Oxygen and Neon Abundances of Planetary Nebulae in the Elliptical Galaxy NGC 4697

Abstract

This paper presents deep spectra of 14 planetary nebulae (PNs) at a variety of angular distances from the center of the flattened elliptical galaxy NGC 4697, which is located at similar to 11 Mpc from us. Both near the center and among the most outlying PNs, we have found several examples of very strong [O III] lambda 5007, about 20 times brighter than H beta. This, together with strong [ Ne III] lines, implies a lower limit for O and Ne abundances near solar at the center and also at more than 2 effective radii from the center of NGC 4697. Thus we have found, for the first time from individual stars, direct evidence of the existence of a metal-rich population in elliptical galaxies, confirming metallicities derived from absorption-line indices and extending the information to angular distances where the stellar surface brightness is too low for absorption-line studies. A comparison with abundances obtained closer to the center of NGC 4697 from integrated absorption-line spectra indicates that the metallicity gradient, which is quite steep within 1 effective radius, becomes much less pronounced beyond 1 effective radius. The only alternative to this interpretation would be the presence of a very metal-poor PN population with [Z/H] below -1, which we cannot rule out, but it would require an extremely bimodal metallicity distribution with almost no intermediate metallicities.