O objetivo desta tese é analisar aspectos do sistema de satélites da Via Láctea de relevância cosmológica. Dentre estes aspectos destacam-se dois: o censo de satélites da Galáxia – onde constata-se que a quantidade destes objetos é muito inferior ao predito por modelos cosmológicos do tipo Matéria Escura Fria – e a frequência anômala de satélites luminosos, como a Pequena e Grande Nuvem de Magalhães (SMC e LMC respectivamente). Além disso, a determinação dos parâmetros estruturais da LMC pode impor vínculos a sua formação, histórico orbital e sobre a massa de nossa Galáxia. Neste trabalho é desenvolvida uma técnica de busca por satélites da Via Láctea. Esta técnica foi otimizada para utilizar dados da nova geração de grandes surveys de maneira eficiente. Este código, o FindSat, foi validado em uma amostra de galáxias anãs conhecidas e se mostrou eficiente em detectar as galáxias anãs mais tênues de que se tem registro. A aplicação desse código a uma região do Sloan Digital Sky Survey (SDSS) ainda não explorada nesse sentido revelou uma série de candidatos a novos satélites. Foram selecionados os candidatos mais promissores para observação de follow-up. Estas observações revelaram que um destes candidatos é de fato um novo satélite da Via Láctea. Este novo objeto é muito provavelmente um aglomerado globular do halo em estágio avançado de dissolução, porém, seu tamanho e magnitude integrada colocamno em um domínio limítrofe entre aglomerado e galáxia anã. Através da cuidadosa análise dos demais candidatos, constatou-se que nenhum outro é de fato um novo satélite da Galáxia. Além disso, foi realizado o estudo do perfil de densidade e geometria da LMC. Este estudo utilizou dados de verificação científica do Dark Energy Survey (DES). Constatase que o perfil de densidades para estrelas jovens (< 3 Gyr) possui um raio de escala cerca de 50% menor que o da população velha (> 3 Gyr), favorecendo o cenário de formação tipo outside-in. O estudo da extensão da componente estelar da LMC revela um raio de maré de cerca de 18 kpc, permitindo o cálculo da massa dinâmica total da LMC. O valor de massa obtido favorece a hipótese onde as Nuvens de Magalhães estariam por sua primeira passagem pelo perigaláctico. Além disso, a distância heliocêntrica e espessura do disco da LMC foram determinadas utilizando estrelas do Red Clump (RC). Notou-se que regiões no extremo norte da LMC estão sistematicamente mais próximas de nós do que o esperado, este efeito evidencia o warp no disco dessa galáxia. Observou-se que a espessura do disco aumenta na periferia da LMC, caracterizando o fenômeno de flare. O aumento na espessura juntamente com a maior extensão da população velha da LMC é interpretado como a presença de dois componentes discoidais. Esta é a primeira evidência desse tipo baseada apenas em métodos de contagem de estrelas. / The goal of this thesis is to analyse comologically relevant aspects of the Milky Way (MW) satellite system. Among these we may highlight two: the census of MW satellites – where the observed number of these objects is much less than what is expected by Cold Dark Matter (CDM) models – and the anomalous frequency of luminous satellites, such as the Small and Large Magellanic Clouds (SMC and LMC respectively). Besides the cosmological importance of the Clouds, the determination of its structural parameters may help to constraint models for their formation, orbital history, and ultimately the mass assembly in our Galaxy. In this work a technique to search newMWsatellites is developed. This technique was optimized to run efficiently on large datasets, such as the ones being generated by the new generation of surveys. The code, FindSat was validated in a sample of well known MW satellites and has proven to be well succeeded even for the most faint of these objects. The application of this code to an unexplored region of the Sloan Digital Sky Survey (SDSS) revealed a large amount of new dwarf galaxy candidates, some of which where selected for follow-up observation. These observations led to the discovery of a new MW satellite. This new object is most likely a globular cluster in an extreme stage of dissolution. However, its integrated magnitude and size makes it difficult to discern it from a dwarf galaxy. By a careful analysis of the remaining candidates, it was shown that no other new satellite was in the sample. The density profile and geometry of the LMC was also analysed. This study used the recent science verification data from the Dark Energy Survey (DES). It was found that the density profile for young stars (< 3 Gyr) has a scale radius 50% smaller when compared to the one obtained for older stars (> 3 Gyr). This result favours the outsidein galaxy formation scenario. The total extension of the LMC stellar component was measured, allowing the estimate of a truncation radius of about 18 kpc. Assuming that this truncation has tidal origins the dynamical mass of the LMC is inferred. The mass value found favours the case for the first perigalactic passage of the Clouds. Besides that, the heliocentric distance and thickness of the LMC disk was determined using Red Clump (RC) stars. Evidence for warp was found in the North edge of the LMC, in the sense that the disk is systematically more distance than expected. While the thickness of the disk increases towards the outer parts of the LMC, which is a phenomena known as flare. This effect joined with the fact that the older LMC stellar population is more extended, favours the presence of two disk components in this galaxy. This is the first evidence of this kind based only on star counts.

