SNe feedback and the formation of elliptical galaxies

The processes governing both the formation and evolution of elliptical galaxies are discussed by means of a new multi-zone photo-chemical evolution model for elliptical galaxies, taking into account detailed nucleosynthetic yields, feedback from supernovae, Pop III stars and an initial infall episode. By comparing model predictions with observations, we derive a picture of galaxy formation in which the higher is the mass of the galaxy, the shorter are the infall and the star formation timescales. In particular, by means of our model, we are able to reproduce the overabundance of Mg relative to Fe, observed in the nuclei of bright ellipticals, and its increase with galactic mass. This is a clear sign of an anti-hierarchical formation process. Therefore, in this scenario, the most massive objects are older than the less massive ones, in the sense that larger galaxies stop forming stars at earlier times. Each galaxy is created outside-in, i.e. the outermost regions accrete gas, form stars and develop a galactic wind very quickly, compared to the central core in which the star formation can last up to ~1.3 Gyr. This finding will be discussed at the light of recent observations of the galaxy NGC 4697 which clearly show a strong radial gradient in the mean stellar [] ratio. The role of galactic winds in the IGM/ICM enrichment will also be discussed.Comment: 3 pages, 2 figures, contributed paper to be published in the proceedings of" Origin of Matter and Evolution of Galaxies - New Horizon of Nuclear Astrophysics and Cosmology", Ed.S.Kubon

Chemical evolution of bulges at high redshift

We present a new class of hydrodynamical models for the formation of bulges (either massive elliptical galaxies or classical bulges in spirals) in which we implement detailed prescriptions for the chemical evolution of H, He, O and Fe. Our results hint toward an outside-in formation in the context of the supernovae-driven wind scenario. The build-up of the chemical properties of the stellar populations inhabiting the galactic core is very fast. Therefore we predict a non significant evolution of both the mass-metallicity and the mass-[alpha/Fe] relations after the first 0.5 - 1 Gyr. In this framework we explain how the observed slopes, either positive or negative, in the radial gradient of the mean stellar [alpha/Fe], and their apparent lack of any correlation with all the other observables, can arise as a consequence of the interplay between star formation and metal-enhanced internal gas flows.Comment: 4 pages, 6 figures, to appear on the IAU Symposium 245 Proceedings, Eds. M. Bureau, E. Athanassoula, B. Barbu

Photo-Chemical Evolution of Elliptical Galaxies I. The high-redshift formation scenario

In this paper we compute new multi-zone photo-chemical evolution models for elliptical galaxies, taking into account detailed nucleosynthetic yields, feedback from supernovae and an initial infall episode. By comparing model predictions with observations, we derive a picture of galaxy formation in which the higher is the mass of the galaxy, the shorter are the infall and the star formation timescales. Therefore, in this scenario, the most massive objects are older than the less massive ones, in the sense that larger galaxies stop forming stars at earlier times. Each galaxy is created outside-in, i.e. the outermost regions accrete gas, form stars and develop a galactic wind very quickly, compared to the central core in which the star formation can last up to ~1.3 Gyr. In particular, we suggest that both the duration of the star formation and the infall timescale decrease with galactic radius. (abridged) By means of our model, we are able to match the observed mass-metallicity and color-magnitude relations for the center of the galaxies as well as to reproduce the overabundance of Mg relative to Fe, observed in the nuclei of bright ellipticals, and its increase with galactic mass. Furthermore, we find that the observed Ca underabundance relative to Mg can be real, due to the non-neglibile contribution of type Ia SN to the production of this element. We predict metallicity and color gradients inside the galaxies which are in good agreement with the mean value of the observed ones. (abridged)Comment: 19 pages, 14 figures, MNRAS accepte

The outside-in formation of elliptical galaxies

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 [] 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 [] 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)Comment: 15 pages, 10 figures, ApJ accepte

The hunt for the Milky Way's accreted disc

The Milky Way is expected to host an accreted disc of stars and dark matter. This forms as massive >1:10 mergers are preferentially dragged towards the disc plane by dynamical friction and then tidally shredded. The accreted disc likely contributes only a tiny fraction of the Milky Way's thin and thick stellar disc. However, it is interesting because: (i) its associated `dark disc' has important implications for experiments hoping to detect a dark matter particle in the laboratory; and (ii) the presence or absence of such a disc constrains the merger history of our Galaxy. In this work, we develop a chemo-dynamical template to hunt for the accreted disc. We apply our template to the high-resolution spectroscopic sample from Ruchti et al. (2011), finding at present no evidence for accreted stars. Our results are consistent with a quiescent Milky Way with no >1:10 mergers since the disc formed and a correspondingly light `dark disc'. However, we caution that while our method can robustly identify accreted stars, our incomplete stellar sample makes it more challenging to definitively rule them out. Larger unbiased stellar samples will be required for this.Comment: 14 pages; 8 figures; 1 table. Accepted for publication in MNRA