Observations and analysis of globular cluster systems associated with three galaxy types are presented. Spectroscopy of globular cluster (GC) candidates in the Sculptor spirals NGC 253 and NGC 55 has identified 15 GCs in these galaxies. This spectroscopic sample, combined with plate scans, indicates total GC populations consistent with that expected for their luminosity and morphological type. From these data, we define new GC samples for spectroscopy. Radial velocities of 87 GCs in the Virgo elliptical NGC 4472 have been obtained, yielding data for 144 GCs when combined with previous studies. We find the blue GCs have significantly higher velocity dispersion than the red GCs, with little rotation in either population. The GCs dispersion profile declines slowly, yielding mass profiles consistent with X-ray data. We find a steeply rising M/L ratio, indicative of a massive dark halo surrounding this galaxy. From line-strengths of the GCs, we derive ages and metallicities for the GCs using simple stellar population (SSP) models. We find that the GCs are old and coeval and the bimodality seen in then- colours reflects metallicity rather than age differences. The GCs exhibit solar abundance ratios and both subpopulations show evidence for radial metallicity gradients. We have obtained high S/N spectra for 64 star clusters in the Large Magellanic Cloud. We measure their Lick indices to test the age and metallicity calibration of SSP models by comparison with literature values. We find our metallicities are consistent, although the values from our integrated spectra are slightly higher. The agreement of the ages for the old GCs is good, but is somewhat poorer for the youngest clusters. We obtain an age-metallicity relation for the clusters consistent with the galaxy's field stars. We show first results of a project to investigate the age and metallicity distributions of globular cluster systems using semi-analytic models of galaxy formation.

Tudo que vive por muito tempo está apto a contribuir com boas histórias sobre o passado. Isto não é diferente com as estrelas azuis tardias que são encontradas em todos os ambientes estelares. Essas estrelas velhas mostram-se aparentemente muito jovens, e talvez por isso, nunca tenham sido ouvidas em um contexto maior que suas próprias vidas. Este trabalho interpreta a história que elas contam sobre a natureza do halo galáctico, através de seus parâmetros físicos fundamentais: coordenadas, temperaturas, gravidades superficiais, metalicidades, cores, distâncias e idades. Este trabalho utiliza dados do Sloan Digital Sky Survey para reunir candidatas a estrelas azuis tardias (BSSs) através de critérios espectrofotométricos. Ao todo, 8001 candidatas a BSSs sobreviveram aos diversos métodos de seleção aplicados, constituindo a base de dados deste estudo. Essa amostra permitiu estimar a frequência média de BSSs no halo em relação ao número de estrelas azuis do ramo horizontal (BHBs) em 2.15±0.13 BSS/BHB, valor similar ao encontrado em galáxias anãs próximas (~2.24±0.17). Verificou-se também que as BSSs apresentam um gradiente de cor em função da distância ao centro Galáctico, aparentemente independente da metalicidade. À variação de cor foi atribuída uma variação de idade, que forneceu um gradiente médio de -0.034±0.002 Ganos/kpc no halo. Esse resultado mostra que as regiões mais velhas se concentram preferencialmente no centro da Galáxia, e ficam cada vez mais jovens para distâncias maiores. O gradiente de cor das BSSs possibilitou a construção de mapas de idade do halo galáctico, que foram superpostos às posições centrais de uma coleção de subestruturas encontrada na literatura. Aproximadamente 60% delas tem posições que concordam com as flutuações de cor observadas nos mapas, além de apresentarem propriedades cinemáticas e químicas similares às BSSs nessas regiões (em 2). / Everything that lives long enough is able to contribute with good stories about the past. This statement also applies to the blue straggler stars (BSSs), which can be found in all stellar environments. These old stars appear to be very young and perhaps because of this have never been properly addressed in a context larger than their own lives. This work interprets the story they tell about the nature of the Galactic halo, through their fundamental physical parameters: coordinates, temperatures, surface gravities, metallicities, colors, distances and ages. This work uses the Sloan Digital Sky Survey (SDSS) database to select BSS candidates through photometric and spectroscopic criteria. Altogether, 8001 BSS candidates survived the various selection methods applied and were used as the database for this study. This large sample allowed the determination of the average frequency of BSSs in the halo, compared to the number of blue horizontal branch stars (BHBs). The average frequency of BSS/BHB found in the galactic halo by this work is 2.15±0.13, very similar to this ratio in nearby dwarf galaxies (~2.24±0.17). In addition, this work verified that the BSSs show a color gradient as a function of distance to the galactic center, which appears to be independent from metallicity. To this color variation was assigned an age variation, yielding an average gradient of -0.034±0.002 Gyr/kpc in the halo. This result shows that the older regions preferentially occur in the center of the Galaxy and get younger for larger distances. The BSSs color gradient allowed the construction of galactic halo age maps. These maps were superimposed to the central positions of a collection of substructures found in the literature. Approximately 60% of them have positions that agree with the color fluctuations observed in the maps, as well as having similar kinematic and chemical properties to the BSSs in those regions (within 2).

