Chemical and kinematical evolution in nearby dwarf spheroidal galaxies

Koch, Andreas (2006) Chemical and kinematical evolution in nearby dwarf spheroidal galaxies. PhD thesis, University of Basel.

Abstract

Dwarf spheroidal (dSph) galaxies are the least massive and luminous objects known to exist. These galaxies are often considered as the fossil building blocks of massive systems predicted by some cosmological models. Nonetheless, evidence has been mounting that this idea of hierarchical assembly may be too simplistic a picture, since a number of the dSphs' characteristics, such as the a-element abundance patterns, stand in contradiction to the properties of stars in the Galactic halo. Further, yet unsolved, puzzles include the missing satellite problem, the influence of feedback and reionization, and the nature of dark matter. How then do the dSphs form, to what extent do they contribute to the build-up of massive galaxies, what can we say about their dark matter content and how can one characterize their role in cosmology and galactic evolution? These systems' intriguing properties, such as the omnipresence of old stellar populations, their gas deficiency, their high velocity dispersion and at dispersion profiles provide stringent tests of the paradigm of galaxy formation and render these systems important benchmarks for studying galactic evolution from the earliest epochs on. In particular, the proximity of the dSphs in the Local Group (LG) allows us to resolve their stellar populations and to pursue near-field cosmology on the smallest scales. In this Thesis I address several evolutionary aspects of these galaxies by concentrating on three Galactic satellites and additionally investigating the global satellite galaxy system of the Andromeda galaxy, M31. In this context, I spectroscopically analyzed the Carina dSph, which stands out among the LG dSphs because of its unusual, episodic star formation (SF) history. Carina bears evidence of at least three prominent stellar populations. Hence, I aimed at studying the metallicity spread of such systems, investigating potential age-metallicity relations, searching for spatial gradients and exploring its evolutionary history accounting for chemical enrichment. This was achieved by obtaining medium-resolution spectroscopy of ~1200 targets in Carina. Based on the near-infrared calcium triplet as a well established metallicity indicator, I was able to compile the metallicity distribution function (MDF) of this galaxy from a large sample of stars. Despite the wide spread in stellar ages present in Carina, originating from its episodic SF, it exhibits a remarkably narrow red giant branch (RGB). On the other hand, I found a wide spread in the metallicities, reaching from -3 dex to near-solar. Hence, I could show that age and metallicity conspire to produce old, metal poor stars at the same locus on the RGB as young, metal rich ones. This manifestation of an age-metallicity degeneracy generically explains the observed narrowness of Carina's RGB. In addition, I could also derive the age of each single red giant from isochrone fits. The resultant age distribution indicates the presence of three major peaks, with a prevailing intermediate-age population. These populations may in fact be related to Carina's three well established SF episodes. By correlating metallicities and spatial information, I could confirm a radial population gradient in the metallicities, in the sense that more metal rich stars are found towards the center of the galaxy. This phenomenon points to a deep central potential well in the dSphs, where gas is longer retained for SF and enrichment. In order to analyze the shape of the observed MDF, several simple models of chemical evolution were calculated, which support the view that the galaxy's early SF must have occurred from pre-enriched gas. Moreover, all the models in use tend to overestimate the number of metal poor stars, i.e., there is a persistent G-dwarf problem. This medium-resolution study was complemented by measurements of iron- and a-element abundance ratios from high-resolution spectra of ten stars in Carina. These elements are important tracers of SF and thus reflect the evolutionary status of any stellar system. It could be shown that the calibration of the metallicity [Fe/H] via the calcium triplet reproduces the "true" stellar iron abundance well for moderately metal-poor stars, but the calibration tends to fail towards the most metal-poor populations. Carina's [a/Fe]-ratios are well consistent with those measured in other dSphs of the LG and confirm that they are systematically lower than those in Galactic halo stars of comparable metallicities. The overall abundance patterns are not inconsistent with an episodic SF, but the accuracy and small number statistics of such measurements impedes quantifying the underlying evolution. It is, however, safe from the present data to say that also Carina inheres the typical characteristics of other dSphs in terms of a low SF efficiency and the occurrence of strong galactic winds. In an analoguous manner I determined metallicities in the remote Galactic satellite Leo II. The resulting MDF also shows a deficiency in very metal-poor stars. Furthermore it turns out to be rather asymmetric, with a rapid decline towards higher metallicities. By comparing my measurements with model predictions of LG dSphs I illustrated that Leo II's MDF bears resemblence to the UMi and Scl dSphs, albeit none of the models succeed in reproducing all features of the MDF simultaneously. By additionally determining the ages of the RGB stars in Leo II, I showed that the age-metallicity relation in this galaxy is essentially at over a long time interval, while there is evidence for an enrichment during the last 2-4 Gyr. The overall wide spread in ages present in this dSph support earlier views that Leo II is in fact a galaxy with a prominent old and a prevalent intermediate-age population. Contrary to Carina, I could not detect any radial metallicity nor age gradient in Leo II. Another important aspect of the nature and evolution of dSphs is the study of their kinematical properties. These low-mass galaxies are believed to be the smallest cosmological structures containing dark matter. By measuring radial velocities in the remote dSph Leo I I showed that the resulting velocity dispersion profile is essentially at out to the nominal tidal radius. The non-detection of any apparent velocity gradient across the galaxy supports the negligible role of Galactic tides in the course of its whole evolution. The application of dynamical modeling under the assumption of an isotropic velocity distribution then yielded mass and density profiles. Moreover, the behaviour of the velocity anisotropy was analysed. The resulting high mass to light ratio of Leo I is supportive of the idea that all dSphs share a common dark halo mass-scale of ~4 _ 107M , so that the pure velocity (dispersion) information of such a system is a direct proxy for mass. All this argues in favour of a general dark matter dominance in the dSphs and renders the hypothesis that these systems are of tidal origin less likely.Lastly, information about the origin and evolution of the dSphs can be gleaned by examining their spatial distribution around their host galaxies. It has often been reported that, in the Milky Way system, the dSphs are aligned along one or more great circles or polar planes. Hence, I reconstructed the three-dimensional distribution of the entire M31 satellite sample. By applying detailed statistical methods I could demonstrate that seven out of 16 satellites are located within a thin polar sheet. One reason for this planar alignment can be the break up of a common progenitor, which was orbiting M31. Also plausible is that the dSphs fell in along the filamentary dark matter structure of the cosmic web, which is underscored by the fact that the plane extends in the direction of nearby galaxy groups. All in all, my studies of the chemical and kinematical properties of a sample of nearby dSph galaxies do confirm that these are dark matter dominated systems, which are governed by highly complex chemical enrichment processes and thus warrant detailed investigations. These turn out to be invaluable for drawing a global picture of galaxy formation in a cosmological context.