<p>The currently-favoured cosmological paradigm, Lamda-CDM, predicts that galaxies are built up from smaller galaxies in a bottom-up process known as hierarchical merging. Lambda-CDM is extremely successful for large-scale structures, but is less so for the detailed features of individual galaxies. We can study these features - the galaxies’ foundations and the remnants of the smaller components that built them - only in the closest galaxies in which we can resolve individual stars. In this thesis, we use data from the Canada-France-Hawaii Telescope (CFHT)/MegaCam as part of the Pan-Andromeda Archaeological Survey (PAndAS) to observe M33 (the Triangulum Galaxy) and the detailed features of its old stellar population. The study of these details is vital for our understanding of galaxy formation and evolution. We search for two types of components within the old stellar population: globular star clusters and the faint, diffuse stellar halo. We find only one new unambiguous outer halo star cluster, in addition to the five previously known in the M33 outer halo (r=10-50 kpc). A further 2440 cluster candidates are identified, which we analyse using two different types of simulated clusters. We are able to describe the type of clusters that are likely to remain hidden from our searches. Our study of a population of red giant branch (RGB) stars far from the M33 disk reveals a low-luminosity, centrally concentrated component which we interpret as the discovery of M33’s halo. It is everywhere in our data fainter than mu_V ~ 33 mag arcsec^−2, with scale length r_exp ~ 20 kpc, an overall luminosity not more than a few percent of the total luminosity of M33, and is possibly also not azimuthally symmetric. For M33 to have so few outer halo clusters compared to M31 and to have such a low-luminosity halo, with the possible asymmetry that we see, suggests tidal stripping of M33’s halo components by M31 - a view that is also favoured by the morphology of the disk substructure and recent modelling.</p> / Doctor of Philosophy (PhD)

Dark matter (DM), whose nature and interaction mechanisms are still an open issue, constitutes about 25 % of the Universe energy density. Weakly Interacting Massive Particles (WIMPs) are considered as strong candidates for particle DM and their search is conveniently carried out through the detection of gamma rays. The newly discovered ultra-faint dwarf spheroidal galaxies (dSphs), located in the vicinity of the Galaxy, exhibit high values of the mass to luminosity ratio, and are therefore considered as strongly dominated by DM. These objects are within reach of the Cherenkov Telescope Array (CTA), which is the future project for gamma-ray astronomy, with an better sensitivity (one order of magnitude) with respect to the current generation experiments. The main goal of the present work is the study of the sensitivity of CTA to WIMPs DM particles, by simulating the observation of the ultra-faint dwarf spherical galaxies Triangulum II, Reticulum II and Carina III, as well as of the classical dwarf galaxy Sculptor, for different annihilation channels, between 70 GeV and 100 TeV. The sensitivity curve in the WIMPs parameter space (velocity-averaged annihilation cross section < &sigma; &nu; > and DM mass mDM) was computed. We found that, within the sample of dwarf galaxies tested, Triangulum II is the most promising source, able to reach the thermal freeze-out values in the annihilation channel &tau;+&tau;- for only 50 hours of observation. Our result, the first estimation of the sensitivity for DM searches in ultra-faint dwarfs with CTA, is consistent with results from current generation experiments, showing better performance over an extended energy range. The limited sample of available stars in the targets induces uncertainties on the DM content. Future measurements, leading to a better understanding of the sources dynamic equilibrium, can improve this situation. Even so, the combination of the high DM content in the ultra-faint dwarf galaxies, together with the excellent expected performance of the future CTA, provides a promising result for indirect DM searches. / A matéria escura, cuja natureza e mecanismos de interação ainda estão em aberto, compõe 25% da densidade de energia do Universo. Weakly Interacting Massive Particles (WIMPs) apresentam-se como forte candidatas e sua busca é convenientemente conduzida através de raios gama. As recém descobertas galáxias esferoidais anãs ultra-fracas, situadas nos arredores da Galáxia, apresentam altos valores da razão entre massa e luminosidade, sendo portanto consideradas objetos fortemente dominados por matéria escura. Esses objetos estão ao alcance do Cherenkov Telescope Array (CTA), que é o futuro projeto da astronomia gama, com sensibilidade de uma order de grandeza melhor do que os experimentos atuais. O presente trabalho teve como objetivo estudar o potencial de detecção indireta de WIMPs através de raios gama com o futuro observatório CTA, observando as galáxias esferoidais anãs ultra-fracas Triangulum II, Retículum II e Carina III e a galáxia anã clássica Sculptor, para diferentes canais de aniquilação, entre 70 GeV e 100 TeV. A curva de sensibilidade no espaço de parâmetros livres de WIMPs (massa da partícula mDM e médida da seção de choque de aniquilação ponderada pela velocidade < &sigma; &nu; >) foi calculada. Nós encontramos que dentro da amostra de galáxias anãs testadas, Triangulum II é a fonte mais promissora, capaz de testar os valores térmicos no canal de aniquilação &tau;+&tau;- considerando apenas 50 horas de observação pelo CTA. Nosso resultado, a primeira estimativa da sensibilidade para busca de matéria escura em galáxias esferoidais anãs ultra-fracas com CTA, é consistente com resultados de experimentos da geração atual, e mostra um melhor desempenho em uma faixa de energia estendida. Os resultados são afetados pelas incertezas devido à pequena amostra de estrelas dos alvos escolhidos, que se reflete no conhecimento do conteúdo de matéria escura. Novas medidas podem ajudar a esclarecer essa situação. Ainda assim, a combinação de galáxias anãs ultra-fracas, aliada às melhorias do futuro CTA, apresenta-se como um passo muito promissor para buscas indiretas de matéria escura.