The Large and Small Magellanic Clouds are unique local laboratories for studying the formation and evolution of small galaxies in exquisite detail. The Survey of the MAgellanic Stellar History (SMASH) is an NOAO community Dark Energy Camera (DECam) survey of the Clouds mapping 480 deg2 (distributed over similar to 2400 square degrees at similar to 20% filling factor) to similar to 24th. mag in ugriz. The primary goals of SMASH are to identify low surface brightness stellar populations associated with the stellar halos and tidal debris of the Clouds, and to derive spatially resolved star formation histories. Here, we present a summary of the survey, its data reduction, and a description of the first public Data Release (DR1). The SMASH DECam data have been reduced with a combination of the NOAO Community Pipeline, the PHOTRED automated point-spread-function photometry pipeline, and custom calibration software. The astrometric precision is similar to 15 mas and the accuracy is similar to 2 mas with respect to the Gaia reference frame. The photometric precision is similar to 0.5%-0.7% in griz and similar to 1% in u with a calibration accuracy of similar to 1.3% in all bands. The median 5s point source depths in ugriz are 23.9, 24.8, 24.5, 24.2, and 23.5 mag. The SMASH data have already been used to discover the Hydra II Milky Way satellite, the SMASH 1 old globular cluster likely associated with the LMC, and extended stellar populations around the LMC out to R. similar to. 18.4 kpc. SMASH DR1 contains measurements of similar to 100 million objects distributed in 61 fields. A prototype version of the NOAO Data Lab provides data access and exploration tools.

Stars on the asymptotic giant branch (AGB) lose a significant fraction of their mass to their surroundings through stellar winds. As a result, they are surrounded by circumstellar shells of gas and dust. This stellar mass loss replenishes and enriches the interstellar medium (ISM) with the products of stellar nucleosynthesis, progressively increasing its metallicity and thereby driving galactic chemical evolution. In this thesis I present a comprehensive study of oxygen-rich (O-rich) AGB stars and red supergiants (RSG) observed with the Spitzer Infrared Spectrograph and Infrared Space Observatory Short Wavelength Spectrometer in the Milky Way, the Large and Small Magellanic Clouds, and Galactic globular clusters; focusing on the composition of the dust in the circumstellar envelopes surrounding these stars. Combining spectroscopic and photometric observations with the GRAMS grid of radiative transfer models to derive (dust) mass-loss rates, I detect crystalline silicates in stars with dust mass-loss rates which span over a factor of 1000, down to rates of ~10^{-9} Msun/yr. Detections of crystalline silicates are more prevalent in higher mass-loss rate objects, and our results indicate that the dust mass-loss rate has a greater influence on the crystalline fraction than the gas mass-loss rate, suggesting that thermal annealing of amorphous silicate grains is the primary formation mechanism of crystalline silicates in such environments rather than the direct condensation of crystalline silicates from the gas phase. I also find that metallicity influences the composition of crystalline silicates, with enstatite seen increasingly at low metallicity, while forsterite becomes depleted at these metallicities due to the different chemical composition of the gas. To trace the evolution of alumina and silicate dust along the AGB, I present an alternative grid of MODUST radiative transfer-models for a range of dust compositions, mass-loss rates, dust shell inner radii and stellar parameters. Our analysis shows that the AKARI [11]-[15] versus [3.2]-[7] colour is a robust indicator of the fractional abundance of alumina in O-rich AGB stars. From the modelling, I show that a grain mixture consisting primarily of amorphous silicates, with contributions from amorphous alumina and metallic iron provides a good fit to the observed spectra of O-rich AGB stars in the LMC. In agreement with previous studies, we find a correlation between the dust composition and mass-loss rate; the lower the mass-loss rate the higher the percentage of alumina in the shell. Finally, I present mid-infrared observations of the Local Group dwarf elliptical galaxy M32; where I find a large population of dust-enshrouded stars. These observations will act as a pathfinder for future observations with the JWST and SPICA.

Neste projeto de pesquisa planeja-se obter indícios da correlação entre o conteúdo metálico estelar deduzido para a Via Láctea e os indíces metálicos obtidos para as Nuvens de Magalhães. O ponto de apoio para tal comparação é que cada encontro deixará importantes traços na eficiência de formação dos membros do tripleto. À medida que os encontros dependem da história dinâmica, suas ``impressões digitais\'\' deixadas nos conteúdos estelares colocam importantes limitações na história dinâmica e vice-versa. Para tanto, são utilizados os dados espectroscópicos já obtidos com o telescópio Magellan Clay, para uma amostra contendo 28 estrelas ricas em carbono encontradas nas Nuvens de Magalhães. A quantidade de carbono bem como a existência ou não de binaridade nas estrelas da amostra são indícios das possíveis fontes da sobreabundância do carbono. A caracterização da amostra é essencial para o estudo da relação entre duas populações estelares aparentemente distintas: a de estrelas de carbono e a de estrelas pobres em metais enriquecidas em carbono. Para tanto, são utilizados catálogos de espectros assim como critérios fotométricos. A descoberta de alguma relação entre as duas populações pode ajudar a esclarecer o problema dos processos de enriquecimento de carbono, notado nas atmosferas estelares. A futura determinação dos parâmetros físicos das estrelas que compõem a amostra pode revelar detalhes sobre a história de formação estelar dos objetos das Nuvens e, portanto, vincular a evolução dessas duas galáxias-satélites à história da Via Láctea. / This project searches for signs of correlation between metallic stellar content, available for the Milky Way, and the metallicities indices obtained for the Magellanic Clouds. This comparison is supported by the traces on the formation of these three galaxies, that should have been left by each triplet members encounter. Since these crosses depend on the dynamical history, their fingerprints left by stellar content can estabilish constraints to the Galactic dynamical history. Spectroscopic data for a sample of carbon stars, obtained on the Magellanic Telescope, are used in this work. The amount of carbon, as the existence or not of binary stars in this sample, indicates possible sources of this element. The determination of sample properties is essential for studying the constraints between two stellar populations that are apparently distincts: carbon stars and carbon enhanced metal-poor stars. To do this, spectral catalogues and photometric criteria are used. Finding the correlations between both populations will bring some light to the unknown carbon enrichment processes that occured at the stellar atmospheres. Variability, emission lines, and binarity are studied for the sample. Stellar parameters are discussed, although there is no method applicable to this sample. By estimating the physical parameters of the stars presented in this sample and by analysing their carbon abundances, one can provide hints of the star formation history of objects in the Magellanic Clouds and therefore constraint the evolution of these satellite-galaxies to the Milky Way.

