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Title: Cosmologically motivated gas infall laws and galactic chemical evolution
Authors: Colavitti, Edoardo
Keywords: chemical evolution of galaxiesRaggi CosmiciMuoniPamelaRapporto Di CaricaDati A Terra
Issue Date: 3-Apr-2009
Publisher: Università degli studi di Trieste
Abstract: The main aim of this thesis has been to find a cosmologically motivated infall law to understand if the CDM cosmology can reproduce the main chemical characteristics of a Milky Way-like spiral galaxy. We have tested several gas infall laws, starting from that suggested in the two-infall model of Chiappini et al. (1997) for the chemical evolution of the Milky Way, but focusing on laws derived from cosmological simulations which follow a concordance CDM cosmology. By means of a detailed chemical evolution model for the solar vicinity, we have studied the effects of the different gas infall laws on the abundance patterns and the G-dwarf metallicity distribution. Our best cosmological infall law, derived from dark matter halos having properties compatible with the formation of a disk galaxy like the Milky Way, and assuming that the baryons assemble like dark matter, resembles the infall law suggested by the two-infall model. It predicts two main gas accretion episodes. Minor infall episodes are predicted to have followed the second main one but they are of little significance compared to the previous two. By means of this cosmologically motivated infall law, we have studied the star formation rate, the SNIa and SNII rate, the total amount of gas and stars in the solar neighbourhood and the behaviour of several chemical abundances (O, Mg, Si, C, N, Fe). We have found that the results of the two-infall model are fully compatible with the evolution of the Milky Way with cosmological accretion laws. We have derived that the timescale for the formation of the stellar halo and the thick disk must have not been longer than 2 Gyr, whereas the disk in the solar vicinity assembled on a much longer timescale (∼ 6 Gyr). Then we have studied the abundance gradients along the Galactic disk produced by our best cosmological model and their dependence upon several parameters: a threshold in the surface gas density regulating star formation, the star formation efficiency, the timescale for the formation of the thin disk and the total surface mass density of the stellar halo. We have found that to reproduce at the same time the abundance, star formation rate and surface gas density gradients along the Galactic disk it is necessary to assume an inside-out formation for the disk. The threshold in the gas density is not necessary and the same effect could be reached 2 by assuming a variable star formation efficiency. However the derived new cosmological infall law contains a mild inside-out formation and is still not enough to reproduce the disk properties at best. We have also studied the effect of a cosmologically motivated infall law for the formation of a massive elliptical galaxy in order to understand the impact on the formation of the spheroids. We have found that such a model predicts a too low mean stellar value for the [Mg/Fe] ratio. This is, according to us, the most important result of our cosmological model applied to an early-type galaxy indicating that perhaps the hierarchical paradigm of galaxy formation should be revised for ellipticals. Moreover we have found that models for ellipticals without a galactic wind predict a too large current SNIa rate. In particular, the cosmological model produces a current SNIa which is about ten times higher than the observed values and predicts a large current SNII rate which is not observed. The predicted SNII rate for models with galactic wind is also in contrast with the actual star formation mesured by GALEX. The conclusions of our work are that a gas assembly history derived from a DM halo, compatible with the formation of a late-type galaxy from the morphological point of view, can produce chemical properties in agreement with the available observations. Moreover, a cosmologically derived infall law with an inside-out process for the disk formation and a variable star formation efficiency can indeed well reproduce all the properties of the disk. Higher resolution cosmological simulations, however, are necessary to better trace the radial properties of disk galaxies. Finally, a cosmologically derived infall law for an elliptical galaxy cannot well reproduce all the chemical constraints given by observations and this is an important result.
Description: 2007/2008
NBN: urn:nbn:it:units-7391
Appears in Collections:Scienze fisiche

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