NEAR-INFRARED SPECTRAL GRADIENTS IN ELLIPTICALS AND BULGES, AND THE NATURE OF THE NA FEATURE NEAR 8200-A

Spatially resolved spectrophotometric data in the 8000-10 000 angstrom spectral interval is presented for a sample of 10 bulges and ellipticals. We have found that in all galaxies observed the IR Na feature near 8200 angstrom extends further to the red than expected from the wavelength of the Na I doublet lines alone, confirming the presence of additional spectral components. A study of the radial dependence of the Faber-French Na I spectral index and of the IR Ca II triplet shows that the former decreases with radius while the latter either decreases or remains constant, but never increases. Furthermore, we have found by measurements at the center of galaxies that the strength of the Na indices correlates both with the strength of the Mg2 metallicity indicator and with the absolute magnitude of the galaxy. It is concluded that the evidence claimed for dwarf enrichment of the central regions of early type galaxies on the basis of the Na feature, as measured by spectral indices, was very likely the result of metallicity effects. Comparison of the galaxy spectra at different radial positions provides indications that this metallicity effect can be due, at least in part, to Na lines arising in metal-rich M giants. We have identified a TiO feature that increases with radius for all galaxies studied. We speculate that this may be the signature of a stacking of giant branches with metallicity analogous to that seen in color-magnitude diagrams of the Galactic Bulge.