A Pequena Nuvem de Magalhães (PNM) é uma galáxia irregular e rica em gás, que juntamente com a Grande Nuvem de Magalhães (GNM) orbita a Via Láctea (VL). Elas formam um sistema triplo em constante interação. A PNM possui metalicidade baixa, e consequentemente seu meio interestelar (MI) apresenta propriedades particularmente diferentes das observadas para o MI da Galáxia. Mais do que isso, a importância do campo magnético em escalas galácticas vem sendo evidenciada cada vez mais. Então, o objetivo desta dissertação foi estudar a estrutura do campo magnético na PNM, e sua relação com componentes do MI desta galáxia. Para este fim, utilizamos dados de polarimetria no óptico, obtidos no Cerro Tololo Inter American Observatory. Construímos um catálogo polarimétrico que contém 7.207 estrelas em 28 campos distribuídos nas secções Nordeste e da Asa da PNM. Os mapas de polarização traçam o campo magnético no plano do céu diretamente, e pode-se obter sua intensidade utilizando o método de Chandrasekhar & Fermi. A partir do catálogo polarimétrico gerado neste trabalho, conseguimos observar que o campo magnético na PNM possui direção bastante irregular, porém é provável a existência de dois padrões em larga escala o primeiro alinhado com a Ponte pan-Magelânica e o segundo alinhado com a Barra da PNM. Obtivemos para o campo magnético regular Bcéu = (1,84 ± 0,11) uG e para o campo turbulento dB = (2,920 ± 0,098) uG. Esse resultado evidencia que na PNM o campo aleatório domina com relação ao de larga escala, justificando a observação de uma configuração tão irregular para os vetores de polarização. Correlacionando os mapas de polarização com estruturas presentes no MI da PNM, pudemos verificar a presença de diversos shells que podem possuir campos magnéticos da ordem de algumas dezenas de uG. Também foi possível observar ambientes onde o campo regular parece ter sido destruído pela turbulência e outros onde ele pode ainda não ter tido tempo de se formar. Derivamos a relação entre polarização e avermelhamento, e obtivemos como resultado que ela é da ordem de P/Av ~ 2, o que indica que na PNM a eficiência para polarização pode ser menor do que na Galáxia, talvez devido a alta turbulência e/ou ao fato de que nela o campo regular é muito baixo. Por fim, a partir da estimativa para as densidades de energia do campo magnético e para o movimento de rotação e de turbulência do gás, pudemos mostrar que o campo magnético possui importância dinâmica para PNM, sendo a componente turbulenta a maior responsável pela pressão magnética. / The Small Magellanic Cloud (SMC) is a gas rich irregular galaxy which, together with the Large Magellanic Cloud (LMC), orbit the Milky Way (MW). They form a triple system in constant interaction. The SMC is a metal poor galaxy and, due to this, its interstellar medium (ISM) presents different properties from the Galaxy\'s ISM. In addition to that, the importance of magnetic fields on galactic scales is being recognized nowadays. Therefore, the aim of this project was to study the magnetic field structure of the SMC and its relationship with other components of SMC\'s ISM. For this purpose we have used starlight optical polarimetric data, obtained at Cerro Tololo Inter-American Observatory. We have constructed a polarization catalog containing a total of 7,207 stars in 28 fields in the Northeast/Wing sections of the SMC. The polarimetric vector maps trace the ISM magnetic field component in the plane of the sky and one can estimate its intensity towards a given region using the Chandrasekhar & Fermi method. Making use of the polarimetric catalog from this work, we have found that the magnetic field in the SMC, although varying from region to region, nevertheless shows two large scale patterns - the first one aligned with the Magellanic Bridge and a second one aligned with the SMC\'s Bar. We derived for the regular sky-projected magnetic field a value of Bsky = (1.84 ± 0.11) uG, and for the turbulent magnetic field dB = (2.920 ± 0.098) uG. These results evidence that in the SMC the random field prevails over the large scale field, which explains the irregular configuration of the polarization vectors often seen. Correlating the polarization maps with structures present on the SMC\'s ISM, we could identify the presence of several shells which may have magnetic fields up to a few tens uG. It was also possible to observe environments where the regular field seems to have been destroyed due to turbulence, and others where it seems that the large scale magnetic field has not enough time to be formed. Studying the relationship with polarization and reddening, we have obtained a value for P/Av ~ 2, which may indicate that the polarization efficiency in the SMC is smaller than in the Galaxy, perhaps due to a higher turbulence and/or because of a smaller regular magnetic field. Lastly, we have estimated the energy density for the magnetic field and for the rotation and turbulent gas motions. We showed that the magnetic field is dynamically important in the SMC\'s ISM, and that the turbulent component is the largest contributor to the magnetic pressure.

