LINE-STRENGTH GRADIENTS IN ELLIPTIC GALAXIES

We have measured line-strength gradients in Mg2, [Fe] and Hbeta for 13 galaxies, using two instruments on the KPNO 4-m telescope. The metal- line strengths are consistent with an average abundance gradient of DE LTA[Fe/H]/DELTA log r= - 0.2 +/- 0.1, or a reduction in mean metallici ty of the stellar population of 40 per cent over a factor of 10 in rad ius. Although line-strength gradients cannot be simply transformed to metallicity gradients, these values are the same as those inferred fro m colour gradients. These shallow gradients present a significant chal lenge to current theories of galaxy formation. The dissipative models of Larson & Carlberg produce gradients that are steeper than those mea sured in giant ellipticals. On the other hand, models of the formation of ellipticals that rely only on stellar interactions, such as dissip ationless hierarchical merging, have no mechanism for the generation o f metallicity gradients. White has shown that an existing gradient wou ld be diluted by about a factor of 2 over three merger events. Togethe r, these considerations support the hypothesis that giant ellipticals form by stellar mergers, and that the fine-strength gradients originat e in their lower mass progenitors which formed predominantly by dissip ational collapse. We find no correlations between the size of the grad ient and any other global parameter of the galaxies, such as luminosit y or flattening. Our sample, however, spans only about a factor of 10 in luminosity. We find that the contours of constant fine strength hav e the same shape as the isophotes. The correlations between local colo urs and local line strengths indicate that dust does not play a signif icant role in the generation of colour gradients in elliptical galaxie s. We confirm the findings of previous workers that the slope of the [ Fe] versus Mg2 relation within ellipticals is steeper than the equival ent relation for the nuclei of ellipticals. This can be interpreted as evidence for an enrichment of Mg over Fe compared to the solar values used in the models. Age differences or a younger population cannot be invoked to account for the difference. Mg could be enhanced with resp ect to Fe over the solar value in the giant ellipticals either by enha nced early star formation or by skewing of the initial mass function t o produce more massive stars. Alternatively, a deficiency of [Fe] coul d be produced if the binary fraction in elliptical galaxies were lower than the local value. We find that Hbeta absorption is constant or in creases with increasing radius in almost all of the galaxies we observ e. We suggest that this is due to the dilution of the Hbeta absorption feature by emission in the centres of these galaxies, an effect which decreases with increasing radius. Our results are consistent with no gradient in the age of the stellar population. We find that the galaxi es with the bluest 1550 - V colours have the strongest Hbeta emission. We speculate that the inverse relationship between Mg2 and 15 50 - V might be due to the presence of an otherwise undetected blue ionizing nucleus in galaxies with high values of Mg2, and that both the activit y and the high metallicity might result from the star formation associ ated with the formation of a core. We investigate the relationship bet ween metallicity and escape velocity proposed by Franx & Illingworth. Their suggestion that metallicity is a function of the local escape ve locity is confirmed by using a more direct indicator of metallicity an d a separate calculation of V(esc) for each galaxy. While this result supports the view that abundance gradients and the colour-magnitude re lation for ellipticals arise from a common physical cause, the decoupl ing of Mg2 and [Fe] gradients indicates that other effects, depending on the details of the star formation history, are at work. NGC 4472 an d 7626 have anomalous core kinematics and enhanced Mg2 within their co res. This suggests that the formation of the cores was accompanied by significant star formation activity that generated a high-metallicity population. This high metallicity does not support models in which the anomalous core is the remnant of a captured low-mass galaxy, as even the most metal-rich low-mass galaxies have lower central line strength s than those found in the centres of NGC 4472 and 7626.