Formation of radial gradients in the stars of elliptical galaxies

¹ Astrophysics, University of Oxford, Denys Wilkinson Building, Keble Road, Oxford OX1 3RH, UK e-mail: axp@astro.ox.ac.uk
² Dipartimento di Astronomia, Università di Trieste, via G.B. Tiepolo 11, 34100 Trieste, Italy
³ INAF - Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy

Received: 20 June 2007
Accepted: 1 April 2008

Abstract

Aims. We aim: i) to test and improve our previous models of an outside-in formation for the majority of ellipticals in the context of the SN-driven wind scenario, by means of a careful study of gas inflows/outflows; ii) to explain the observed slopes, either positive or negative, in the radial gradient of the mean stellar , and their apparent lack of correlation with all other observables.

Methods. We present a new class of hydrodynamical simulations for the formation of single elliptical galaxies in which we implement detailed prescriptions for the chemical evolution of H, He, O and Fe.

Results. We find that all the models that predict chemical properties (such as the central mass-weighted abundance ratios, the colours or the gradient) that lie within the observed ranges for a typical elliptical, also exhibit a variety of gradients in the ratio, in agreement with the observations (namely positive, null or negative). All these models undergo an outside-in formation, in the sense that star formation stops earlier in the outermost than in the innermost regions, due to the onset of a galactic wind. We find that the predicted variety of gradients in the ratio can be explained by physical processes generally not taken into account in simple chemical evolution models, such as radial flows coupled with different initial conditions for the galactic proto-cloud. The typical gradients predicted by our models have a slope of –0.3 dex per decade variation in radius, consistent with the mean values of several observational samples. However, we also find a quite extreme model in which this slope is –0.5 dex per decade, thus explaining some recent data on gradients in ellipticals.

Conclusions. We conclude that the history of star formation is fundamental for the creation of abundance gradients in ellipticals but that radial flows with different velocity in conjunction with the duration and efficiency of star formation in different galactic regions are responsible for the gradients in the ratios.