O objetivo desta tese é analisar aspectos do sistema de satélites da Via Láctea de relevância cosmológica. Dentre estes aspectos destacam-se dois: o censo de satélites da Galáxia – onde constata-se que a quantidade destes objetos é muito inferior ao predito por modelos cosmológicos do tipo Matéria Escura Fria – e a frequência anômala de satélites luminosos, como a Pequena e Grande Nuvem de Magalhães (SMC e LMC respectivamente). Além disso, a determinação dos parâmetros estruturais da LMC pode impor vínculos a sua formação, histórico orbital e sobre a massa de nossa Galáxia. Neste trabalho é desenvolvida uma técnica de busca por satélites da Via Láctea. Esta técnica foi otimizada para utilizar dados da nova geração de grandes surveys de maneira eficiente. Este código, o FindSat, foi validado em uma amostra de galáxias anãs conhecidas e se mostrou eficiente em detectar as galáxias anãs mais tênues de que se tem registro. A aplicação desse código a uma região do Sloan Digital Sky Survey (SDSS) ainda não explorada nesse sentido revelou uma série de candidatos a novos satélites. Foram selecionados os candidatos mais promissores para observação de follow-up. Estas observações revelaram que um destes candidatos é de fato um novo satélite da Via Láctea. Este novo objeto é muito provavelmente um aglomerado globular do halo em estágio avançado de dissolução, porém, seu tamanho e magnitude integrada colocamno em um domínio limítrofe entre aglomerado e galáxia anã. Através da cuidadosa análise dos demais candidatos, constatou-se que nenhum outro é de fato um novo satélite da Galáxia. Além disso, foi realizado o estudo do perfil de densidade e geometria da LMC. Este estudo utilizou dados de verificação científica do Dark Energy Survey (DES). Constatase que o perfil de densidades para estrelas jovens (< 3 Gyr) possui um raio de escala cerca de 50% menor que o da população velha (> 3 Gyr), favorecendo o cenário de formação tipo outside-in. O estudo da extensão da componente estelar da LMC revela um raio de maré de cerca de 18 kpc, permitindo o cálculo da massa dinâmica total da LMC. O valor de massa obtido favorece a hipótese onde as Nuvens de Magalhães estariam por sua primeira passagem pelo perigaláctico. Além disso, a distância heliocêntrica e espessura do disco da LMC foram determinadas utilizando estrelas do Red Clump (RC). Notou-se que regiões no extremo norte da LMC estão sistematicamente mais próximas de nós do que o esperado, este efeito evidencia o warp no disco dessa galáxia. Observou-se que a espessura do disco aumenta na periferia da LMC, caracterizando o fenômeno de flare. O aumento na espessura juntamente com a maior extensão da população velha da LMC é interpretado como a presença de dois componentes discoidais. Esta é a primeira evidência desse tipo baseada apenas em métodos de contagem de estrelas. / The goal of this thesis is to analyse comologically relevant aspects of the Milky Way (MW) satellite system. Among these we may highlight two: the census of MW satellites – where the observed number of these objects is much less than what is expected by Cold Dark Matter (CDM) models – and the anomalous frequency of luminous satellites, such as the Small and Large Magellanic Clouds (SMC and LMC respectively). Besides the cosmological importance of the Clouds, the determination of its structural parameters may help to constraint models for their formation, orbital history, and ultimately the mass assembly in our Galaxy. In this work a technique to search newMWsatellites is developed. This technique was optimized to run efficiently on large datasets, such as the ones being generated by the new generation of surveys. The code, FindSat was validated in a sample of well known MW satellites and has proven to be well succeeded even for the most faint of these objects. The application of this code to an unexplored region of the Sloan Digital Sky Survey (SDSS) revealed a large amount of new dwarf galaxy candidates, some of which where selected for follow-up observation. These observations led to the discovery of a new MW satellite. This new object is most likely a globular cluster in an extreme stage of dissolution. However, its integrated magnitude and size makes it difficult to discern it from a dwarf galaxy. By a careful analysis of the remaining candidates, it was shown that no other new satellite was in the sample. The density profile and geometry of the LMC was also analysed. This study used the recent science verification data from the Dark Energy Survey (DES). It was found that the density profile for young stars (< 3 Gyr) has a scale radius 50% smaller when compared to the one obtained for older stars (> 3 Gyr). This result favours the outsidein galaxy formation scenario. The total extension of the LMC stellar component was measured, allowing the estimate of a truncation radius of about 18 kpc. Assuming that this truncation has tidal origins the dynamical mass of the LMC is inferred. The mass value found favours the case for the first perigalactic passage of the Clouds. Besides that, the heliocentric distance and thickness of the LMC disk was determined using Red Clump (RC) stars. Evidence for warp was found in the North edge of the LMC, in the sense that the disk is systematically more distance than expected. While the thickness of the disk increases towards the outer parts of the LMC, which is a phenomena known as flare. This effect joined with the fact that the older LMC stellar population is more extended, favours the presence of two disk components in this galaxy. This is the first evidence of this kind based only on star counts.

