Gradients of Absorption Line Strengths in Elliptical Galaxies

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Abstract
We have re-studied line-strength gradients of 80 elliptical galaxies. Typical metallicity gradients are d[Fe/H]/dlogr = -0.3. The metallicity gradients do not correlate with any physical properties of galaxies, including central and mean metallicities, central velocity dispersions sigma0, 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. The mean metallicities correlate well with sigma0 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), and the scatters of which are much smaller than those of the <[Fe/H]>-Re (or MB) relations. The <[Fe/H]>-sigma0 relation is nearly in parallel to the [Fe/H]0- sigma0 relation but systematically lower by 0.3 dex. The metallicity-mass relation, or equivalently, the color-magnitude relation holds not only for the central part but also for the whole part of galaxies. Using Mg2 and Fe1, we find <[Mg/Fe]> = +0.2 in most of ellipticals. <[Mg/Fe]> shows no correlation with galaxy mass tracers. This can be most naturally explained if the star formation had stopped in ellipticals before the bulk of Type Ia supernovae began to explode. Ellipticals 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 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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