The synchrotron emissivity distribution of the Milky Way Galaxy has been modelled from the 408 MHz allsky survey of Haslam et al.(1982) after separation of its thermal component with the help of IRAS 60 micron emission(Broadbent et al 1989). We have refined the spiral arm pattern in the inner part of the Galaxy by including a bar at the Galactic centre and an updated the Galactic distance scale and obtained fitted free parameters. At 408 MHz, there is very little absorption in the interstellar medium and the line of sight distribution of synchrotron emissivity was inferred mainly from its presumed relationship to the other tracers of spiral structure via these fitted free parameters. At lower frequencies, the absorption of synchrotron emission due to thermal electrons becomes significant and can give direct information of the nonthermal distribution along the line of sight. We have modelled the distribution of thermal electrons according to our synchrotron arm model and an alternative model based on pulsar dispersion measures using the Galactic rotation curve and the surveys of the distribution of Hl66a emission We have then used our synchrotron model applied at lower frequencies including the absorption to compare with the surveys of Dwarakanath et al.(l990) at 34.5 MHz and Jones and Finlay(l974) at 29.9 MHz. The result confirms that the absorption model of the synchrotron emissivity in the Galactic plane is broadly correct and illustrates the potentials of the absorption technique. However we were not able to distinguish the two models of ionised hydrogen spiral structure. To do this, recombination line surveys with improved frequency resolution are required. Using a new value of the cosmic ray gradient in the Galaxy from diffuse gamma-ray emission we obtain the separate variation of magnetic field and cosmic ray electron density. We give the global properties of the thermal and nonthermal emission that our model implies.

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I have used blue near-infrared colours to select a group of UKIDSS T dwarfs with spectral types later than T4. From amongst these I identify two kinematic halo T-dwarf candi- dates. Blue near-infrared colours have been attributed to collisionally-induced hydrogen absorption, which is enhanced by either high surface gravity or low metallicity. Proper motions are measured and distances estimated, allowing the determination of tangential velocities. U and V components are estimated for our objects by assuming Vrad = 0. From this, ULAS J0926+0835 is found to have U = 62 kms−1 and V = −140 kms−1 and ULAS J1319+1209 is found to have U = 192 kms−1 and V = −92 kms−1. These values are consistent with potential halo membership. However, surprisingly, these are not the bluest objects in the selection. The bluest is ULAS J1233+1219, with J −K = −1.16±0.07, and surprisingly this object is found to have thin disc-like U and V . Our sample also contains Hip 73786B, which I find to be a companion to the star Hip 73786. Hip 73786 is a metal- poor star, with [Fe/H]= −0.3 ± 0.1 and is located at a distance of 19±0.7 pc. U, V,W space velocity components are calculated for Hip 73786A and B, finding that U = −48±7 kms−1, V = −75 ± 4 kms−1 and W = −44 ± 8 kms−1. From the properties of the pri- mary, Hip 73786B is found to be at least 1.6Gyr old. As a metal poor object, Hip 73786B represents an important addition to the sample of known T dwarf benchmarks. Using mid-infrared data from WISE, I also identify T dwarfs with abnormally-red H − W2 and consider possible causes for their extreme colours. In particular I exam- ine three prominent examples of this phenomenon, ULAS J1416+1348B, 2MASS J0939- 2448 and BD+01o 2920B. A plot of spectral type against MW2-magnitude suggests that ULAS J1416+1348B is potentially an unresolved binary, similar to 2MASS J0939-2448. However, the plot also indicates that BD+01o 2920B is not an unresolved binary. I also present new FIRE spectroscopy for ULAS J1416+1348B and 2MASS J0939-2448. These data show that ULAS J1416+1348B has a similar shape to the Y -band spectrum to that of BD+01o 2920B, thus suggesting that the two objects have a similar metallicity, whereas 2MASS J0939-2448 appears to be a more metal-rich object. Using a new parallactic dis- tance, I derive a luminosity of (6.9±0.7)×1020W for ULAS J1416+1348B. I also find a radial velocity of −39 ± 1 kms−1 for this object. The agreement between this and that of the L dwarf SDSS J1416+1348A confirms that these two objects are physically-associated. I also present a set of simulated unresolved binaries; the colours of these systems do not appear to redden significantly with the addition of cooler companions. From this, I suggest that the colours of ULAS J1416+1348B and BD+01o 2920B cannot be solely attributed to any possible unresolved companions; for these two objects, composition and/or surface gravity must be playing a substantial role. Consideration of model predictions provides extra evidence for this argument, showing as it does that high log g and low metallicity can redden H − W2 colours by as much as »0.5mag as compared to a high-metallicity and low log g object of the same effective temperature. I also present kinematics and photometry for several more new candidate low-metallicity T dwarfs. Spectra are also presented, where available. In addition I provide new follow-up JHK spectroscopy for ULAS J0926+0835, ULAS J1233+1219 and ULAS J1319+1209. These new spectra allow full JHK-based spectral typing for these objects.