Tudo que vive por muito tempo está apto a contribuir com boas histórias sobre o passado. Isto não é diferente com as estrelas azuis tardias que são encontradas em todos os ambientes estelares. Essas estrelas velhas mostram-se aparentemente muito jovens, e talvez por isso, nunca tenham sido ouvidas em um contexto maior que suas próprias vidas. Este trabalho interpreta a história que elas contam sobre a natureza do halo galáctico, através de seus parâmetros físicos fundamentais: coordenadas, temperaturas, gravidades superficiais, metalicidades, cores, distâncias e idades. Este trabalho utiliza dados do Sloan Digital Sky Survey para reunir candidatas a estrelas azuis tardias (BSSs) através de critérios espectrofotométricos. Ao todo, 8001 candidatas a BSSs sobreviveram aos diversos métodos de seleção aplicados, constituindo a base de dados deste estudo. Essa amostra permitiu estimar a frequência média de BSSs no halo em relação ao número de estrelas azuis do ramo horizontal (BHBs) em 2.15±0.13 BSS/BHB, valor similar ao encontrado em galáxias anãs próximas (~2.24±0.17). Verificou-se também que as BSSs apresentam um gradiente de cor em função da distância ao centro Galáctico, aparentemente independente da metalicidade. À variação de cor foi atribuída uma variação de idade, que forneceu um gradiente médio de -0.034±0.002 Ganos/kpc no halo. Esse resultado mostra que as regiões mais velhas se concentram preferencialmente no centro da Galáxia, e ficam cada vez mais jovens para distâncias maiores. O gradiente de cor das BSSs possibilitou a construção de mapas de idade do halo galáctico, que foram superpostos às posições centrais de uma coleção de subestruturas encontrada na literatura. Aproximadamente 60% delas tem posições que concordam com as flutuações de cor observadas nos mapas, além de apresentarem propriedades cinemáticas e químicas similares às BSSs nessas regiões (em 2). / Everything that lives long enough is able to contribute with good stories about the past. This statement also applies to the blue straggler stars (BSSs), which can be found in all stellar environments. These old stars appear to be very young and perhaps because of this have never been properly addressed in a context larger than their own lives. This work interprets the story they tell about the nature of the Galactic halo, through their fundamental physical parameters: coordinates, temperatures, surface gravities, metallicities, colors, distances and ages. This work uses the Sloan Digital Sky Survey (SDSS) database to select BSS candidates through photometric and spectroscopic criteria. Altogether, 8001 BSS candidates survived the various selection methods applied and were used as the database for this study. This large sample allowed the determination of the average frequency of BSSs in the halo, compared to the number of blue horizontal branch stars (BHBs). The average frequency of BSS/BHB found in the galactic halo by this work is 2.15±0.13, very similar to this ratio in nearby dwarf galaxies (~2.24±0.17). In addition, this work verified that the BSSs show a color gradient as a function of distance to the galactic center, which appears to be independent from metallicity. To this color variation was assigned an age variation, yielding an average gradient of -0.034±0.002 Gyr/kpc in the halo. This result shows that the older regions preferentially occur in the center of the Galaxy and get younger for larger distances. The BSSs color gradient allowed the construction of galactic halo age maps. These maps were superimposed to the central positions of a collection of substructures found in the literature. Approximately 60% of them have positions that agree with the color fluctuations observed in the maps, as well as having similar kinematic and chemical properties to the BSSs in those regions (within 2).

