Galactic formation models have long sought to reproduce the observed chemical and kinematical properties of the Milky Way's stellar halo and disk. Recently it is the so-called "intermediate population", the stellar thick disk, that is driving advances in our understanding of the formation of spiral galaxies. The thick disk is kinematically more like the thin disk than the halo, for all the thick disk has a velocity dispersion twice that of the thin disk and rotates ~40 km/s more slowly. It is generally accepted that the thick disk's metallicity distribution function peaks at a lower metallicity than the thin disk but at higher metallicity than the halo. The lower bound of the thick disk is still uncertain, as many observational studies have found only a few thick disk candidate stars or clusters that are more metal poor than (Fe/H)=1. Beers et al. (2002) have so far proposed the largest sample of metal poor thick disk candidates, presenting 9 stars at (Fe/H)= -1.2 or lower and 46 more stars at (Fe/H)= -1 or lower, all of which are believed to belong to the thick disk. Beers et al. (2002) present possible thick disk stars as metal poor as (Fe/H)~ -2.5, roughly 1 dex lower than is suggested by current Galactic formation models (Brook et al., 2005). This study is a high-resolution spectroscopic follow-up of 29 of the stars Beers et al. (2002) and Chiba & Beers (2000) identify as potential metal poor members of the thick disk and an additional 40 stars from the cannonical thick disk, halo, and thin disk. None of the very metal-poor stars identified by Beers et al. (2002) can be confirmed as members of the thick disk and many are not metal poor at all. Only two stars more metal poor than (Fe/H)= 1.2 retain their thick disk membership. These two stars exhibit some of the chemical characteristics of the cannonical thick disk: high α-element abundances and a relatively low s--/r-- process element ratio. Also of interest are six stars with thin disk kinematic signatures but thick disk α-element abundances. That only a small number of metal poor thick disk stars could be confirmed in this study indicates that the thick disk is neither as populous nor as metal poor as has been proposed by Beers et al. (2002). / text

In this thesis I present an investigation into the presence, nature and origin of the thick disk component in late-type galaxies. I use ground-based wide-field observations to study two edge-on low-mass galaxies in the Local Universe: NGC 4244 and NGC 55. The large field-of-view of the ground-based data enables me to inspect the radial and vertical structure of each galaxy. The vertical profiles are studied up to larger distances fromthemid-plane than in any previous study and the presence of a second disk component beyond the thin disk with a larger scale height is revealed for both galaxies. The high-quality data allows me to carry out stellar population and metallicity studies for stars above and below the plane. Furthermore, direct comparisons with two simulated low-mass galaxies provided by the Preston group at the University of Central Lancashire are carried out. By putting the results for NGC 4244 and NGC 55 into context with the thick disk properties from these simulations and from the literature, the most likely thick disk formation scenarios can be pointed out. The bulgeless low-mass systemNGC 4244 lies at a distance of 4.4 Mpc and is studied using V- and I-band wide-field images taken with Suprime-Cam on the Subaru telescope, Hawaii. The extra-planar regions of NGC 4244 show the presence of a large population of Red Giant Branch (RGB) stars and some Asymptotic Giant Branch (AGB) stars. The best strategy to study the presence and structure of a thick disk component is to use the vertical diffuse light profiles in the crowded central regions and RGB star counts in the sparser and sky background dominated outskirts. The profiles show evidence for the presence of a sparsely populated second structural component beyond ∼ 2 kpc above and below the mid-plane. The profiles are fitted with a twodisk model, where each disk is approximated by an isothermal, self-gravitating sheet. A Bayesian model comparison confirms the need for a second disk component in the profile fit. Furthermore, the AGB profiles are inspected and are found to have a lower scale height than the RGB profiles. Metallicity studies of the RGB population in the thick disk component reveal that the metallicity is much lower than the solar metal-licity. Last, the scale length of the thin disk is quantified from the diffuse light radial profiles. At a distance of 1.9 Mpc the Magellanic type low-mass galaxy NGC 55 is studied using V- and I-band images from the VIsible MultiObject Spectrograph (VIMOS) in imaging mode on the Very Large Telescope (VLT, UT3) on Cerro Paranal, Chile. The very central regions of the galaxy are not covered by the VIMOS pointings so I use additional images from the Curtis-Schmidt telescope on Cerro Tololo. As for NGC 4244, I find that the RGB star count profiles extend to larger scale heights than the AGB profiles. The combined diffuse light + RGB profiles show evidence for a very prominent second disk component beyond ∼ 2 kpc above and below the plane. The metallicity studies of the RGB population show, that there is no trend in themetallicity with height above or below the plane. As for NGC 4244 the metallicity is significantly lower than the solar metallicity. Furthermore, the scale length of the thin and thick disk is derived from the radial profiles. The properties of thick disks in galaxies of all masses is studied by compiling the results fromobserved and simulated galaxies in the literature in addition to the results for NGC 4244, NGC 55 and the two simulated low-mass galaxies provided by the Preston group. By studying the similarities, differences and global trends with mass in the thick disk properties it is possible to confront the thick disk formation models. I find that none of the formation scenarios can be ruled out and even a hybrid scenario is possible.

