The Outside‐In Formation of Elliptical Galaxies

In this paper we compare the predictions of a detailed multi-zone chemicalevolution model for elliptical galaxies with the very recent observations ofthe galaxy NGC 4697. As a consequence of the earlier development of the wind inthe outer regions with respect to the inner ones, we predict an increase of themean stellar [] ratio with radius, in very good agreement with the datafor NGC4697. This finding strongly supports the proposed outside-in formationscenario for ellipticals. We show that, in spite of the good agreement foundfor the [] ratio, the predicted slope of the mass-weighted metallicitygradient does not reproduce the one derived from observations, once acalibration to convert indices into abundances is applied. This is explained asthe consequence of the different behaviour with metallicity of theline-strength indices as predicted by a Single Stellar Population (SSP) andthose derived by averaging over a Composite Stellar Population (CSP). In orderto better address this issue, we calculate the theoretical ``G-dwarf''distributions of stars as functions of both metallicity ([Z/H]) and [Fe/H],showing that they are broad and asymmetric that a SSP cannot correctly mimickthe mixture of stellar populations at any given radius. We find that thesedistributions differ from the ``G-dwarf'' distributions especially at largeradii,except for the one as a function of [Mg/Fe]. Therefore, we conclude thatin ellipticals the [Mg/Fe] ratio is the most reliable quantity to be comparedwith observations and is the best estimator of the star formation timescale ateach radius.(abridged)Comment: 15 pages, 10 figures, ApJ accepte