<p> Globular clusters (GCs) can be divided into two subpopulations when plotted on a colour-magnitude diagram: one red and metal-rich (MR), and the other blue and metal-poor (MP). For each subpopulation, any correlation between colour and luminosity can then be converted into mass-metallicity relations (MMRs).</p> <p> Tracing the MMRs for fifteen GC systems (GCSs) - all around Brightest Cluster Galaxies - we see a nonzero trend for the MP subpopulation but not the MR. This trend is characterised by p in the relation Z=M^p. We find p ~ 0.35 for the MP GCs, and a relation for the MR GCs that is consistent with zero. When we look at how this trend varies with the host galaxy luminosity, we extend previous studies (e.g., Mieske et al, 2006b) into the bright end of the host galaxy sample.</p> <p> In addition to previously presented (B-I) photometry for eight GCSs obtained with ACS/WFC on the HST, we present seven more GCSs. Four of these are newly analysed from HST data, one is previously presented (g-i) photometry obtained with GMOS on Gemini South, and two are the author's newly reduced and analysed (g-i) photometry also obtained with GMOS on Gemini South.</p> <p> Interpretation of these results is important for further understanding the formation and evolution of galaxies within the hierarchical-merging picture, and what process enables the production of two subpopulations.</p> / Thesis / Master of Science (MSc)

Thesis (Ph.D.)--Boston University / This dissertation studies the interstellar medium (ISM) in dwarf galaxies. Dwarf galaxies are important because: 1) they constitute the largest fraction of extragalactic systems, and 2) they provide templates for primordial galaxies. Indeed, local active dwarf galaxies resemble primitive systems, since they are poor in dust and heavy elements and they are profusely forming massive stars. Because dwarf galaxies are nearby, however, they can be observed in much greater detail than distant primordial systems. Therefore studies of the ISM in nearby dwarf galaxies can be used to understand the processes at work in primitive galaxies. This work focuses on the effects of low heavy element abundances (i.e., low metallicities) on the star-forming ISM. Low metallicities are known to drastically affect the ISM. With decreasing metallicity, an increasingly large fraction of the molecular ISM is photodissociated into atoms and ions. We modeled and observed the emission of a sample of low metallicity dwarf galaxies in the millimeter, submillimeter, and far-infrared wavebands. The submillimeter waveband allows us to observe the mid-J rotational transitions of carbon monoxide (CO), the usual tracer of the molecular ISM, and the fine structure transit ions of neutral carbon ([C I]), a tracer of translucent and photodissociated material. We studied regions in the Large and Small Magellanic Clouds and the Northern Hemisphere dwarf galaxy IC 10. We find that the preponderant mechanism producing neutral carbon inside molecular clouds is photodissociation. We observe a moderate increase in the ratio of [C I] to CO emission for decreasing metallicity. Our models of clumpy, unresolved photo dissociation regions explain these observations as the natural result of an augmented fraction of photo dissociated material. Finally, our observations of the submillimeter thermal dust continuum in IC 10 find an abnormally low emissivity exponent for its graybody emission. We conclude that the unusual dust continuum is caused by the selective destruction of small grains, brought about by the combined effects of low metallicities and high radiation fields.

