In this thesis we explored the use of nuclear stellar discs as tracers of the merging history of early-type galaxies. These small structures, just a few tens to a few hundreds of parsecs across, are a common but poorly studied feature of early-type galaxies. They are formed during or shortly after merging events due to the infall of gas, which settles in a disc and leads to the formation of new stars. Initial simulations showed that they should not survive a following major merger and could, therefore, be used to trace the epoch when their host galaxies experienced their last major merger event. We produced the first census of nuclear discs and established that their incidence is 20%, fairly independent of the host-galaxy mass or galactic environment. Furthermore, we have more than doubled the sample of nuclear discs with known photometric properties, finding that they give a hint of possessing different characteristics from those of large, galactic discs. Using these nuclear discs as clocks for the assembly history of galaxies requires dating their stellar populations. By combining the use of integral-field spectroscopy with the a priori knowledge of the relative bulge- and disc-light contribution to the observed spectra, as determined by a photometric disc-bulge decomposition, we have shown that it is possible to reduce the degeneracies that affect the study of two superimposed populations and thus that the age of stellar discs can be measured more precisely. To illustrate our method, we present VLT-VIMOS data for NGC 4458, a low-mass slowly rotating early-type galaxy with a disc that we found to be at least 5-6 Gyr old. The presence of such an old central disc in such a small, slowly-rotating and, mostly likely, round galaxy is particularly puzzling and presents a challenge to existing models. Disc fragility is central to our studies and we have expanded the limited initial simulations to study it in more detail. By means of N-body simulations, we have reproduced the final stages of a galaxy encounter by exposing a nuclear disc rotating in the gravitational potential of its host bulge and central supermassive black hole to the impact of a secondary massive black hole. We explored not only major mergers (1:1 mass ratio), but also large minor mergers (1:5 and 1:10), across a variety of collision angles, and assessed the survival of the disc, as perceived by current observational limits, both for photometry and spectroscopy. As expected, the discs do not survive a major merger whereas it is in general possible to detect their presence after a 1:5 or 1:10 encounter, in particular when looking at kinematic signatures with spectroscopy. This thesis has demonstrated that nuclear discs constitute both a common and accurate tool for constraining the assembling history of nearby early-type galaxies. The advent of more sensitive integral-field spectrographs, such as MUSE, will make measuring the stellar age of nuclear discs not only more precise, but also more economical in terms of telescope time. This will allow embarking on a more systematic age dating campaign for nuclear stellar discs across a wider range of type, mass and galactic environments for their host galaxies. Combining such a census with a larger set of numerical simulations aimed at calibrating better the range of merger event that would erase any photometric or kinematic signature of a nuclear disc, should finally allow us to put firm constraints on the merging history of early-type galaxies.

The Apache Point Observatory Galactic Evolution Experiment (APOGEE), one of the programs in the Sloan Digital Sky Survey III (SDSS-III), has now completed its systematic, homogeneous spectroscopic survey sampling all major populations of the Milky Way. After a three-year observing campaign on the Sloan 2.5 m Telescope, APOGEE has collected a half million high-resolution (R similar to 22,500), high signal-to-noise ratio (>100), infrared (1.51-1.70 mu m) spectra for 146,000 stars, with time series information via repeat visits to most of these stars. This paper describes the motivations for the survey and its overall design-hardware, field placement, target selection, operations-and gives an overview of these aspects as well as the data reduction, analysis, and products. An index is also given to the complement of technical papers that describe various critical survey components in detail. Finally, we discuss the achieved survey performance and illustrate the variety of potential uses of the data products by way of a number of science demonstrations, which span from time series analysis of stellar spectral variations and radial velocity variations from stellar companions, to spatial maps of kinematics, metallicity, and abundance patterns across the Galaxy and as a function of age, to new views of the interstellar medium, the chemistry of star clusters, and the discovery of rare stellar species. As part of SDSS-III Data Release 12 and later releases, all of the APOGEE data products are publicly available.

