GALACTIC COSMIC EVOLUTION AND IDENTIFICATION OF HIGH REDSHIFT OBJECTS

CALURA, FRANCESCO

GALACTIC COSMIC EVOLUTION AND IDENTIFICATION OF HIGH REDSHIFT OBJECTS

CALURA, FRANCESCO

2004-04-05

http://thesis2.sba.units.it/store/handle/item/12544

http://hdl.handle.net/10077/11573

Doctoral Thesis

Contributor(s)

SENATORE, GAETANO

Abstract

The present work is aimed at the study of the evolution of "cosmic quantities", such as the luminosity density, star formation rate density, supernova rate and metal production rate density. In some cases, these quantities are calculated per unitary comoving volume, hence they are called "densities". All these quantities are investigated in detail by means of chemical and spectrophotometric models for galaxies of different morphological types, namely ellipticals, spirals and irregulars. The chemical evolution models allow one to calculate the time evolution of the production rate of any chemical element, along with the element abundances in the interstellar gas and in the stars. Starting from the matter reprocessed by the stars and restored into the ISM through stellar winds and supernova explosions, it is possible to model the chemical enrichment of the universe in all of its components, namely the interstellar medium (ISM), the stars and the intergalactic medium (IGM). It is possible to calculate the time evolution of the abundances of several chemical species, such as C, O, N, Fe and Zn. In this thesis, the predicted abundances are compared to the observed ones: such task can provide precious hints on galactic evolution and element production both in the local and in the distant universe. The light emitted by stars in galaxies is studied by means of population synthesis models, which allow one to calculate the galaxy spectra, magnitudes and colors. The cosmic evolution of the luminosity density in various optical bands has been calculated starting from the luminosity function observed in the local universe, which allows us to determine the number of galaxies per comoving volume. We have assumed that the galaxy densities remain constant throughout the whole cosmic epoch and that the galaxies evolve as isolated systems only in luminosity. This scenario represents a pure luminosity evolution (PLE) one and it is equivalent to assume that merging has had no dominant effect on galaxy evolution throughout the Hubble time. Hence, we have developed a galaxy evolution picture which is opposed to the galaxy formation models based on the current "concordance" cosmological paradigm, which assume a cold dark matter universe whose energy is dominated by the cosmological constant. Such models indicate that the formation of large scale structures occurs in a hierarchical fashion: the first objects to form are the smallest ones, which then merge and build larger and larger objects. The baryonic matter, which builds up the galaxies, has the same behaviour of the dark matter, so that the first galaxies to form are the dwarf ones, which then by means of several merging processes give place to the giant galaxies. Thus, in contrast to our scenario, these models predict a strong galaxy number evolution. The strength of the PLE scenario described in this thesis is that, at variance with other approaches such as semi-analytical galaxy formation modeling or hydrodynamical/SPH cosmological simulations, it allows to study the evolution of the galaxy morphological types and the contributions brought by each type to cosmic star formation and element production in the universe. This point represents the first main novelty of my thesis. The PLE picture has also been used to predict the evolution of the cosmic Supernova rate which, thanks to the next generation space telescopes, will be soon measurable up to the highest redshifts, providing other fundamental hints to constrain galaxy formation and evolution. Furthermore, the comparison between the predictions obtained by the PLE scenario described in this thesis and the hierarchical represents an important benchmark to study galaxy evolution. When compared to the observations, it is possible to infer which picture is better to describe galaxy evolution. This is another aim of the present thesis, along with the suggestion of new observational strategies to disentangle between the hierarchical and the PLE picture if the current data do not allow us to perform such task. The predictions on the cosmic element production rates are then used to calculate the metal and baryonic budget in the local universe. Part of the elements synthesized so far remains locked up in long living stars and remnants, part is restored into the galactic ISM through supernova (SN) explosions and stellar winds while another fraction is expelled into the IGM through galactic winds and outflows. The formalism described in this thesis allows us a detailed computation of the element fractions present in each of the three main components of the local universe. The investigation of the amounts of different elements locked up in various phases (stars, IGM, ISM) and in different galaxies represents the second main novelty of this thesis. Finally, the chemical evolution models for galaxies of different morphological types allow us to perform an analysis of the chemical abundances measured in high-redshift objects. Of particular importance are the Damped-Lyman Alpha (DLA) systems, often referred to as the most likely progenitors of the local spiral and irregular galaxies. With the present work, it has been possible to study first the chemical evolution of the DLA population as a whole, in order to infer which morphological galactic types can be associated to DLAs and which are to be ruined out. Secondly, by focusing on individual systems and by reproducing at the same time as many observed abundances as possible, it has been possible to infer the star formation history of some single DLA systems and to have important hints on their ages. This kind of analysis, never performed so far, represents the third major novelty of this thesis. The present thesis is organized as follows: in chapter l a general overview on galaxy evolution is performed. The observational properties of the different galactic types are described, along with their interpretation and current theoretical approaches to galaxy formation and evolution. Particular emphasis is put on the most intriguing debate in galaxy evolution, namely: how and when did the formation of spheroids occurred in the universe occurred? Chapter 2 is a description of the chemical evolution models used in the present work: the main physical assumptions at the basis of the models for ellipticals, spiral and irregular galaxies and some results concerning the predictions which these models provide. Chapter 3 is a description of the spectro-photometric models used to study galaxy spectra, magnitudes and colors, with a comparison between results coming from different models and a description of the method used to investigate the dust extinction effects. In chapter 4 the results concerning the cosmic star formation history, the galaxy luminosity density and cosmic supernova rate are presented, along with a comparison between the predictions provided by the PLE model developed with this thesis and the hierarchical model by Mencietal. (2002). In chapter 5 we present our detailed calculations of the metal production rates. We perform a budget of the metals in the local universe and investigate the metal abundances in stars, ISM and IGM. In chapter 6 we apply our chemicai evoiution models to the study of damped Lyman alpha systems. Finally, in chapter 7 some conclusions are drawn.