As estrelas azuis tardias (blue straggler stars ou estrelas BS) são estrelas de sequência principal que apresentam um aparente atraso evolutivo em relação às suas vizinhanças. Elas foram identificadas inicialmente na sequência principal de aglomerados globulares acima do ponto de turnoff no diagrama HR. Desde então, têm sido encontradas em todos os ambientes estelares: aglomerados abertos e globulares, galáxias anãs próximas e entre as estrelas de campo na Galáxia. O principal objetivo deste trabalho é construir uma grande amostra de estrelas BS no campo galáctico. Para tanto, métodos de separação desses objetos de estrelas BHB foram comparados com critérios de seleção que envolvem parâmetros atmosféricos estimados pelo SEGUE Stellar Parameter Pipeline (SSPP). Tal procedimento permitiu incluir também estrelas com magnitudes g maiores que 18, antes excluídas pelos métodos tradicionais para objetos com razão sinal-ruído menores que 9. Os métodos apresentados neste trabalho permitiram a seleção de uma amostra de 8001 candidatas a estrelas BS que foram analisadas cinematicamente, através de suas velocidades radiais. Verificou-se que aproximadamente um quinto dessa amostra (cerca de 1500 objetos) possui características que a associa à corrente de Sagitário, sugerindo uma origem extragaláctica para tais objetos. / Blue straggler stars (BSSs) are main sequence stars that exhibit an apparent evolutionary delay over the region where they are found in the HR diagram. They were initially identified in the main sequence of globular clusters above the turnoff point. Since then, they have been found in many different stellar environments: globular and open clusters, near dwarf galaxies, and also in the field of the Galaxy. The main goal of this work is to build a large sample of BSSs in the galactic field region. In order to accomplish this task, classical methods of separation of BSS from BHB stars were compared with proposed restrictions based on their atmospheric parameters, which are estimated by the SEGUE Stellar Parameter Pipeline (SSPP). The latter procedure allowed to include stars with magnitudes g greater than 18, not handled by traditional methods when signal-noise ratio is smaller than 9. The selection methods gathered a sample of 8001 BS stars. These stars were kinematically analyzed through their radial velocities. The results suggest that many of them (about 1500) can have extragalactic origin, associated with Sagittarius stream.

Este trabalho envolve um estudo espectroscópico voltado para estimativas de metalicidades de uma amostra de estrelas anãs M. As estrelas M de baixa massa constituem os objetos estelares mais numerosos na Galáxia e com tempos de vida de sequência principal que excedem a atual idade do Universo. Sendo assim, podem ser vistos como grandes laboratórios para estudo da estrutura e evolução da Galáxia. Os esforços deste trabalho se concentraram em traçar paralelos entre a força de algumas bandas moleculares presentes nos espectros dessas estrelas e a metalicidade. Seguiu-se para isso metodologias presentes na literatura. A motivação para o trabalho foi dar continuidade a um estudo iniciado por Sebastien Lépine e colaboradores, em 2007, utilizando, pela primeira vez uma amostra de anãs M do hemisfério sul. Além disso, uma reavaliação da calibração de metalicidade para anãs M norteou o estudo. O trabalho utilizou o espectrógrafo Goodman do observatório SOAR para obtenção dos espectros estelares. As estimativas de metalicidade da amostra foram obtidas a partir do n-sspp, uma adaptação do Segue Stellar Parameters Pipeline (sspp), pipeline do SDSS. Os objetos considerados são sistemas binários constituídos por uma estrela de tipo espectral F ou G e outra de tipo espectral M. A estrela primária (F ou G) é utilizada para se determinar a metalicidade da estrela M (secundária), diante da hipótese de que o sistema se formou a partir de uma mesma nuvem mãe. Apesar de todos esforços empregados neste estudo, não foi possível refinar a calibração de metalicidades para as estrelas em questão. O índice calibrador desenvolvido por Lépine et al. (2007) mostrou-se um fraco indicador de metalicidades, resultado já documentado por eles em 2012. Com isso, os estudos para calibrações de metalicidade de estrelas anãs M devem ser incentivados. / The aim of this work is to obtain metallicity calibration for a sample of M dwarfs stars through spectroscopic studies. The low-mass M stars are the most numerous objects in the Galaxy and their main sequence lifetimes exceed the current age of the Universe. Thus, they reveal themselves as large laboratories for studying the structure and evolution of the Galaxy. The efforts of this work are focused in the construction of relations between the strengths of some molecular bands and metallicities for M stars. For that, methodologies in the literature were followed. The motivation of this work was to continue a study initiated by Lépine et al. (2007) using for the first time a sample of M dwarf from southern hemisphere. Besides that, there is yet no definitive metallicity calibration for M dwarfs. Goodman spectrograph at SOAR was used to obtain the stellar spectra. Metallicities estimates for the sample were obtained from an adapted version of the Segue Stellar Parameters Pipeline (sspp), the SDSS\'s pipeline. The studied objects are binary systems consisting of a F or G spectral type star and a M stars. The primary star (F or G) is used to estimate the metallicity of the M star (secondary), since it is assumed that the system is formed from the same parent cloud. Despite all efforts made in this work, it was not possible to refine the metallicities calibration for the analyzed sample. The index originally developed by Lépine et al. (2007) proved to be a poor metallicity indicator, as already documented by them in 2012. Thus, metallicity calibration studies for M dwarf stars should be encouraged.