Here, we present the results from our analysis of 6 yr of optical photometry taken by the Siding Spring Survey (SSS). This completes a search for periodic variable stars within the 30 000 deg(2) of the sky covered by the Catalina Surveys. The current analysis covers 81 million sources with declinations between -20 degrees. and -75 degrees. with median magnitudes in the range 11 < V < 19.5. We find approximately 34 000 new periodic variable stars in addition to the similar to 9000 RR Lyrae that we previously discovered in SSS data. This brings the total number of periodic variables identified in Catalina data to similar to 110 000. The new SSS periodic variable stars mainly consist of eclipsing binaries, RR Lyrae, LPVs, RS CVn stars, delta Scutis, and Anomalous Cepheids. By cross-matching these variable stars with those from prior surveys, we find that similar to 90 per cent of the sources are new discoveries and recover similar to 95 per cent of the known periodic variables in the survey region. For the known sources, we find excellent agreement between our catalogue and prior values of luminosity, period, and amplitude. However, we find many variable stars that had previously been misclassified. Examining the distribution of RR Lyrae, we find a population associated with the Large Magellanic Cloud (LMC) that extends more than 20 degrees from its centre confirming recent evidence for the existence of a very extended stellar halo in the LMC. By combining SSS photometry with Dark Energy Survey data, we identify additional LMC halo RR Lyrae, thus confirming the significance of the population.

Galaxy evolution depends on a diverse suite of factors, from the environment in which the galaxy exists to the number of supernovae that explode throughout its history. The structure and stellar populations present will also be altered by a galaxy's merger history, stellar mass, star formation rate, among other influences. Some factors, like mergers, are dependent on the environment of the galaxy, while others, like feedback from star formation, are intrinsic to the galaxy themselves. Dwarf galaxies are sensitive to many of these factors due to their smaller masses, hence shallower potential wells. Dwarfs are also interesting in themselves as the least massive structures that can form stars, forming the faint limit of galaxy types. There is some indication that the evolutionary pathway of dwarfs might be different than their more massive counterparts. Indeed, some dwarfs may be the stripped remnants of larger galaxy after a major interaction. Regardless, dwarfs are thought to be the building blocks of larger galaxies via hierarchical galaxy formation and understanding these small dwarfs helps us build a more complete picture of galaxy formation and evolution at all masses. As dwarfs generally have low stellar mass, they are very faint. Our most complete sample of dwarfs is therefore restricted to those that are nearby. These nearby systems are dominated by dwarfs satellite to the Milky Way and M31. However, the evolution of these satellites will be greatly influenced by their massive host. By studying nearby isolated dwarfs, we can try to separate the secular evolutionary processes of dwarfs from the influence of their larger host. Additionally, stellar populations can be resolved in these nearby galaxies, and so their structures can be probed to much fainter regimes than integrated light studies allow. The Sagittarius Dwarf Irregular Galaxy (Sag DIG) is one of the most isolated, low mass galaxies, located at the edge of the Local Group. Its isolation from other galaxies coupled with its relative proximity provide an excellent opportunity to study the intrinsic properties of this low mass system. We preform an in-depth analysis of its resolved stellar populations and its structural properties as the first galaxy in the larger dataset, Solitary Local Dwarfs Survey (Solo). Solo is a wide field photometric study targeting every isolated dwarf galaxy within 3 Mpc of the Milky Way. Solo is based on (u)gi multi-band imaging from CFHT/MegaCam for northern targets, and Magellan/Megacam for southern targets. All galaxies fainter than M ~ -18 (in the V band) situated beyond the nominal virial radius of the Milky Way and M31 (> 300 kpc) are included in this volume-limited sample, for a total of 42 targets. For