Dwarf Galaxies and Galaxy Halos: the First Stages of Galaxy Evolution

The various projects presented in this thesis contribute to the expansion of our knowledge and understanding of the fundamental processes involved in the formation of a galaxy and its stellar populations. For this purpose, two different techniques are applied: A photometric study of the properties of the stellar halo surrounding the edge-on galaxy NGC 3957, and spectroscopic studies of the chemical compositions of stars in the nearby dwarf spheroidal galaxies Sextans, Fornax, and Sculptor. Due to their extreme weakness, studies of halos outside the Local Group are very challenging, and available data are still very scarce with limited quality. The results of previous studies are quite puzzling. Lequeux et al. (1996, 1998); Zibetti et al. (2004); Zibetti & Ferguson (2004), for example, report the discovery of external stellar halos with extremely red integrated colors, which were neither in agreement with the halo stars observed in the Milky Way or M31, nor with any other realistic stellar population. To investigate this problem, we obtained deep images of the edge-on galaxy NGC 3957, to extract minor axis surface brightness profiles, allowing the study of the halo properties in regions which are not contaminated by the much brighter disk or bulge component. While the central regions of our profiles are very well described by a de Vaucouleurs law, we find a clear excess of light at larger radii, indicating the presence of a stellar halo. Our color profiles rise to very red colors at large radii, however, a careful error analysis shows that systematic errors are mainly responsible for this rise. In the radial range where the halo component is dominant and the systematics are still small, our results are compatible with the expected integrated halo colors of the Milky Way, M31 and the slightly more distant galaxy NGC 891. Even though this is no proof, it suggests that the stellar population of the halo of NGC 3957 is a rather typical one. It is shown that the limiting factor is definitely a sufficiently accurate determination of the background level. We propose a few ways how this could be improved. Nevertheless, the sky subtraction remains the largest challenge for future studies of this type, especially if the goal is to trace the stellar halo to larger radii, comparable to the halo of the Milky Way or M31. The targets for the spectroscopic analysis of stars in dwarf galaxies were selected from an extended low resolution survey of the Ca II triplet (CaT) region, containing ∼2000 stars in Sextans, Sculptor, and Fornax. This is the largest survey of this kind done so far targeting stars outside the Milky Way. One of the most striking results of this survey was the absence of stars with metallicity estimations from the strength of the CaT lines which were below -2.8 (Helmi, 2006). This is particularly interesting, since the Milky Way halo contains a wealth of stars clearly below this limit (Cayrel et al., 2004). Such a difference would have consequences for our understanding of galaxy formation, implying that at early epochs dwarf galaxies must have been fundamentally different from the Milky Way halo. However, in some of the recently discovered, so called "ultra faint dwarfs" (UFDs) several stars with very low metallicities ([Fe/H] ∼ -4) have been found (Norris et al., 2009; Kirby et al., 2008; Frebel et al., 2010, 2009). It is therefore essential to test the reliability of the CaT metallicity estimate and confirm or disprove the existence of extremely metal poor stars in classical dwarf spheroidal galaxies (dSphs). For this purpose, the stars with the lowest CaT metallicities were followed up with high resolution spectroscopy in order to accurately determine metallicities and abundances of several other elements. Our analysis shows unambiguously, that these stars are more metal poor than suggested by the CaT estimations, setting the metallicity floor of dwarf galaxies on a level comparable with the Milky Way halo. The chemical abundance patterns of these stars and their comparison with various other dwarf galaxies and the Milky Way provide valuable insights in the earliest chemical enrichment processes. For carbon, we find signs of a large abundance scatter in Sextans, indicating inhomogeneities in the early ISM of this galaxy. Most of the other elements, in contrast, have very small abundance spreads, implying that there must be a specific process leading to a dispersion in carbon abundances. We also find evidence, that the abundance spread is larger for low mass galaxies, indicating that mixing processes in such systems are less efficient. Below [Fe/H] = -3, the abundances of the α- and iron peak elements are very similar in all galaxies, suggesting formation mechanisms independent on the properties of the host galaxy. Other elements show a more ambiguous picture. The neutron-capture elements strontium and barium are similar in all galaxies at the lowest metallicities ([Fe/H] ≲ -3.5). At higher metallicities, however, the more massive galaxies like Sculptor or the Milky Way increase their abundances to solar values (though with some scatter), while the less massive ones like Ursa Major II or Draco stay very low both in [Ba/Fe] and [Sr/Fe]. This is another indication for different chemical evolution, depending on the size of the host galaxy. It shows the importance of studying stars in various galaxies, spanning a large range of masses. Additionally, abundance patterns for a large sample of stars in the Sextans dSph were derived. Previous spectroscopic studies include only a very small number of objects owing to the large observation times required to obtain the spectra, star by star, with single slit spectrographs. This thesis utilizes the multi-object spectrograph GIRAFFE to obtain spectra of a large number of stars simultaneously, allowing us to study chemical abundances for the biggest sample of Sextans stars observed so far. Our targets, which are complemented with literature data, span a metallicity range from -1 to -3, covering different stages of the the chemical evolution history of Sextans. We find that the decrease in [α/Fe] starts at very low metallicities ([Fe/H] ≲ -2), confirming the low star formation rate of Sextans (e.g. Revaz et al., 2009). Comparing our results with other galaxies, some elements (e.g. those of the Fe-peak) have virtually the same abundance patterns everywhere. Others show signs of differential evolution in the metallicity range -1 to -2. One example is Mg, which decreases to lower values in Sextans than in the more massive Sculptor dSph or the Milky Way. Another one is the abundance ratio of 2nd to 1st s-process elements ([Ba/Y]) which is higher in low mass systems such as Sextans and draco than in Sculptor or the Milky Way.


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