As part of the McDonald Observatory M dwarf planet search program, we present the results and detection limits for our high-precision radial velocity survey of 99 M dwarf stars. We also detail our efforts to improve the precision of our RV measurements as well as our frequency analysis methods. For any RV program, it is essential to obtain as high a precision as possible; increasing sensitivity can realistically reveal terrestrial-mass planets with our data. M dwarfs provide a unique opportunity to study these lower-mass planets (the so-called "super-Earths") from ground-based facilities; such planets are mostly undetectable around FGK stars, whose larger masses result in much smaller RV amplitudes. However, the low intrinsic luminosities of the M spectral type make it difficult to obtain high S/N measurements for a statistically significant sample, making our analysis improvements especially critical. Finally, we conduct a statistical analysis of the 21 known M dwarf planets. In particular, we use the photometric metallicity calibration for M dwarfs described in Johnson and Apps (2009) to further explore the frequency of planetary systems as a function of stellar metallicity. Our analysis confirms the correlation between stellar mass and the presence of giant planets, but also reveals a significant metallicity dependence on the presence of high-mass planets for M dwarfs. We show that the metallicities of our target sample are evenly distributed around solar [M/H], eliminating the possibility that the results of our survey will be biased due to metallicity effects. The frequency and characteristics of planets around M stars provides important insight into planet formation theories, especially for giant planets, which appear to form less easily around low-mass primaries. While previous results suggesting a dearth of short-period Jovian planets around M stars still holds, there is now a long enough observational time baseline to begin to characterize the frequency of planets with lower masses and larger orbital separations around these stars as opposed to other main sequence stars. / text

As estrelas azuis tardias (blue straggler stars ou estrelas BS) são estrelas de sequência principal que apresentam um aparente atraso evolutivo em relação às suas vizinhanças. Elas foram identificadas inicialmente na sequência principal de aglomerados globulares acima do ponto de turnoff no diagrama HR. Desde então, têm sido encontradas em todos os ambientes estelares: aglomerados abertos e globulares, galáxias anãs próximas e entre as estrelas de campo na Galáxia. O principal objetivo deste trabalho é construir uma grande amostra de estrelas BS no campo galáctico. Para tanto, métodos de separação desses objetos de estrelas BHB foram comparados com critérios de seleção que envolvem parâmetros atmosféricos estimados pelo SEGUE Stellar Parameter Pipeline (SSPP). Tal procedimento permitiu incluir também estrelas com magnitudes g maiores que 18, antes excluídas pelos métodos tradicionais para objetos com razão sinal-ruído menores que 9. Os métodos apresentados neste trabalho permitiram a seleção de uma amostra de 8001 candidatas a estrelas BS que foram analisadas cinematicamente, através de suas velocidades radiais. Verificou-se que aproximadamente um quinto dessa amostra (cerca de 1500 objetos) possui características que a associa à corrente de Sagitário, sugerindo uma origem extragaláctica para tais objetos. / Blue straggler stars (BSSs) are main sequence stars that exhibit an apparent evolutionary delay over the region where they are found in the HR diagram. They were initially identified in the main sequence of globular clusters above the turnoff point. Since then, they have been found in many different stellar environments: globular and open clusters, near dwarf galaxies, and also in the field of the Galaxy. The main goal of this work is to build a large sample of BSSs in the galactic field region. In order to accomplish this task, classical methods of separation of BSS from BHB stars were compared with proposed restrictions based on their atmospheric parameters, which are estimated by the SEGUE Stellar Parameter Pipeline (SSPP). The latter procedure allowed to include stars with magnitudes g greater than 18, not handled by traditional methods when signal-noise ratio is smaller than 9. The selection methods gathered a sample of 8001 BS stars. These stars were kinematically analyzed through their radial velocities. The results suggest that many of them (about 1500) can have extragalactic origin, associated with Sagittarius stream.

