Gradients of Absorption‐Line Strengths in Elliptical Galaxies

We have re-studied line-strength gradients of 80 elliptical galaxies. Typicalmetallicity gradients are d[Fe/H]/dlogr = -0.3. The metallicity gradients donot correlate with any physical properties of galaxies, including central andmean metallicities, central velocity dispersions sigma0, absolute B-magnitudesMB, absolute effective radii Re, and dynamical masses of galaxies. By using themetallicity gradients, we have calculated mean stellar metallicities forindividual ellipticals. Typical mean stellar metallicities are <[Fe/H]> = -0.3.The mean metallicities correlate well with sigma0 and dynamical masses, thoughrelations for MB and Re include significant scatters. We find fundamentalplanes defined by surface brightnesses SBe, <[Fe/H]>, and Re (or MB), and thescatters 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-massrelation, or equivalently, the color-magnitude relation holds not only for thecentral part but also for the whole part of galaxies. Using Mg2 and Fe1, wefind <[Mg/Fe]> = +0.2 in most of ellipticals. <[Mg/Fe]> shows no correlationwith galaxy mass tracers. This can be most naturally explained if the starformation had stopped in ellipticals before the bulk of Type Ia supernovaebegan to explode. Ellipticals can have significantly different metallicitygradients and <[Fe/H]> even if they have the same galaxy mass. This may resultfrom galaxy mergers, but no evidence is found from presently available data tosupport the same origin for metallicity gradients, the scatters aroundmetallicity-mass relation, and dynamical disturbances. This may suggest thatthe scatters have their origin at the formation epoch of galaxies.Comment: 33 pages LaTeX, 18 PostScript figures, Accepted for publication in the Astrophysical Journa