Malgré des années de travaux théoriques et observationnels intensifs, nous sommes toujours loin d’une complète compréhension de l’univers proche, la Voie Lactée (MW) et ses galaxies voisines. Parmi les satellites de la MW, le Petit et le Grand Nuage de Magellan (LMC) sont particulièrement intéressants puisqu’ils forment le plus proche exemple de galaxies en interaction gravitationnelle et hydrodynamique, et partant, constituent un laboratoire unique pour étudier les effets des marées et l’échange de matière sur l’évolution chimique et l’histoire de la formation stellaire d’une galaxie. Le LMC est une galaxie de petite masse barrée à disque, prototype des galaxies riches en gaz que l’on pense jouer un rôle important dans la construction des grandes galaxies dans le cadre du ΛCDM. De plus, avec sa métallicité actuelle d’environ le tiers de la métallicité solaire, le chemin d’enrichissement chimique suivi par le LMC donne un grand poids aux yields des générations stellaires pauvres en métaux, ce qui fait du LMC un environnement idéal pour étudier la nucléosynthèse aux basses métallicités. Ce travail de doctorat vise à: 1) caractériser chimiquement la population de la barre du LMC, 2) comparer les tendances des éléments de la MW et du LMC et interpréter les différences ou ressemblance en termes d’évolution chimique et/ou de processus nucléosynthétiques (contraintes sur les sites et les processus nucléosynthétiques), 3) comparer l’évolution chimique de la barre et du disque interne du LMC et interpréter les différence ou ressemblance dans le contexte de la formation de la barre. Nos résultats montrent que l’histoire chimique du LMC a connu un forte contribution des supernovae de type I ainsi qu’un fort enrichissement en éléments s par les vents d’étoiles AGB pauvres en métaux. Par rapport à la MW, les étoiles massives ont eu une contribution plus petite à l’enrichissement chimique du LMC. Les différences observées entre la barre et le disque parlent en faveur d’un épisode de formation stellaire accrue il y a quelques Gyr, ayant lieu dans les zones centrales du LMC et conduisant à la formation de la barre. Ceci est en accord avec les histoires de la formation stellaire récemment dérivées. / Despite decades of intensive observational and theoretical work, we are still far from a complete and clear understanding of the nearby universe, the Milky Way (MW) and its neighbours. Among the satellites of the MW, the Small and Large Magellanic Cloud (LMC) are of particular interest since they form the closest example of galaxies in gravitational and hydrodynamical interaction, and therefore constitute a unique laboratory to study the effect of tides and matter exchange on the chemical evolution and star formation history of a galaxy. The LMC is a low-mass barred disc galaxy, prototypical of gas-rich galaxies that are thought to play an important role in the build-up of large galaxies in the ΛCDM framework. Furthermore, with its present day metallicity of only third of solar, the chemical enrichment path followed by the LMC gives a heavy weight to the yields of metal-poor stellar generations, which makes the LMC an ideal environment to study nucleosynthesis at low metallicities. This thesis work aims at: 1) chemically characterizing the LMC bar population, 2) comparing the elemental trends of the MW and the LMC and interpreting the differences or similarities in terms of chemical evolution and/or nucleosynthesis processes (constraints on the nucleosynthetic sites and processes), 3) comparing the chemical evolution of the LMC bar and inner disc and interpreting the differences or similarities between the LMC bar and inner disc in the context of the bar formation. Our results show that the chemical history of the LMC experienced a strong contribution from type Ia supernovae as well as a strong s-process enrichment from metal-poor AGB winds. Massive stars made a smaller contribution to the chemical enrichment compared to the MW. The observed differences between the bar and the disc speak in favour of an episode of enhanced star formation a few Gyr ago, occurring in the central parts of the LMC and leading to the formation of the bar. This is in agreement with recently derived star formation histories.