Este trabalho envolve um estudo espectroscópico voltado para estimativas de metalicidades de uma amostra de estrelas anãs M. As estrelas M de baixa massa constituem os objetos estelares mais numerosos na Galáxia e com tempos de vida de sequência principal que excedem a atual idade do Universo. Sendo assim, podem ser vistos como grandes laboratórios para estudo da estrutura e evolução da Galáxia. Os esforços deste trabalho se concentraram em traçar paralelos entre a força de algumas bandas moleculares presentes nos espectros dessas estrelas e a metalicidade. Seguiu-se para isso metodologias presentes na literatura. A motivação para o trabalho foi dar continuidade a um estudo iniciado por Sebastien Lépine e colaboradores, em 2007, utilizando, pela primeira vez uma amostra de anãs M do hemisfério sul. Além disso, uma reavaliação da calibração de metalicidade para anãs M norteou o estudo. O trabalho utilizou o espectrógrafo Goodman do observatório SOAR para obtenção dos espectros estelares. As estimativas de metalicidade da amostra foram obtidas a partir do n-sspp, uma adaptação do Segue Stellar Parameters Pipeline (sspp), pipeline do SDSS. Os objetos considerados são sistemas binários constituídos por uma estrela de tipo espectral F ou G e outra de tipo espectral M. A estrela primária (F ou G) é utilizada para se determinar a metalicidade da estrela M (secundária), diante da hipótese de que o sistema se formou a partir de uma mesma nuvem mãe. Apesar de todos esforços empregados neste estudo, não foi possível refinar a calibração de metalicidades para as estrelas em questão. O índice calibrador desenvolvido por Lépine et al. (2007) mostrou-se um fraco indicador de metalicidades, resultado já documentado por eles em 2012. Com isso, os estudos para calibrações de metalicidade de estrelas anãs M devem ser incentivados. / The aim of this work is to obtain metallicity calibration for a sample of M dwarfs stars through spectroscopic studies. The low-mass M stars are the most numerous objects in the Galaxy and their main sequence lifetimes exceed the current age of the Universe. Thus, they reveal themselves as large laboratories for studying the structure and evolution of the Galaxy. The efforts of this work are focused in the construction of relations between the strengths of some molecular bands and metallicities for M stars. For that, methodologies in the literature were followed. The motivation of this work was to continue a study initiated by Lépine et al. (2007) using for the first time a sample of M dwarf from southern hemisphere. Besides that, there is yet no definitive metallicity calibration for M dwarfs. Goodman spectrograph at SOAR was used to obtain the stellar spectra. Metallicities estimates for the sample were obtained from an adapted version of the Segue Stellar Parameters Pipeline (sspp), the SDSS\'s pipeline. The studied objects are binary systems consisting of a F or G spectral type star and a M stars. The primary star (F or G) is used to estimate the metallicity of the M star (secondary), since it is assumed that the system is formed from the same parent cloud. Despite all efforts made in this work, it was not possible to refine the metallicities calibration for the analyzed sample. The index originally developed by Lépine et al. (2007) proved to be a poor metallicity indicator, as already documented by them in 2012. Thus, metallicity calibration studies for M dwarf stars should be encouraged.