Please use this identifier to cite or link to this item: http://hdl.handle.net/10077/3069
Title: Cosmologically motivated gas infall laws and galactic chemical evolution
Authors: Colavitti, Edoardo
Supervisore/Tutore: Matteucci, Maria Francesca
Matteucci, Maria Francesca
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.
Ciclo di dottorato: XXI Ciclo
metadata.dc.subject.classification: FISICA
Description: 
2007/2008
Keywords: chemical evolution of galaxiesRaggi CosmiciMuoniPamelaRapporto Di CaricaDati A Terra
Type: Doctoral Thesis
Language: en
Settore scientifico-disciplinare: FIS/05 ASTRONOMIA E ASTROFISICA
NBN: urn:nbn:it:units-7391
Appears in Collections:Scienze fisiche

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