A Pequena Nuvem de Magalhães (PNM) é uma galáxia irregular e rica em gás, que juntamente com a Grande Nuvem de Magalhães (GNM) orbita a Via Láctea (VL). Elas formam um sistema triplo em constante interação. A PNM possui metalicidade baixa, e consequentemente seu meio interestelar (MI) apresenta propriedades particularmente diferentes das observadas para o MI da Galáxia. Mais do que isso, a importância do campo magnético em escalas galácticas vem sendo evidenciada cada vez mais. Então, o objetivo desta dissertação foi estudar a estrutura do campo magnético na PNM, e sua relação com componentes do MI desta galáxia. Para este fim, utilizamos dados de polarimetria no óptico, obtidos no Cerro Tololo Inter American Observatory. Construímos um catálogo polarimétrico que contém 7.207 estrelas em 28 campos distribuídos nas secções Nordeste e da Asa da PNM. Os mapas de polarização traçam o campo magnético no plano do céu diretamente, e pode-se obter sua intensidade utilizando o método de Chandrasekhar & Fermi. A partir do catálogo polarimétrico gerado neste trabalho, conseguimos observar que o campo magnético na PNM possui direção bastante irregular, porém é provável a existência de dois padrões em larga escala o primeiro alinhado com a Ponte pan-Magelânica e o segundo alinhado com a Barra da PNM. Obtivemos para o campo magnético regular Bcéu = (1,84 ± 0,11) uG e para o campo turbulento dB = (2,920 ± 0,098) uG. Esse resultado evidencia que na PNM o campo aleatório domina com relação ao de larga escala, justificando a observação de uma configuração tão irregular para os vetores de polarização. Correlacionando os mapas de polarização com estruturas presentes no MI da PNM, pudemos verificar a presença de diversos shells que podem possuir campos magnéticos da ordem de algumas dezenas de uG. Também foi possível observar ambientes onde o campo regular parece ter sido destruído pela turbulência e outros onde ele pode ainda não ter tido tempo de se formar. Derivamos a relação entre polarização e avermelhamento, e obtivemos como resultado que ela é da ordem de P/Av ~ 2, o que indica que na PNM a eficiência para polarização pode ser menor do que na Galáxia, talvez devido a alta turbulência e/ou ao fato de que nela o campo regular é muito baixo. Por fim, a partir da estimativa para as densidades de energia do campo magnético e para o movimento de rotação e de turbulência do gás, pudemos mostrar que o campo magnético possui importância dinâmica para PNM, sendo a componente turbulenta a maior responsável pela pressão magnética. / The Small Magellanic Cloud (SMC) is a gas rich irregular galaxy which, together with the Large Magellanic Cloud (LMC), orbit the Milky Way (MW). They form a triple system in constant interaction. The SMC is a metal poor galaxy and, due to this, its interstellar medium (ISM) presents different properties from the Galaxy\'s ISM. In addition to that, the importance of magnetic fields on galactic scales is being recognized nowadays. Therefore, the aim of this project was to study the magnetic field structure of the SMC and its relationship with other components of SMC\'s ISM. For this purpose we have used starlight optical polarimetric data, obtained at Cerro Tololo Inter-American Observatory. We have constructed a polarization catalog containing a total of 7,207 stars in 28 fields in the Northeast/Wing sections of the SMC. The polarimetric vector maps trace the ISM magnetic field component in the plane of the sky and one can estimate its intensity towards a given region using the Chandrasekhar & Fermi method. Making use of the polarimetric catalog from this work, we have found that the magnetic field in the SMC, although varying from region to region, nevertheless shows two large scale patterns - the first one aligned with the Magellanic Bridge and a second one aligned with the SMC\'s Bar. We derived for the regular sky-projected magnetic field a value of Bsky = (1.84 ± 0.11) uG, and for the turbulent magnetic field dB = (2.920 ± 0.098) uG. These results evidence that in the SMC the random field prevails over the large scale field, which explains the irregular configuration of the polarization vectors often seen. Correlating the polarization maps with structures present on the SMC\'s ISM, we could identify the presence of several shells which may have magnetic fields up to a few tens uG. It was also possible to observe environments where the regular field seems to have been destroyed due to turbulence, and others where it seems that the large scale magnetic field has not enough time to be formed. Studying the relationship with polarization and reddening, we have obtained a value for P/Av ~ 2, which may indicate that the polarization efficiency in the SMC is smaller than in the Galaxy, perhaps due to a higher turbulence and/or because of a smaller regular magnetic field. Lastly, we have estimated the energy density for the magnetic field and for the rotation and turbulent gas motions. We showed that the magnetic field is dynamically important in the SMC\'s ISM, and that the turbulent component is the largest contributor to the magnetic pressure.

