COLOR DISTRIBUTIONS IN EARLY TYPE GALAXIES .3. RADIAL GRADIENTS IN SPECTRAL FEATURES

Long-slit spectra have been obtained of three Virgo Cluster ellipticals in the spectral range lambda-lambda 5000-9500. By ratioing spectra from different galactocentric distances within the same galaxy, very small changes in the strengths of spectral features can be measured. The original intent of this study was to test the idea that the inward bluing of several hundredths of a magnitude in the r-i color of NGC 4472 and NGC 4486 (M87) discovered in previous work is due to strong TiO absorption in the i band. Using these spectra we conclude that the r-i blueing was predominantly an instrumental effect, with the exception of NGC 4486, in which nuclear emission lines account for one-fourth of the observed gradient. In addition, we use the ratioing technique to extract information relevant to a number of other problems. (1) We detect a very weak emission-line spectrum in the nucleus of NGC 4472, corresponding to 300 M- of ionized gas. A comparison with M31 shows that the ionized gas mass to stellar luminosity ratio in corresponding regions around the nuclei are the same. (2) We detect very weak emission lines in the nucleus of M87 including the ]N II[ lambda 5755 line which can be used with the ]N ii[ lines lambda-lambda 6548, 6584 to determine an electron temperature of 8800 K. (3) We measure the gradients and nuclear values of a number of absorption features in NGC 4472. After velocity dispersion corrections, it is found that the Na I-lambda-8190 feature is much stronger in the nucleus than any models in the literature. This finding is inconsistent with a recent study by Terndrup et al. (1990). This line and Na D 5895 show quite large increases in strength in the nucleus, while all other features measured, with the exception of the Ca triplet, show modest gradients. It is impossible to distinguish whether this is due to very high metallicity, possibly involving an enhancement of the Na abundance, or is due to dwarf enrichment in the nucleus.