Sag DIG, we provide updated estimates of its central surface brightness and integrated luminosity, and trace its surface brightness profile to a level fainter than 30 mag./sq.arcsec. Sag DIG is well described by a highly elliptical (disk-like) system following a single component Sersic model. However, a low-level distortion is present at the outer edges of the galaxy that, were Sag DIG not so isolated, would likely be attributed to some kind of previous tidal interaction. Further, we find evidence of an extremely low level, extended distribution of stars beyond ~5 arcmins (> 1.5 kpc) that suggests Sag DIG may be embedded in a very low density stellar halo. We compare the stellar and HI structures of Sag DIG, and discuss results for this galaxy in relation to other isolated, dwarf irregular galaxies in the Local Group. Sag DIG, and the similarly isolated dwarf Aquarius, both have HI distributions that are more circular than their stellar components. In contrast, Wolf - Lundmark - Mellote (WLM), another isolated but slightly more massive dwarf, has stellar and HI components that trace each other well. Sag DIG and Aquarius also differ in that there is no signature of rotation in the HI of Sag DIG, while there is clear rotation in both the HI and stellar component for Aquarius. These preliminary comparisons demonstrate some of the potential analysis which will be possible on a much larger scale with the full Solo Survey. / Graduate / 0606 / higgs@uvic.ca

A Milky Way region in the direction of the North America and Pelican Nebulae is investigated using CCD photometry of 2600 stars in the Vilnius seven-color system. Its central part was covered with a CCD camera on the Maksutov wide-field telescope of the Moletai Observatory. Deeper CCD exposures for most interesting selected targets were taken with the 1 m telescope of the USNO Flagstaff Station. The author developed a new method for CCD data processing trying to reduce systematic errors and to increase photometric accuracy. This allowed us to measure M67 stars with high precision and use it as a CCD standard region in all our subsequent observations. To investigate interstellar extinction law in the area we obtained spectra for 33 B-type stars with the 2.3 m telescope of Steward Observatory at Kitt Peak. We also used near-infrared 2MASS data and other data sources when they were available. From photometric results in the Vilnius system we determined two-dimensional spectral types, color excesses, interstellar extinctions and distances of ~1300 stars. For more accurate determination of open cluster ages we recalibrated the zero-age main sequence in absolute magnitude vs. color diagram of the Vilnius system. We also investigated the possible connection between two open clusters in the area and surrounding star-forming region. Young stellar objects and possible ionizing sources of the NAP gas clouds were identified. / Naudojant CCD fotometriją, ištirta Paukščių Tako sritis Šiaurės Amerikos ir Pelikano ūkų kryptimi. Vilniaus fotometrinėje sistemoje išmatuota 2600 žvaigždžių. Didesnė sritis, apimanti abu ūkus ir tarp jų esantį dulkių debesį L935 buvo eksponuojama Molėtų observatorijos plataus lauko Maksutovo sistemos teleskopu. Mažesnės sritys, turinčios svarbių objektų, buvo eksponuojamos JAV laivyno observatorijos Flagstafo stoties 1 m. teleskopu. Autorius sukūrė metodiką. kaip ženkiai sumažinti sistematines redukcijos paklaidas ir padidinti CCD fotometrijos tikslumą. Tiksliai išmatuoti CCD fotometriniai standartai padrikąjame spiečiuje M67 bus naudojami tolimesniuose stebėjimuose. Ekstinkcijos dėsnio tyrimui buvo gauti 33 O-B žvaigždžių spektrai Stiuarto observatorijos 2.3 m teleskopu. Taip pat buvo panaudota 2MASS infraraudonosios fotometrijos katalogas ir kiti duomenys. Iš CCD ekspozicijų Vilniaus sistemoje fotometrijos daugumai fotometruotų žvaigždžių nustatyta: spektrinė ir šviesio klasė (MK sistemoje), spalvos ekscesas, tarpžvaigždinė ekstinkcija ir nuotolis. Tikslesniam padrikųjų spiečių amžiaus nustatymui iš naujo sukalibruota nulinio amžiaus pagrindinė seka Vilniaus fotometrinėje sistemoje. Ištirti du padrikieji žvaigždžių spiečiai esantys Šiaurės Amerikos ūko kryptimi ir patikrintas jų ryšys su aplinkine žvaigždėdaros sritimi. Aptikta apie 30 jaunų emisinių žvaigždžių ir 4 ar 5 žvaigždės, kurios galėtų būti ūkų jonizacijos šaltiniai.

