The nuclear colours of weakly active elliptical galaxies

Astronomy and Astrophysics – Astronomy

Scientific paper

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Active Galaxies, Astronomical Spectroscopy, Elliptical Galaxies, Galactic Nuclei, Radio Galaxies, Abundance, Color, Metallicity, Ubv Spectra

Scientific paper

Long-slit spectroscopy has been obtained of 15 elliptical radio galaxies. Although many of these show radio emission from their nuclei, and might be expected to have weak H-alpha emission lines, this does not seem to be associated with a nonthermal ionizing continuum extending to the optical region. Rather, most nuclear regions are redder than the surrounding regions by a larger amount than expected from the color-aperture effect, although about 20 percent of the nuclei are bluer. Evidence from the magnesium index suggests that the reddening is due to abundance effects, and the colors then imply excess metallicities by factors of about 1.5. These colors and metallicities are not as expected from straightforward predictions by Bailey's (1982) model for active nuclei. It seems more likely that the apparent excess redness over the standard color aperture effect only shows that the color gradient is generally steeper in the nucleus than previously thought, rather than a distinct stellar population being responsible. The blue nuclei may contain either a power-law continuum source, or young stars.

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