2005ApJ...627..767M

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 ∼11 Mpc from us. Both near the center and among the most outlying PNs, we have found several examples of very strong [O III] λ5007, about 20 times brighter than Hβ. 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.