Šio darbo tikslas buvo ištirti Galaktikos storojo disko žvaigždžių cheminę evoliuciją. Devynių F-G-K spektrinės klasės žvaigždžių spektrai buvo gauti SOFIN spektrografu su Šiaurės šalių teleskopu. Iš spektrų nustačiau pagrindinius žvaigždžių atmosferų fizikinius parametrus: efektinę temperatūra Teff., gravitacinį pagreitį žvaigždės paviršiuje lg g, mikroturbulencijos greitį vt. ir deguonies, geležies, α, r- ir s-procesų elementų gausas devyniose storojo Galaktikos disko žvaigždėse. Rezultatus palyginau su kitų mokslininkų grupių darbais: tai Tautvaišienės ir kt. darbas (2001 m.); Bensby ir kt. darbai (2004, 2005, 2007 m.); Reddy ir kt. darbas (2006 m.). Geležies gausai [Fe/H] esant intervale nuo ≈ –0,9 iki ≈ –0,3 dešimtųjų žvaigždės, pasižyminčios storojo Galaktikos disko kinematika, yra turtingesnės deguoniu, magniu ir alfa-proceso elementais, nei plonojo disko žvaigždžių populiacija. Kitų cheminių elementų gausos storajame ir plonajame Galaktikos diskų žvaigždėse yra panašios. / The aim of this research is to study the Galactic thick disk chemical evolution. Using high-resolution spectra I have derived the chemical composition of a sample of Galactic thick disk stars. High resolution spectra of 9 F-G-K spectral type stars in the Galactic thick disk have been obtained by the SOFIN spectrograph on the Nordic Optical Telescope. From these stellar spectra I determined the basics stellar atmosphere parameters: effective temperatures Teff [K], surface gravities lg g, microturbulent velocities vt [km/s], and abundances of oxygen, iron, α, r- and s-process elements in the Galactic thick disk. The results are similar to those of other investigations: Tautvaišienė et al., 2001; Bensby et al., 2004, 2005, 2007; Reddy et al., 2006. For a given [Fe/H] the stars with kinematics typical of the thick disk are more enriched in oxygen and α- process elements (Mg, Si, Ca, Ti) than the stars with kinematics typical of the thin disk. The abundance of other elements (Al, Na, Cr, Ni, V, Sc, Mn, Co, Cu, Y, Zr, Ba, La and Nd) is similar for the two disks. The abundance ratios of oxygen, α, r- and s-process elements to iron provide further evidence that the thick disk star population have a different chemical history compared to the thin disk. The stars formation rate was probably higher in the Galactic thick disk than in the Galactic thin disk.

Le bulbe Galactique, est cruciale pour comprendre les processus physiques responsables de la formationde la galaxie. L'étude spectroscopique des étoiles vieilles de faible masse permettre de caractériser endétail la chimie et la cinématique du bulbe. Dans cette thèse, nous avons utilisé des données provenantdu Gaia-ESO survey pour mener une étude détaillée du système du disque ainsi que du bulbeGalactique. La distribution de métallicité du bulbe est bimodale. La population riche en métaux montreune cinématique typique de la barre. Elle présente une caractéristique de double RC et recouvre laséquence du disque mince à haute métallicité dans le plan [Mg/Fe] vs. [Fe/H]. Nous associons cesétoiles avec celles de la barre formée à la suite de l'évolution séculaire du disque mince primordial.D'autre part, la population pauvre en métaux présente une cinématique chaude et ne participe pas à laforme en X du bulbe. Ces étoiles semblent imiter la distribution de celles du disque épais dans le plan[Mg/Fe] vs. [Fe/H]. Quand nous comparons la position en métallicité du genou de cette distribution,qui se trouve à [Fe/H]=-0.37+/-0.09 dex, elle est plus élevée de 0.6 dex par rapport au disque épais. Unmodèle d'évolution chimique permet de bien ajuster cette distribution pour les étoiles du bulbe ensupposant un épisode de formation stellaire rapide (<1 Gyr) et intense. L'origine du bulbe pauvre enmétaux reste encore relativement incomprise, mais divers projets futurs devraient permettre de faire ladistinction entre les processus violents ou ceux liés à une évolution séculaire qui ont pu contribuer à saformation / The Galactic bulge, as a massive and old Galactic component, is key to understand the physicalprocesses responsibles for the formation of the Galaxy. The spectroscopic study of long lived low massstars represents an opportunity to characterize the detailed chemical and kinematical patterns of theeventual mix of stellar populations building up the bulge. In this thesis we made use of data comingfrom the Gaia-ESO survey to conduct a detailed analysis of the disk system as well as bulge stellarpopulations. The bulge metallicity distribution function is bimodal. The metal-rich population exhibitsbar-like kinematics, displays the double RC feature and overlaps the metal-rich end of the thin disksequence in the [Mg/Fe] vs. [Fe/H] plane. We associate these stars with the bar X-shape bulge formedas the product of secular evolution of the early thin disk. On the other hand, the metal-poor populationpresents isotropic hot kinematics and does not participate in the X-shaped bulge. When compared to thethick disk, bulge stars seem to mimic their distribution in the [Mg/Fe] vs. [Fe/H] plane. Whencomparing the metallicity position of the so called ``knee'', that of the bulge is found to be at [Fe/H]=-0.37+/-0.09 dex, being 0.6 dex higher than that of the thick disk. A chemical evolution model suitablyfits the whole bulge sequence by assuming a fast (<1 Gyr) intense burst of star formation taking place atearly epochs. The origin of the metal-poor bulge still remains unconstrained, but further research shouldallow to distinguish between violent processes or secular evolution for its origin