > 5000 Å in the nuclei of five galaxies (M31, M32, M81, NGC 4472, and NGC 3115) as well as the radial gradients of line strength within these galaxies. A theoretical model for predicting the strengths of features in the integrated light of galaxies is also presented. It is concluded, on the basis of this limited sample of galaxies, that the changes in spectral features between galactic nuclei and within galaxies arise from a single causal factor, which is the overall metallicity of the galaxy. The range in nuclear abundance spanned by the five galaxies is a factor of 4. There is no evidence for nonsolar ratios among the elements with lines that have been studied, namely, Ca, Na, Mg, and Fe; specifically, the approximate constancy of the Fe blends within and between galaxies has a natural explanation. Furthermore, the nuclear metallicity ordering is the same as the ordering of M_v.

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