Naudojant CCD fotometriją, ištirta Paukščių Tako sritis Šiaurės Amerikos ir Pelikano ūkų kryptimi. Vilniaus fotometrinėje sistemoje išmatuota 2600 žvaigždžių. Didesnė sritis, apimanti abu ūkus ir tarp jų esantį dulkių debesį L935 buvo eksponuojama Molėtų observatorijos plataus lauko Maksutovo sistemos teleskopu. Mažesnės sritys, turinčios svarbių objektų, buvo eksponuojamos JAV laivyno observatorijos Flagstafo stoties 1 m. teleskopu. Autorius sukūrė metodiką. kaip ženkiai sumažinti sistematines redukcijos paklaidas ir padidinti CCD fotometrijos tikslumą. Tiksliai išmatuoti CCD fotometriniai standartai padrikąjame spiečiuje M67 bus naudojami tolimesniuose stebėjimuose. Ekstinkcijos dėsnio tyrimui buvo gauti 33 O-B žvaigždžių spektrai Stiuarto observatorijos 2.3 m teleskopu. Taip pat buvo panaudota 2MASS infraraudonosios fotometrijos katalogas ir kiti duomenys. Iš CCD ekspozicijų Vilniaus sistemoje fotometrijos daugumai fotometruotų žvaigždžių nustatyta: spektrinė ir šviesio klasė (MK sistemoje), spalvos ekscesas, tarpžvaigždinė ekstinkcija ir nuotolis. Tikslesniam padrikųjų spiečių amžiaus nustatymui iš naujo sukalibruota nulinio amžiaus pagrindinė seka Vilniaus fotometrinėje sistemoje. Ištirti du padrikieji žvaigždžių spiečiai esantys Šiaurės Amerikos ūko kryptimi ir patikrintas jų ryšys su aplinkine žvaigždėdaros sritimi. Aptikta apie 30 jaunų emisinių žvaigždžių ir 4 ar 5 žvaigždės, kurios galėtų būti ūkų jonizacijos šaltiniai. / A Milky Way region in the direction of the North America and Pelican Nebulae is investigated using CCD photometry of 2600 stars in the Vilnius seven-color system. Its central part was covered with a CCD camera on the Maksutov wide-field telescope of the Moletai Observatory. Deeper CCD exposures for most interesting selected targets were taken with the 1 m telescope of the USNO Flagstaff Station. The author developed a new method for CCD data processing trying to reduce systematic errors and to increase photometric accuracy. This allowed us to measure M67 stars with high precision and use it as a CCD standard region in all our subsequent observations. To investigate interstellar extinction law in the area we obtained spectra for 33 B-type stars with the 2.3 m telescope of Steward Observatory at Kitt Peak. We also used near-infrared 2MASS data and other data sources when they were available. From photometric results in the Vilnius system we determined two-dimensional spectral types, color excesses, interstellar extinctions and distances of ~1300 stars. For more accurate determination of open cluster ages we recalibrated the zero-age main sequence in absolute magnitude vs. color diagram of the Vilnius system. We also investigated the possible connection between two open clusters in the area and surrounding star-forming region. Young stellar objects and possible ionizing sources of the NAP gas clouds were identified.

