paper

The outside-in formation of elliptical galaxies

arXiv:astro-ph/0510556 · doi:10.1086/499033

Abstract

In this paper we compare the predictions of a detailed multi-zone chemical evolution model for elliptical galaxies with the very recent observations of the galaxy NGC 4697. As a consequence of the earlier development of the wind in the outer regions with respect to the inner ones, we predict an increase of the mean stellar [<Mg/Fe>] ratio with radius, in very good agreement with the data for NGC4697. This finding strongly supports the proposed outside-in formation scenario for ellipticals. We show that, in spite of the good agreement found for the [<Mg/Fe>] ratio, the predicted slope of the mass-weighted metallicity gradient does not reproduce the one derived from observations, once a calibration to convert indices into abundances is applied. This is explained as the consequence of the different behaviour with metallicity of the line-strength indices as predicted by a Single Stellar Population (SSP) and those derived by averaging over a Composite Stellar Population (CSP). In order to 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 mimick the mixture of stellar populations at any given radius. We find that these distributions differ from the ``G-dwarf'' distributions especially at large radii,except for the one as a function of [Mg/Fe]. Therefore, we conclude that in ellipticals the [Mg/Fe] ratio is the most reliable quantity to be compared with observations and is the best estimator of the star formation timescale at each radius.(abridged)

arXiv comment15 pages, 10 figures, ApJ accepted

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