Abundance gradients in elliptical galaxies

Dipartimento di Fisica, Università “La Sapienza”, Piazzale A. Moro, 5 00185 Roma, Italy e-mail: lucio.angeletti@uniroma1.it;pietro.giannone@uniroma1.it

Corresponding author: L. Angeletti, lucio.angeletti@uniroma1.it

Received: 27 March 2002
Accepted: 30 January 2003

Abstract

Abundance gradients predicted by dissipative models of galaxy formation are studied with the aid of mass models of spherical galaxies obeying the Sersic law in projection. The link between metallicity and stellar binding energies and angular momenta is derived from the “concentration model” by Lynden-Bell ([CITE]) with the addition of a terminal wind and an age spread among the stellar populations of individual ellipticals. By using the calibration of the Mg₂ index as a function of metallicity and age from Single-burst Stellar Populations models, the radial profiles of Mg₂ are computed for a variety of models to illustrate the general predictions of the present approach. We also matched the observed radial profiles of Mg₂ in a sample of 11 ellipticals. We find that observations are reproduced reasonably well in our adopted approximation and require a considerable degree of dissipation and a metal yield above the solar value. A terminal wind or an age spread make it easier to fit the Mg₂ profiles of the ellipticals with shallow Mg₂ profiles in the innermost regions. However, model results (with or without an age spread) show that the exact determination of the free parameters suffers from degeneracy of their effects. Moreover, model results also suffer from the poorly known effects of non-solar abundance ratios on index Mg₂. Other spectral indices, besides Mg₂, and broad-band colours observed in elliptical galaxies can be examined in the same way with the present scheme; their study is postponed to a further investigation.