One of the key goals of Milky Way science is measuring the distribution of dark matter in the Galaxy. Through the study of Galactic dynamics, inferences can be made about the structure of the Galaxy, and hence the dark matter distribution. To this end, we present a study of methods useful for modelling and understanding dynamical systems in the Galaxy. A natural choice of coordinate system when studying dynamical systems is the canonical system of angle-action coordinates. We present methods for estimating the angle-actions in both axisymmetric and triaxial potentials. These fall into two categories: non-convergent and convergent. The non-convergent methods are fast approaches, mostly based on approximations to Stäckel potentials. We investigate the accuracy of these methods for realistic Galactic potentials. The slower convergent methods operate by constructing generating functions to take us from simple analytically-tractable potentials to our target potential. Tidal streams should prove useful for constraining the large-scale dark matter distribution in the Galaxy. Armed with our new angle-action tools, we investigate the properties of known streams in a realistic Galactic potential. We present a simple algorithm for constraining the Galactic potential using a tidal stream, which exploits the expected structure of a stream in the angle-frequency space of the true potential. We expand this approach into a fully probabilistic scheme that allows for handling of large errors, missing data and outliers. We close by discussing another tool useful for modelling the dynamics of the Galaxy: extended distribution functions for the Galactic disc. We present a simple extension of an action-based distribution function from Binney (2010) that includes metallicity information, and compare the model predictions with current data. These models are essential for incorporating the selection effects of any survey, and reveal the important chemo-dynamic correlations that expose the history and evolution of the Galaxy.

Despite striking evidence for the existence of dark matter from astrophysical observations, dark matter has still escaped any direct or indirect detection until today. Therefore a proof for its existence and the revelation of its nature belongs to one of the most intriguing challenges of nowadays cosmology and particle physics. The present work tries to investigate the nature of dark matter through indirect signatures from dark matter annihilation into electron-positron pairs in two different ways, pressure from dark matter annihilation and multi-messenger constraints on the dark matter annihilation cross-section. We focus on dark matter annihilation into electron-positron pairs and adopt a model-independent approach, where all the electrons and positrons are injected with the same initial energy E_0 ~ m_dm*c^2. The propagation of these particles is determined by solving the diffusion-loss equation, considering inverse Compton scattering, synchrotron radiation, Coulomb collisions, bremsstrahlung, and ionization. The first part of this work, focusing on pressure from dark matter annihilation, demonstrates that dark matter annihilation into electron-positron pairs may affect the observed rotation curve by a significant amount. The injection rate of this calculation is constrained by INTEGRAL, Fermi, and H.E.S.S. data. The pressure of the relativistic electron-positron gas is computed from the energy spectrum predicted by the diffusion-loss equation. For values of the gas density and magnetic field that are representative of the Milky Way, it is estimated that the pressure gradients are strong enough to balance gravity in the central parts if E_0 < 1 GeV. The exact value depends somewhat on the astrophysical parameters, and it changes dramatically with the slope of the dark matter density profile. For very steep slopes, as those expected from adiabatic contraction, the rotation curves of spiral galaxies would be affected on kiloparsec scales for most values of E_0. By comparing the predicted rotation curves with observations of dwarf and low surface brightness galaxies, we show that the pressure from dark matter annihilation may improve the agreement between theory and observations in some cases, but it also imposes severe constraints on the model parameters (most notably, the inner slope of the halo density profile, as well as the mass and the annihilation cross-section of dark matter particles into electron-positron pairs). In the second part, upper limits on the dark matter annihilation cross-section into electron-positron pairs are obtained by combining observed data at different wavelengths (from Haslam, WMAP, and Fermi all-sky intensity maps) with