The Magellanic Clouds (MCs) consist of a pair of galaxies, the Large Magellanic Cloud (LMC) and the Small Magellanic Cloud (SMC), which are located at a distance of 50 kpc and 60 kpc, with stellar masses of 1010 M and 109 M , respectively. Morphologically they are categorized as irregular type galaxies. The MCs are gas rich and metal poor (Z=0.008 for LMC, and 0.004 for SMC) as compared to the Milky Way (MW), and have active star-forming regions. Their proximity and location at high galactic latitude enable us to resolve their individual populations as well as detect faint stellar populations. It is well known that the MCs are interacting with each other, as well as with the MW. The interaction is supported by the presence of the Magellanic Bridge and the Magellanic Stream. The evolved stellar populations in the MCs help us to understand their evolution and interaction process. The MCs host both Population I as well as Population II stars. This extended range of star formation is a valuable source of information to understand the formation and evolution of galaxies in general, and the MCs in particular. Evolved stellar popu-lation means the stars that have evolved o the main sequence and the giants, such as red giants (RGs), red clump stars, and asymptotic giant branch stars. There is a dominant population of evolved stars present in the MCs, in star clusters as well as in the eld. The aim of the thesis is to study the evolved stellar populations for one of the component of the MCs, the LMC. The study is primarily divided into two parts. (1) Study of sparse star clusters in the LMC: To increase our understanding of sparse star clusters in the LMC, with well estimated parameters, using deep Washington photometric data for 45 LMC clusters. (2) To estimate a metallicity map of LMC: In order to understand the metallicity variation across the galaxy. This is done by creating a high spatial resolution metallicity map of the LMC, using red giant branch (RGB) stars, with the help of photometric data and calibrated using spectroscopic studies of RGs in eld and star clusters. The introduction to the thesis study along with the aim are described in Chapter 1 of the thesis. The three sets of photometric data used for this study are described in Chapter 2. The data sets are: CT1 Washington photometric data for 45 star clusters within the LMC, the VI photometric data from the Optical Gravitational Lensing Experiment Phase-III survey (OGLE III), and the Magellanic Cloud Photometric Survey (MCPS). Study of sparse star clusters in the LMC: A systematic study is per-formed to analyse the 45 cluster candidates, to estimate their parameters (radius, reddening, and age) using the main-sequence turn-o (MSTO), as well as the evolved portion of the colour{magnitude diagram (CMD). The basic parameters were estimated for 33 genuine clusters, whereas the other 12 cluster candidates have been classi ed as possible clusters/asterisms. The study of 33 star clusters are presented in Chapter 3. These clus-ters are categorized as genuine star clusters based on their strong density enhancement and cluster features with respect to their surrounding eld regions. Out of the 33 clusters, 23 are identi ed as single clusters and 10 are found to be members of double clusters. Detailed discussions of all the individual clusters are presented. The estimated parameters for the single and double clusters are listed in two di erent tables. About 50% of the clusters are in the age range 100{300 Myr, the rest of them being older or younger. Comparison with previous age estimates shows some agreement as well as some deviation. The remaining 12 clusters which could not be categorized as genuine star clusters are studied in Chapter 4. These clusters have poor (/suspi-cious) density enhancement and cluster features when compared to their surrounding elds. It is important to study such cluster candidates, as these objects probe the lower limit of the cluster mass function. Detailed discussion on these individual objects are presented and their estimated parameters are tabulated in this chapter. A detailed discussion based on the study of all the 45 inconspicuous clusters is presented in this chapter, including the estimated sizes (radii 2{10 pc), reddening with respect to eld, and location in the LMC. The mass limit estimated for genuine clusters is found to be 1000 M , whereas for possible clusters/asterisms it is few 100 M , using synthetic CMDs. The study of sparse clusters enlarged the number of objects con rmed as genuine star clusters (33) and estimated their fundamental parameters. The study emphasizes that the sizes and masses of the studied sample are found to be similar to that of open clusters in the MW. Thus, this study adds to the lower end of cluster mass distribution in the LMC, suggesting that the LMC, apart from hosting rich clusters, also has formed small, less massive open clusters in the 100{300 Myr age range. The 12 cases of possible clusters/asterisms are worthy of attention, in the sense that they can throw light on the survival time of such objects in the LMC. Photometric metallicity map of the LMC using RGB stars: A metallic-ity map of the LMC is estimated using OGLE III and MCPS photometric data. This is a rst of its kind map of metallicity up to a radius of 4{5 de-grees, derived using photometric data and calibrated using spectroscopic data of RGB stars. The RGB is identi ed in the V, (V I) CMDs of small areal subregions of varying sizes in both data sets. The slope of the RGB is used as an indicator of the average metallicity of a subregion, and this RGB slope is calibrated to metallicity using spectroscopic data for eld and cluster RGs in selected subregions. The metallicity map estimated using OGLE III photometric data is presented in Chapter 5. A method to identify the RGB of small subre-gions within the LMC and estimate its slope by using a consistent and automated method was developed. The technique is robust and indepen-dent of reddening and extinction. The details of calibrating the RGB slopes to metallicities, using previous spectroscopic results of RGs in eld and star clusters are presented. The OGLE III metallicity maps are pre sented, based on four cut-o criteria to separate regions with good ts. The OGLE III map has substantial coverage of the bar, the eastern and western LMC, but does not cover the northern and southern regions. The OGLE III metallicity map shows the bar region to be metal rich whereas the eastern and western regions to be relatively metal poor. The mean metallicity is estimated for three di erent regions within the LMC. For the complete LMC the mean [Fe/H] is = 0.39 dex ( [Fe/H] = 0.10); for the bar region it is = 0.35 dex ( [Fe/H] = 0.9); and for the outer LMC it is = 0.46 dex ( [Fe/H] = 0.11). The metallicity histogram for these di erent regions are also estimated. A radial metallicity gradient is estimated in the de-projected plane of the LMC. The metallicity gradient is seen to remain almost constant in the bar region (till a radius of 2.5 kpc) and has a shallow gradient of 0.066 0.006 dex kpc 1 beyond that till 4 kpc. In Chapter 6 the metallicity map based on MCPS photometric data is estimated. The MCPS data covers more of the northern and south-ern LMC (less of eastern and western regions) and is important to be analysed in order to reveal the metallicity trend of the overall disk. The systematic di erences between the lter systems of MCPS and OGLE III are corrected, and the MCPS slopes are then calibrated using the OGLE III slope{metallicity relation. The MCPS metallicity maps are presented, based on four cut-o criteria to separate regions with good ts. The bar region is found to be metal rich as was found using OGLE III data, whereas the northern and southern regions are marginally metal poor. The mean metallicity estimated for the complete LMC is = 0.37 dex ( [Fe/H] = 0.12); and for the outer LMC it is = 0.41 dex ( [Fe/H] = 0.11). The metallicity histogram for these di erent regions are estimated and compared with the OGLE III distribution. The metallicity range of the complete LMC is found to be almost similar for both data sets. The metallicity distribution within the bar has a narrow range as found using both data sets. The slight di erence between mean metallicity of outer LMC for the two data sets is attributed to their coverage. We suggest that the northern and southern regions of the LMC could be marginally more metal rich than the eastern and western regions. The metallicity gradient of the LMC disk, estimated from MCPS data is found to be shallow 0.049 0.002 dex kpc 1 till about 4 kpc. We also constructed a metallicity map of outliers using both OGLE III and MCPS data, and identi ed subregions where the mean metallic-ity di ers from the surrounding areas. We suggest further spectroscopic studies in order to assess their physical significance. The detailed conclusion of the thesis and future work are presented in Chapter 7. From the study of sparse star clusters in the LMC, it is concluded that LMC has open cluster like star cluster systems. It is important to include them to understand the cluster formation history (CFH) and their survival time scale. Presently, our understanding of the CFH is dominated by rich clusters. The bar of the LMC is found to be the most metal rich region, and the LMC metallicity gradient though shallow, resembles the gradient seen in spiral galaxies. The gradient is also similar to that found in our Galaxy. The higher metallicity in the bar region might indicate an active bar in the past.

