Evolution of Dwarf Galaxies

Recent chemo-dynamical simulations for one instantaneous starburst also suggest the possibiliy of galactic winds and that these winds are metal enriched (MacLow Ferrara 1999 Recchi et al. 2001). 

Purely chemical models (no dynamics) have been computed by several authors by varying the number of bursts, the time of occurrence of bursts, tburst, the star formation efficiency, the type of galactic wind, the IMF and the nucleosynthesis prescriptions (Marconi et al. 1994 Kunth et al.1995 Bradamante et al.1998). The main conclusions of these papers can be summarized as follows  

Finally, before concluding this section, we would like to draw the attention upon the fact that there are similarities between BCG, DIG and DLAs or more in general between DLAs and systems with a low level of star formation. 

The nature of DLA systems is under debate and the abundance ratios measured there can be used as a diagnostic to infer their nature and age. Matteucci et al. (1997) 

Plots of [cc/Fe] vs. [Fe/H] and plots of [a/Fe] vs. redshift should be used to infer the nature and the age of these objects, when compared with chemical evolution predictions. 

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Abundances as Tracers of the Formation and Evolution of (Dwarf) Galaxies... 

E.Tolstoy Abundances as Tracers of the Formation and Evolution of (Dwarf) Galaxies . In Chemical Abundances and Mixing in Stars, 13-17 Sep. 2004, Castiglione della Pescaia, Italy, L. Pasquini, S. Randich (eds.)... 

The evolution of dwarf galaxies is discussed further in Section 11.3.3. 

These star formation histories are of vital interest to understanding the evolution of the dEs, and they have raised some puzzles. Among these are the question of how, on the one hand, the dEs lost their gas, and how, on the other hand, they retained gas to experience multiple episodes of star formation Combining the star formation history with the element enrichment history can, in principle, yield the information needed to understand the evolution of dwarf galaxies and their contribution to enrichment of the IGM. 

Abstract. In an effort to determine accurate stellar parameters and abundances for a large sample of nearby stars, we have performed the detailed analysis of 350 high-resolution spectra of FGK dwarfs and giants. This sample will be used to investigate behavior of chemical elements and kinematics in the thick and thin disks, in order to better constrain models of chemical and dynamical evolution of the Galaxy. 

Abstract. This aims to be an overview of what detailed observations of individual stars in nearby dwarf galaxies may teach us about galaxy evolution. This includes some early results from the DART (Dwarf Abundances and Radial velocity Team) Large Programme at ESO. This project has used 2.2m/WFI and VLT/FLAMES to obtain spectra of large samples of individual stars in nearby dwarf spheroidal galaxies and determine accurate abundances and kinematics. These results can be used to trace the formation and evolution of nearby galaxies from the earliest times to the present. 

The yield table thus serves as a basis for modelling the chemical evolution of our Galaxy, or any other galaxy. Three distinct components must be specified the yields of massive stars (8-100 Mq), which become type II supernovas, those of intermediate-mass stars (1-8 Mq), which blossom into planetary nebulas, and finally, those of overfed white dwarfs, which give birth to type la supernovas. 

Concerning gas losses, we must subtract gas transformed into stars and the matter imprisoned in stellar corpses. The latter occur in three forms white dwarfs, neutron stars and black holes. We must also include matter going into planets and aborted stars (brown dwarfs), forever frozen and permanently withdrawn from the (nuclear) chemical evolution of the Galaxy. 

Much attention has been paid recently to an apparent excess of faint blue galaxies observed in photometric surveys. When the models of the B band number counts are normalized at B=16, the data show an excess over the luminosity evolution models of a factor of 2 at B=22. ([Tyson, 1988], [Lilly et al., 1991]) However, the K band number counts do not show this same excess, ([Gardner et al., 1993]). The shape of the number-redshifr distribution of surveys conducted at 20 < B < 22.5 by [Broadhurst et al., 1988] and [CoUess et al., 1990] are fitted by the no-evolution model. The median redshifts of the data from these surveys, and deeper data of [Cowie et al., 1991] and [AUington-Smith et al., 1992] show no evolution as faint as B=24. Proposed explanations for the high B band number counts include massive amounts of merging at intermediate redshifts (z 0.4) ([Broadhurst et al., 1992]) and an excess population of dwarf galaxies which appears at these redshifts, but has dissipated or faded by the present epoch. ([Cowie et al., 1991])... 

Abstract. We have performed the chemical analysis of extragalactic carbon stars from VLT/UVES spectra. The derived individual abundances of metals and s-elements as well as the well known distance of the selected stars in the Small Magellanic Cloud and the Sagittarius dwarf galaxies permit us to test current models of stellar evolution and nucleosynthesis during the Asymptotic Giant Branch phase in low metallicity environments. 

In order to test the current evolution and nucleosynthesis models predicting the formation and the yields of such carbon stars, we have collected high-resolution spectra of stars located in the SMC and the Sagittarius dwarf galaxy, extragalactic systems with low average metallicity and well known distances. 

The new data, while subject to the incompleteness of the 43 upper limits on the fiux, is also fitted by the no-evolution model in Figure 2. At these magnitude levels, pure luminosity evolution makes little difference in the model predictions, and the amount cannot be determined, but the effects of more extreme models such as merging or excess dwarfs are not evident in the data. Thus, in the region 20 < R < 22, the shape of the luminosity function of field galaxies is normal, both in the B band and in the K band. 

The most metal-rich stars in dwarf spheroidals (dSph) have been shown to have significantly lower even-Z abundance ratios than stars of similar metallicity in the Milky Way (MW). In addition, the most metal-rich dSph stars are dominated by an s-process abundance pattern in comparison to stars of similar metallicity in the MW. This has been interpreted as excessive contamination by Type la super-novae (SN) and asymptotic giant branch (AGB) stars ( Bonifacio et al. 2000, Shetrone et al. 2001, Smecker-Hane McWilliam 2002). By comparing these results to MW chemical evolution, Lanfranchi Matteucci (2003) conclude that the dSph galaxies have had a slower star formation rate than the MW (Lanfranchi Matteucci 2003). This slow star formation, when combined with an efficient galactic wind, allows the contribution of Type la SN and AGB stars to be incorporated into the ISM before the Type II SN can bring the metallicity up to MW thick disk metallicities. 

The Composition of the Sagittarius Dwarf Spheroidal Galaxy and Implications for Nucleosynthesis and Chemical Evolution... 

Abstract. We present metallicities for 487 red giants in the Carina dwarf spheroidal (dSph) galaxy that were obtained from FLAMES low-resolution Ca triplet (CaT) spectroscopy. We find a mean [Fe/H] of —1.91dex with an intrinsic dispersion of 0.25 dex, whereas the full spread in metallicities is at least one dex. The analysis of the radial distribution of metallicities reveals that an excess of metal poor stars resides in a region of larger axis distances. These results can constrain evolutionary models and are discussed in the context of chemical evolution in the Carina dSph. 

Detailed elemental abundances are now available for several individual stars in the Galaxy s dwarf satellites (Shetrone et a1. 2001, 2003 Geisler et al. 2005 also see the reviews in this proceedings). A comparison of these abundance ratios to those of stars in the Galaxy can be used to address several questions related to galaxy formation and evolution, as well as stellar nucleosynthesis. 

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