recent measurements of the electron and positron spectra in the solar neighbourhood by PAMELA, Fermi, and H.E.S.S.. We consider synchrotron emission in the radio and microwave bands, as well as inverse Compton scattering and final-state radiation at gamma-ray energies. For most values of the model parameters, the tightest constraints are imposed by the local positron spectrum and synchrotron emission from the central regions of the Galaxy. According to our results, the annihilation cross-section should not be higher than the canonical value for a thermal relic if the mass of the dark matter candidate is smaller than a few GeV. In addition, we also derive a stringent upper limit on the inner logarithmic slope α of the density profile of the Milky Way dark matter halo (α < 1 if m_dm < 5 GeV, α < 1.3 if m_dm < 100 GeV and α < 1.5 if m_dm < 2 TeV) assuming a dark matter annihilation cross-section into electron-positron pairs (σv) = 3*10^−26 cm^3 s^−1, as predicted for thermal relics from the big bang. / Trotz vieler Hinweise auf die Existenz von dunkler Materie durch astrophysikalische Beobachtungen hat sich die dunkle Materie bis heute einem direkten oder indirekten Nachweis entzogen. Daher gehrt der Nachweis ihrer Existenz und die Enthüllung ihrer Natur zu einem der faszinierensten Herausforderungen der heutigen Kosmologie und Teilchenphysik. Diese Arbeit versucht die Natur von dunkler Materie durch indirekte Signaturen von der Paarzerstrahlung dunkler Materie in Elektron-Positronpaare auf zwei verschiedene Weisen zu untersuchen, nämlich anhand des Drucks durch die Paarzerstrahlung dunkler Materie und durch Grenzen des Wirkungsquerschnitts für die Paarzerstrahlung dunkler Materie aus verschiedenen Beobachtungsbereichen. Wir konzentrieren uns dabei auf die Zerstrahlung dunkler Materie in Elektron-Positron-Paare und betrachten einen modellunabhängigen Fall, bei dem alle Elektronen und Positronen mit der gleichen Anfangsenergie E_0 ~ m_dm*c^2 injiziert werden. Die Fortbewegung dieser Teilchen wird dabei bestimmt durch die Lösung der Diffusions-Verlust-Gleichung unter Berücksichtigung von inverser Compton-Streuung, Synchrotronstrahlung, Coulomb-Streuung, Bremsstrahlung und Ionisation. Der erste Teil dieser Arbeit zeigt, dass die Zerstrahlung dunkler Materie in Elektron-Positron-Paare die gemessene Rotationskurve signifikant beeinflussen kann. Die Produktionsrate ist dabei durch Daten von INTEGRAL, Fermi und H.E.S.S. begrenzt. Der Druck des relativistischen Elektron-Positron Gases wird aus dem Energiespektrum errechnet, welches durch die Diffusions-Verlust-Gleichung bestimmt ist. Für Werte der Gasdichte und des magnetischen Feldes, welche für unsere Galaxie repräsentativ sind, lässt sich abschätzen, dass für E_0 < 1 GeV die Druckgradienten stark genug sind, um Gravitationskräfte auszugleichen. Die genauen Werte hängen von den verwendeten astrophysikalischen Parametern ab, und sie ändern sich stark mit dem Anstieg des dunklen Materie-Profils. Für sehr große Anstiege, wie sie für adiabatische Kontraktion erwartet werden, werden die Rotationskurven von Spiralgalaxien auf Skalen von einegen Kiloparsek für die meisten Werte von E_0 beeinflusst. Durch Vergleich der erwarteten Rotationskurven mit Beobachtungen von Zwerggalaxien und Galaxien geringer Oberflächentemperatur zeigen wir, dass der Druck von Zerstrahlung dunkler Materie die Übereinstimmung von Theorie und Beobachtung in einigen Fällen verbessern kann. Aber daraus resultieren auch starke Grenzen für die Modellparameter - vor allem für den inneren Anstieg des Halo-Dichteprofils, sowie die Masse und den Wirkungsquerschnitt der dunklen Materie-Teilchen. Im zweiten Teil werden obere Grenzen für die Wirkungsquerschnitte der Zerstrahlung der dunkler Materie in Elektron-Positron-Paare erhalten, indem die beobachteten Daten bei unterschiedlichen Wellenlängen (von Haslam, WMAP und Fermi) mit aktuellen Messungen von Elektron-Positron Spektren in der solaren Nachbarschaft durch PAMELA, Fermi und H.E.S.S. kombiniert werden. Wir betrachten Synchrotronemission bei Radiound Mikrowellenfrequenzen, sowie inverse Compton-Streuung und Final-State-Strahlung bei Energien im Bereich der Gamma-Strahlung. Für die meisten Werte der Modellparameter werden die stärksten Schranken durch das lokale Positron-Spektrum und die Synchrotronemission im Zentrum unser Galaxie bestimmt. Nach diesen Ergebnissen sollte der Wirkungsquerschnitt für die Paarzerstrahlung nicht größer als der kanonische Wert für thermische Relikte sein, wenn die Masse der dunklen Materie-Kandidaten kleiner als einige GeV ist. Zusätzlich leiten wir eine obere Grenze für den inneren logarithmische Anstieg α des Dichteprofiles des dunklen Materie Halos unserer Galaxie ab.

