Gradients of Absorption‐Line Strengths in Elliptical Galaxies

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Abstract
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 MB, absolute effective radii Re, 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 MB and Re include significant scatters. We find fundamental planes defined by surface brightnesses SBe, [Fe/H], and Re (or MB), the scatters of which are much smaller than those of the [Fe/H]-Re (or [Fe/H]-MB) 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.
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