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DYNAMICAL AND CHEMICAL EVOLUTION IN BLUE COMPACT DWARF GALAXIES
RECCHI, SIMONE
2002-01-17
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Contributor(s)
PAVER, NELLO
•
PAVER, NELLO
Abstract
This thesis work is based on the study of the dynamical and chemical evolution of the interstellar medium (ISM) in Blue Compact Dwarf (BCD) galaxies as a consequence of single or multiple episodes of star formation. After single or multiple star bursts, stars release energy into the ISM through stellar winds and supernova explosions. This energy can drive a galactic wind and thus eject most of gas out of the parent galaxy. The development of galactic winds strongly depends on what fraction of the mechanical energy released by the supernova is available to thermalize the ISM and thus power the galactic wind (a parameter often called Thermalization efficiency). During the expansion of the Supernova Remnant, radiati ve losses can occur, thus most of the initial blast wave energy can be lost. This is a controversia! point, thus we dedicateci part of our work to the determination of the Thermalization efficiency and we found that this efficiency is low for Type II SNe, whereas we expect higher thermalization efficiencies in Type la SNe, because this kind of explosions occur in a medium already heated and diluted by the previous activity of Type II SNe. We then used both purely chemical evolution models and a 2-D hydrodynamical code, coupled with detailed chemical yields, originating from Supernovae of Type II and Type la and from single intermediate-mass stars, to simulate the dynamical and chemical evolution of this kind of objects. There are a lot of hydrodynamical simulations concerning the evolution of the ISM in dwarf galaxies after a starburst episode in the current literature (see e.g. MacLow & Ferrara 1999; D'Ercole & Brighenti 1999; Silich & Tenorio-Tagle 1998). These simulations generally agree on the fact that the ISM is rather robust and only a tiny fraction of the ISM is expelled as a consequence of the galactic wind. However, none of these works consider the effect of Type la, whose role is certainly fundamental in the late evolution of these objects. Given the importance of this kind of Supernovae for our models, a significant part of this work has been devoted to this topic. We found analytical solutions for the rate of Type la SNe under different star formation regimes and for the most popular progenitor models, and we applied these results on the study of the chemical evolution in the salar neighbourhood. We found that the best prescription to obtain SNela rates in agreement with the observations seems to be the so-called single-degenerate scenario. In this scenario, SNela are thought to originate from the thermonuclear deflagration of a White Dwarf reaching the Chandrasekhar mass after accretion of mass from a non-degenerate companion star. We found also that the typical time-scale for chemical enrichment from Type la SNe, often claimed in literature to be of the arder of l Gyr, strongly depends on the star formation history of the galaxy, ranging from 40 -50 Myr for BCD galaxies and 4 - 5 Gyr for spirai galaxies, like the Milky Way. We then performed numerica! hydrodynamical simulations of the evolution of the ISM in BCD galaxies as a consequence of single or multiple instantaneous star bursts. The aims of this kind of study are: • evaluate the impact of a single or multiple instantaneous star bursts on the dynamics of the ISM and study under what conditions a galactic wind could develop. • Follow the evolution, in space and time, of some chemical elements of particular astrophysical interest (namely H, He, C, N, O, Mg, Si, Fe). Most of chemical evolution models do not take into account a dynamical treatment, thus evaluating the mass of gas which escapes from the parent galaxy in a simplistic manner. With our model instead we are ab le t o follow the dynamical evolution of the concentration of several chemical elements, by taking into account stellar evolution and nucleo synthesis prescriptions as well as stellar lifetimes. Moreover, the mass of gas ejected from the galaxy in the form of the various chemical elements is computed in detail. • Apply these simulations to a galaxy model whose structural parameters are similar to IZw18, the most metal-poor galaxy locally known, in arder to put constraints on its age and its past star formation history. Main results of this kind of work can be summarized as follows: • As a consequence of the energy injected during the star formation activity ( either single or multiple instantaneous starbursts), a galactic wind develops. This galactic wind is differential, in the sense t ha t the newly formed metals are ejected more easily t han the pristine gas. In particular, metals produced by Type la supernovae are ejected more easily than those produced by SNeii, because this kind of explosions occurs in a medium already heated and diluted by the previous activity of Type II SNe. As a consequence of this, the [a/Fe] abundance ratios outside the galaxy are found to be lower than inside, an important prediction for galactic evolutionary models. • We are able to reproduce the chemical abundances found in literature for IZw18, either with a single burst or with two instantaneous bursts, separated by a quiescent period. The predicted age for the single-burst model is rv 30 Myr, but we found better agreement for models with two bursts of star formation, the first with an age of 300-500 Myr and the second with an age between 4 and 70 Myr, depending on the adopted IMF and nucleo synthesis prescriptions. A first burst with an age of 300 Myr and a second, more intense o ne, with an age of few Myr, is particularly appealing since it agrees with the derived ages from stellar population studies (Aloisi et al. 1999; Òstlin 2000) and with the spectral energy distribution of this galaxy (Mas-Hesse & Kunth 1999). • W e find t ha t the majority of metals are in a col d phase, especially for the single-burst model. This is due to the fact that most of energy produced by the supernovae is quickly radiated away, thus the hot hubble of gas created after the star burst (the so-called super bubble) evolves slowly, having time to significantly cool before break-aut. This result is the first of this kind, since in previous works i t was concluded that most of the metals should reside in a hot gas phase, virtually undetectable with the optical spectroscopy. Some recent studies of Color-Magnitude diagrams in BCD galaxies seem to indicate that these objects experienced star formation episodes of non-negligible duration. We are not able at the moment to simulate this kind of star formation regimes with our chemo-dynamical code, thus we ran simulations by means of a purely chemical evolution code. We assumed the hypothesis of the "Differential wind", namely, when a galactic wind develops, it expel mostly metals. The fraction of various chemical elements ejected by the parent galaxy are assumed in agreement with the results of our chemo-dynamical simulations. Main results of this kind of simulations are that models of bursts of star formation with duration shorter than 100 Myr do not develop a galactic wind. We then concentrate mainly on the be haviour of nitrogen. A large spread of N/0 as a function of 0/H is observed in a large sample of BCD galaxies, whereas for a small subsample of metal-poor BCD (i.e. with log(O/H) < -4.5) the N/0 abundance ratio seems to be almost constant (Izotov & Thuan 1999). W e can explain the spread in the N/ O vs. O /H diagram as due to different star formation efficiencies, which means also different wind efficiencies, different burst ages or differences in the burst durations. The nearly flat trend of N/0 at low 0/H (if true) could be explained by primary production of N in massive stars (a suggestion already introduced by Matteucci & Greggio 1986), or alternatively by a couple of bursts separated by a short quiescent period, or more extended burst, in which a differential wind develops and, in the following evolution, both the O abundance and the N/0 abundance ratio decrease with time.
Insegnamento
Publisher
Università degli studi di Trieste
Languages
en
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