Remarkably, the central regions of galaxies are very important in shaping and influencing galaxies as a whole. As such, galaxy cores can be used for classification, to determine which processes may be important in galaxy formation and evolution. Past studies, for example, have found a dichotomy in the inner slopes of early-type galaxy surface brightness profiles. Using deprojections of the galaxies from the ACS Virgo and Fornax Cluster Surveys (ACSVCS/FCS), we show that, in fact, this dichotomy does not exist. Instead, we demonstrate that the brightest early-type galaxies tend to have central light deficits, a trend which gradually transitions to central light excesses – also known as compact stellar nuclei – as we go to fainter galaxies. This effect is quantified, and can be used to determine what evolutionary factors are important as we move along the galaxy luminosity function. The number of stellar nuclei that we observe is, in fact, an unexpected result emerging from the ACSVCS/FCS. Being three times more common than previously thought, they are present in the vast majority of intermediate and low-luminosity galaxies. Conversely, it has been known for over a decade that there is likely a supermassive black hole weighing millions to billions of solar masses at the center of virtually every galaxy of sufficient size. These black holes are known to follow scaling relations with their host galaxies. Using the ACSVCS, along with new kinematical data from long-slit spectroscopy, we measure the dynamical masses of 83 galaxies, and show that supermassive black holes and nuclei appear to fall along the same scaling relation with host mass. Both represent approximately 0.2% of their host’s mass, implying an important link between the two types of central massive objects. Finally, we extract elliptical isophotes and fit parameterized models to the surface brightness profiles of new Hubble Space Telescope imaging of the ACSVCS galaxies, observed in infrared and ultraviolet bandpasses. Taken together, the two surveys represent an unprecedented collection of isophotal and structural parameters of early-type galaxies, and will allow us to learn a great deal about the stellar populations and formation histories of galaxy cores. / Graduate

La formation et l'évolution du disque épais de la Voie Lactée restent controversées. Nous avons utilisé un modèle de synthèse de la population de la Galaxie, le Modèle de la Galaxie de Besançon (Robin et al., 2003), qui peut être utilisé pour l'interprétation des données, étudier la structure galactique et tester différents scénarios de formation et évolution Galactique. Nous avons examiné ces questions en étudiant la forme et la distribution de métallicité du disque mince et du disque épais en utilisant l'approche de synthèse de la population. Nous avons imposé sur des simulations les erreurs d'observation et les biais afin de les rendre directement comparables aux observations. Nous avons corrigé les magnitudes et les couleurs des étoiles de la simulation, en utilisant un modèle d'extinction. Les modèles d'extinction disponibles ne reproduisent pas toujours la quantité exacte d'extinction le long de la ligne de visée. Un programme a été développé pour corriger la distribution de l'extinction en fonction de la distance le long de ces lignes. Les extinctions correctes ont ensuite été appliquées sur les simulations du modèle. Nous avons étudié la forme du disque mince en utilisant des données photométriques aux basses latitudes du sondage SDSS-SEGUE. Nous avons comparé qualitativement et quantitativement les observations et les simulations et nous avons essayé de contraindre la fonction de masse initiale. En utilisant la spectroscopie du relevé SEGUE, nous avons sélectionné les étoiles du turn-off de la séquence principale (MSTO) (Cheng et al 2012) et des géantes K pour étudier la distribution de métallicité du disque mince et du disque épais. Nous avons calculé une estimation de distance pour chaque étoile à partir de la relation entre les températures effectives et magnitudes absolues pour les catalogues observés et simulés. Ces deux catalogues ont les mêmes biais sur les distances, elles sont donc comparables. Nous avons développé un outil basé sur une méthode MCMC-ABC pour déterminer la distribution de la métallicité et étudier les corrélations entre les paramètres ajustés. Nous avons confirmé la présence