The Outside-In Formation of Elliptical Galaxies

In this paper we compare the predictions of a detailed multizone chemical evolution model for elliptical galaxies with the very recent observations of the galaxy NGC 4697. In particular, the model allows for an initial gas infall and a subsequent galactic wind; it takes into account detailed nucleosynthesis prescriptions of both Types II and Ia supernovae and reproduces the main photochemical properties of normal elliptical galaxies. 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 NGC 4697. This finding strongly supports the proposed outside-in formation scenario for elliptical galaxies. We also 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 so that a SSP cannot correctly mimic the mixture of stellar populations at any given radius. We also compute the stellar distribution as a function of the [Mg/Fe] ratio, which has negligible "skewness" and is narrower than functions of [Z/H] and [Fe/H] and hence can be better represented by a SSP with an abundance ratio given by the average [⟨Mg/Fe⟩] ratio. Moreover, we compute the luminosity distributions of stars for a typical elliptical galaxy as functions of the [Z/H], [Fe/H], and [Mg/Fe] ratios. We find that these distributions differ from the G-dwarf distributions especially at large radii, except for that as a function of [Mg/Fe]. Therefore, we conclude that in elliptical galaxies 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.