Gradients of Absorption-Line Strengths in Elliptical Galaxies

We have restudied line-strength gradients of 80 elliptical galaxies. Typical metallicity gradients of elliptical galaxies are Δ[Fe/H]/Δ log r ≃ -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 σ₀, 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]⟩ ≃ -0.3 and range from ⟨[Fe/H]⟩ ≃ -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 σ₀ and dynamical masses, though relations for M_B and R_e include significant scatters. We find fundamental planes defined by surface brightnesses SB_e, ⟨[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 σ₀ relation is nearly parallel to the [Fe/H]₀-log σ₀ 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 Mg₂ and Fe₁ give [Mg/H] and [Fe/H], respectively, we find ⟨[Mg/Fe]⟩ ≃ +0.2 in most of elliptical galaxies. ⟨[Mg/Fe]⟩ shows no correlation with galaxy mass tracers such as σ₀, 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.