Two-Dimensional Line Strength Maps in Three Well-studied Early-Type Galaxies

Abstract

Integral field spectroscopy has been obtained for the nuclear regions of 3 large, well-studied, early-type galaxies. From these spectra we have obtained line strength maps for about 20 absorption lines, mostly belonging to the Lick system. An extensive comparison with multi-lenslet spectroscopy shows that accurate kinematic maps can be obtained, and also reproducible line strength maps. Comparison with long-slit spectroscopy also produces good agreement. We show that Mg is enhanced with respect to Fe in the inner disk of one of the three galaxies studied, the Sombrero. [Mg/Fe] there is larger than in the rest of the bulge. The large values of Mg/Fe in the central disk are consistent with the centres of other early-type galaxies, and not with large disks, like the disk of our Galaxy, where [Mg/Fe] is approximately 0. We confirm with this observation a recent result of Worthey (1998) that Mg/Fe is determined by the central kinetic energy, or escape velocity, of the stars, only, and not by the formation time scale of the stars. A stellar population analysis using the models of Vazdekis et al. (1996) shows that our observed H gamma agrees well with what is predicted based on the other lines. Using the line strength of the Ca II IR triplet as an indicator of the abundance of Ca, we find that Ca follows Fe, and not Mg, in these galaxies. This is peculiar, given the fact that Ca is an alpha-element. Finally, by combining the results of this paper with those of Vazdekis et al. (1997) we find that the line strength gradients in the three galaxies are primarily caused by variations in metallicity.