Gradients of absorption-line strengths in elliptical galaxies

Abstract

We have restudied line-strength gradients of 80 elliptical galaxies. Typical metallicity gradients of elliptical galaxies are Delta[Fe/H]/Delta log r similar or equal to -0.3, which is flatter than the gradients predicted by monolithic collapse simulations. The metallicity gradients do not correlate with any physical properties of galaxies, including central and mean metallicities, central velocity dispersions sigma(0), absolute B magnitudes M-B, absolute effective radii R-e, and dynamical masses of galaxies. By using the metallicity gradients, we have calculated mean stellar metallicities for individual ellipticals. Typical mean stellar metallicities are [[Fe/H]] similar or equal to -0.3 and range from [[Fe/H]] similar or equal to -0.8 to +0.3, which is contrary to what Gonzalez & Gorgas claimed; the mean metallicities of ellipticals are not universal. The mean metallicities correlate well with sigma(0) and dynamical masses, though relations for M-B and R-e include significant scatters. We find fundamental planes defined by surface brightnesses SBe, [[Fe/H]], and R-e (or M-B), the scatters of which are much smaller than those of the [[Fe/H]]-R-e (or [[Fe/H]]-M-B) relations. The [[Fe/H]]-log sigma(0) relation is nearly parallel to the [Fe/H](0)-log sigma(0) relation but systematically lower by 0.3 dex; thus the mean metallicities are about one-half of the central values. The metallicity-mass relation or, equivalently, the color-magnitude relation of ellipticals holds not only for the central parts of galaxies but also for entire galaxies. Assuming that Mg-2 and Fe-1 give [Mg/H] and [Fe/H], respectively, we find [[Mg/Fe]] similar or equal to +0.2 in most of elliptical galaxies. [[Mg/Fe]] shows no correlation with galaxy mass tracers such as sigma(0), in contrast to what was claimed for the central [Mg/Fe]. This can be most naturally explained if the star formation had stopped in elliptical galaxies before the bulk of Type Ia supernovae began to occur. Elliptical galaxies can have significantly different metallicity gradients and [[Fe/H]], even if they have the same galaxy mass. This may result from galaxy mergers, but no evidence is found from presently available data to support the same origin for metallicity gradients, the scatters around the metallicity-mass relation, and dynamical disturbances. This may suggest that the scatters have their origin at the formation epoch of galaxies.