Gradients of absorption-line strengths in elliptical galaxies

We have restudied line-strength gradients of 80 elliptical galaxies. Typica l 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 metal licities, central velocity dispersions sigma(0), absolute B magnitudes M-B, absolute effective radii R-e, and dynamical masses of galaxies. By using t he metallicity gradients, we have calculated mean stellar metallicities for individual ellipticals. Typical mean stellar metallicities are [[Fe/H]] si milar 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 metalli cities of ellipticals are not universal. The mean metallicities correlate w ell with sigma(0) and dynamical masses, though relations for M-B and R-e in clude significant scatters. We find fundamental planes defined by surface b rightnesses 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 metallicitie s are about one-half of the central values. The metallicity-mass relation o r, equivalently, the color-magnitude relation of ellipticals holds not only for the central parts of galaxies but also for entire galaxies. Assuming t hat Mg-2 and Fe-1 give [Mg/H] and [Fe/H], respectively, we find [[Mg/Fe]] s imilar 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 i f the star formation had stopped in elliptical galaxies before the bulk of Type Ia supernovae began to occur. Elliptical galaxies can have significant ly 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 g radients, the scatters around the metallicity-mass relation, and dynamical disturbances. This may suggest that the scatters have their origin at the f ormation epoch of galaxies.