Globular clusters are tracers of the formation and evolution of their host galaxies. Kinematics, chemical abundances, age and position of the clusters allows tracing interactions between Milky Way and surrounding galaxies and outlines their chemical enrichment history. In this thesis we analyse mid-resolution spectra of about 800 red giant stars in 51 Galactic globular clusters. It is the first time that [Fe/H] and [Mg/Fe] derived in a consistent way are published for such a huge sample of globular clusters, almost 1/3 of the total number of catalogued clusters. Our metallicities are showed to be more precise than previous works based on mid-resolution spectroscopy. A turnover at [Fe/H] ~ -1.0 is found in the plot [Fe/H] vs. [Mg/Fe] for bulge and halo, although bulge seems to have a more metal-rich turnover, i.e, bulge has more efficient formation than the halo. Comparing the abundances with age the timescale for SNIa to start to become important is 1Gyr. [Fe/H] vs. age corroborates the different star formation efficiency of bulge and halo while [Mg/Fe] does not follow that. Halo was formed in mini halos or dwarf galaxies, and two multiple population clusters had their origin analysed to check it. M 22 seems to have been formed in the Milky Way while NGC 5824 possibly was originated in a dwarf galaxy, although our results are inconclusive for NGC 5824. The Galactic bulge seems to have been formed fast i.e., probably the oldest globular cluster is there. In fact HP 1 has a bluer horizontal branch than expected for its metallicity and we interpret that as an age effect. We determine its distance using light curves of variable stars in order to constrain future age determinations via colour-magnitude diagram. Finally, we investigate interaction between Milky Way and its neighbour galaxy SMC. We find that some star clusters are being stripped out of the SMC main body, which is consistent with tidal stripping scenario for the interaction between the galaxies, instead of ram pressure that would only affect gas. / Aglomerados globulares são traçadores da formação e evolução de suas galáxias. Cinemática, abundâncias químicas, idades e posições dos aglomerados permitem traçar interações entre Via Láctea e galáxias vizinhas e suas histórias de enriquecimento químico. Nesta tese analisamos espectros de média resolução de mais de 800 estrelas gigantes vermelhas em 51 aglomerados globulares Galácticos. É a primeira vez que [Fe/H] and [Mg/Fe] determinados de modo consistente são publicados para uma amostra desse porte, ~1/3 dos objetos catalogados. Nossas metalicidades são mais precisas que trabalhos anteriores similares. Uma quebra em [Fe/H] ~ -1.0 é encontrada no gráfico [Fe/H] vs. [Mg/Fe] para o bojo e halo, embora bojo parece ter uma quebra em [Fe/H] maior, i.e, bojo tem formaçãao mais eficiente que o halo. Comparando abundâncias com idade, a escala de tempo para SNIa ficar importante é 1Gano. [Fe/H] vs. idade corrobora diferentes eficiências de formação do bojo e halo, mas [Mg/Fe] vs. idade não mostra isso. O halo foi formado em mini halos ou galáxias anãs, e dois aglomerados com dispersão em [Fe/H] tiveram suas origens analisadas. M 22 parece ter sido formado na Via Láctea e NGC 5824 possivelmente foi originado em uma galáxia anã, embora os resultados são inconclusivos para NGC 5824. O bojo parece ter sido formado rapidamente e deve possuir o aglomerado mais velho. De fato, HP 1 tem um ramo horizontal mais azul que o esperado para sua metalicidade e vemos isso como um efeito da idade. Determinamos sua distância usando curvas de luz de RR Lyrae de maneira a restringir futuras determinações de idade via diagrama cor-magnitude. Finalmente, investigamos a interação entre Via Láctea e sua galáxia vizinha SMC. Encontramos aglomerados sendo removidos do corpo central da SMC, consistente com cenário de remoção por força de maré para a interação entre as galáxias, em vez de ``ram pressure\'\' que afeta só gás.