STELLAR POPULATION GRADIENTS IN EARLY-TYPE GALAXIES

Long-slit spectroscopy, covering the wavelength range 3600-4600 angstrom at 5.3 angstrom FWHM resolution, has been obtained for a sample of seven elliptical and SO galaxies. The two-dimensional spectra have been spatially binned to provide good signal-to-noise ratio (S/N approximately 40 per 5.3 A spectral resolution element), spatially resolved spectra out to 0.5-1 half-light radii. Spectral indices have been extracted from the binned spectra, and correlations among the indices have been examined. The CN-D(4000) diagram is shown to be effective at separating the effects of metallicity and age on the integrated spectra. All galaxies in the sample have lower metallicity at larger radii, with gradients of similar magnitude to those found in previous studies. Radial gradients in the age of the most recent epoch of significant star formation have been searched for. The lack of adequate stellar population synthesis models covering the necessary metallicity range prevents a measurement of the absolute age gradient in these galaxies, however, empirically it is shown that four of the seven galaxies possess similar, and probably small or null, age gradients, while the remaining three galaxies show evidence for a decrease in age with increasing radius. Whether the radial age gradients are due simply to a difference in age between the disk and bulge components or also reflect age gradients within the individual components has not been determined. The mean ages of the individual galaxies vary by several gigayears, with the youngest bulges belonging to the most bulge-dominated systems.