d'un gradient de métallicité radiale de -0.079 ± 0.015 dex kpc−1 pour le disque mince. Nous avons obtenu une métallicité du disque épais au voisinage solaire de -0.47 ± 0.03 dex, compatible avec les résultats obtenus par les études précédentes. De plus, le disque épais ne montre pas de gradient, mais les données sont compatibles avec un gradient positif intérieur suivi d'un négatif extérieur. Nous avons ensuite appliqué les outils développés au relevé spectroscopique Gaia-ESO et calculé la distribution de métallicité des étoiles F/G/K dans le disque mince et épais en supposant une formation en deux époques du disque épais de la Voie Lactée. Nous avons obtenu une métallicité locale dans le disque épais de -0.23 ± 0.04 dex légèrement plus élevée que celle obtenue avec SEGUE mais en accord avec Adibekyan et al. (2013) et un gradient de métallicité radiale du disque épais en accord avec notre analyse précédente des données de SEGUE et la littérature. La métallicité locale est en accord avec la littérature au niveau de 3σ mais parce que les données GES sont préliminaires, une analyse plus approfondie avec plus de données et de meilleurs calibrations doit être faite. L'existence d'un gradient plat dans le disque épais peut être une conséquence d'une formation à partir d’un gaz turbulent et bien homogène, ou bien un fort mélange radial a brassé après coup les étoiles. / The formation and evolution of the thick disc of the Milky Way remain controversial. We made use of a population synthesis model of the Galaxy, the Besançon Galaxy Model (Robin et al. 2003), which can be used for data interpretation, study the Galactic structure and test different scenarios of Galaxy formation and evolution. We examined these questions by studying the shape and the metallicity distribution of the thin and thick disc using the population synthesis approach. We imposed on simulations observational errors and biases to make them directly comparable to observations. We corrected magnitudes and colors of stars, from the simulation, using an extinction model. The available extinction models do not always reproduce the exact quantity of extinction along the line of sight. A code to correct the distribution of extinction in distance along these lines have been developed and the corrected extinctions have been applied on model simulations. We studied the shape of the thin disc using photometric data at low latitudes from the SDSS-SEGUE survey. We compared qualitatively and quantitatively observations and simulations and try to constrain the Initial Mass Function. Using the spectroscopic survey SEGUE we selected Main Sequence Turnoff (MSTO) stars (Cheng et al 2012) and K giants to study the metallicity distribution of the thin and thick discs. We computed a distance for each star from the relation between effective temperatures and absolute magnitudes for the observed and simulated catalogs. These two catalogues have the same biases in distances, therefore are comparable. We developed a tool based on a MCMC-ABC method to determine the metallicity distribution and study the correlations between the fitted parameters. We confirmed a radial metallicity gradient of -0.079 ± 0.015 dex kpc−1 for the thin disc. We obtained a solar neighborhood metallicity of the thick disc of -0.47 ± 0.03 dex similar to previous studies and the thick disc shows no gradient but the data are compatible with an inner positive gradient followed by a outer negative one. Furthermore, we have applied the developed tools to the Gaia-ESO spectroscopic survey and computed the metallicity distribution of F/G/K stars in the thin and thick disc assuming a two epoch formation for the thick disc of the Milky Way. We obtained a local metallicity in the thick disc of -0.23 ± 0.04 dex slightly higher than the one obtained with SEGUE but in agreement with Adibekyan et al. (2013) and a radial metallicity gradient for the thick disc in agreement with our previous analysis of SEGUE data and the literature. The local metallicity is in fair agreement with literature at the 3σ level but because the GES data is an internal release under testing further analysis with more data and better calibrations have to be done. The existence of a flat gradient in the thick disc can be a consequence of an early formation from a highly turbulent homogeneous well mixed gas, unless it has suffered heavy radial mixing later on.