LINE-STRENGTH GRADIENTS IN ELLIPTIC AND BRIGHTEST CLUSTER GALAXIES

Line strengths and their gradients in Mg, Fe, and H beta have been determined for seven elliptical and nine brightest cluster galaxies (BCGs) in order to study their stellar populations and investigate their relationship to one another. We find that BCGs follow the same relationship between central Mg b line strength and central velocity dispersion found for elliptical galaxies. Brightest cluster galaxies are in agreement with the known trend toward more massive elliptical galaxies having larger [Mg/Fe] ratios, while the internal gradients within our BCG and E galaxies are consistent with a roughly constant [Mg/Fe] ratio. We find that a correlation exists between the central [Mg/Fe] ratio and average H beta line strength in the sense that both BCG and elliptical galaxies with larger [Mg/Fe] ratios have lower [H beta] strengths. For our sample, H beta is the best predictor of [Mg/Fe] ratio. If the dominant contribution to the H beta feature is from turnoff stars then this relation predicts that more massive elliptical galaxies are older than less massive ones. If, however, the main source of the H beta index is from horizontal-branch stars, then the observed {[Mg/Fe],H beta} relation could be the result of more massive elliptical galaxies having flatter IMFs for high-mass stars than less massive elliptical galaxies. The [H beta] line strengths of the objects in our sample span a range of values. The BCGs generally have low global H beta line strengths, which, under the assumption that the H beta feature can be used as an age discriminant, indicates that the bulk of these systems underwent their last major episode of star formation greater than or similar to 8-10 Gyr ago. For both the elliptical galaxies and BCGs we find that within a galaxy, the H beta profile is flat for objects whose H beta absorption can be reliably measured. In the presence of a declining metallicity gradient this suggests that the centers of elliptical galaxies and BCGs are similar to 1-3 Gyr younger than their outer regions. The metal line strength gradients for the elliptical galaxies and BCGs are similar and consistent with an average gradient of Delta[Fe/H]/Delta log r = -0.25 +/- 0.1, corresponding to a reduction in the mean metallicity of the stellar population by a factor of similar to 2 over a factor of 10 in radius. No strong correlations are found between the metallicity gradient sizes and either kinematic or line strength parameters of the galaxies. For the galaxy NGC 4073, the brightest member of the poor cluster MKW 4, we find enhanced H beta absorption in the region coincident with the counterrotating stellar core. These observations are consistent with a scenario in which the accreted object was a younger system and/or a recent episode of star formation was triggered by an interaction.