Numerical simulations of galaxy formation and observational Galactic Astronomy are two fields of research that study the same objects from different perspectives. Simulations try to understand galaxies like our Milky Way from an evolutionary point of view while observers try to disentangle the current structure and the building blocks of our Galaxy. Due to great advances in computational power as well as in massive stellar surveys we are now able to compare resolved stellar populations in simulations and in observations. In this thesis we use a number of approaches to relate the results of the two fields to each other. The major observational data set we refer to for this work comes from the Radial Velocity Experiment (RAVE), a massive spectroscopic stellar survey that observed almost half a million stars in the Galaxy. In a first study we use three different models of the Galaxy to generate synthetic stellar surveys that can be directly compared to the RAVE data. To do this we evaluate the RAVE selection function to great detail. Among the Galaxy models is the widely used Besancon model that performs well when individual parameter distribution are considered, but fails when we study chemodynamic correlations. The other two models are based on distributions of mass particles instead of analytical distribution functions. This is the first time that such models are converted to the space of observables and are compared to a stellar survey. We show that these models can be competitive and in some aspects superior to analytic models, because of their self-consistent dynamic history. In the case of a full cosmological simulation of disk galaxy formation we can recover features in the synthetic survey that relate to the known issues of the model and hence proof that our technique is sensitive to the global structure of the model. We argue that the next generation of cosmological galaxy formation simulations will deliver valuable models for our Galaxy. Testing these models with our approach will provide a direct connection between stellar Galactic astronomy and physical cosmology. In the second part of the thesis we use a sample of high-velocity halo stars from the RAVE data to estimate the Galactic escape speed and the virial mass of the Milky Way. In the course of this study cosmological simulations of galaxy formation also play a crucial role. Here we use them to calibrate and extensively test our analysis technique. We find the local Galactic escape speed to be 533 (+54/-41) km/s (90% confidence). With this result in combination with a simple mass model of the Galaxy we then construct an estimate of the virial mass of the Galaxy. For the mass profile of the dark matter halo we use two extreme models, a pure Navarro, Frenk & White (NFW) profile and an adiabatically contracted NFW profile. When we use statistics on the concentration parameter of these profile taken from large dissipationless cosmological simulations we obtain an estimate of the virial mass that is almost independent of the choice of the halo profile. For the mass M_340 enclosed within R_340 = 180 kpc we find 1.3 (+0.4/-0.3) x 10^12 M_sun. This value is in very good agreement with a number of other mass estimates in the literature that are based on independent data sets and analysis techniques. In the last part of this thesis we investigate a new possible channel to generate a population of Hypervelocity stars (HVSs) that is observed in the stellar halo. Commonly, it is assumed that the velocities of these stars originate from an interaction with the super-massive black hole in the Galactic center. It was suggested recently that stars stripped-off a disrupted satellite galaxy could reach similar velocities and leave the Galaxy. Here we study in detail the kinematics of tidal debris stars to investigate the probability that the observed sample of HVSs could partly originate from such a galaxy collision. We use a suite of $N$-body simulations following the encounter of a satellite galaxy with its Milky Way-type host galaxy. We quantify the typical pattern in angular and phase space formed by the debris stars and develop a simple model that predicts the kinematics of stripped-off stars. We show that the distribution of orbital energies in the tidal debris has a typical form that can be described quite accurately by a simple function. The main parameters determining the maximum energy kick a tidal debris star can get is the initial mass of the satellite and only to a lower extent its orbit. Main contributors to an unbound stellar population created in this way are massive satellites (M_sat > 10^9 M_sun). The probability that the observed HVS population is significantly contaminated by tidal debris stars appears small in the light of our results. / Ein häufig verfolgter Ansatz Galaxien wie unsere Milchstraße besser zu verstehen, sind numerische Simulationen, d.h. das Nachvollziehen ihrer Entstehung und Entwicklung mit Hilfe von Computern. Dieses Vorgehen erlaubt das Betrachten solcher Objekte von einem evolutionären Standpunkt aus. Eine andere Herangehensweise verfolgt die Galaktische Astronomie, welche über Sternbeobachtungen den aktuellen Zustand der Milchstraße untersucht. Hier wird versucht, die konstitutiven Bestandteile unserer Galaxie zu erkennen, um dadurch ein besseres Verständnis ihrer Struktur zu erlangen. Die enorme Rechenleistung moderner Supercomputer und die Entwicklungssprünge im Bereich der digitalen Himmelsdurchmusterungen haben dazu geführt, dass inzwischen mit beiden Ansätzen vergleichbare Populationen von einzeln beobachtbaren Sternen studiert werden können. In der vorliegenden Arbeit werden verschiedene Möglichkeiten untersucht, die Ergebnisse dieser beiden astrophysikalischen Disziplinen, welche bislang weitgehend getrennt von einander betrieben wurden, sinnvoll zu kombinieren. Der überwiegende Teil der Beobachtungsdaten, die dabei verwendet werden, wurde im Zuge des Radial Velocity Experiments (RAVE) gesammelt, einer spektroskopischen Durchmusterung der Sterne fast des gesamten Südhimmels. Um die Daten des RAVE-Projekts statistisch auswerten zu können, musste zuerst die detaillierte Auswahlfunktion der Durchmusterung rekonstruiert werden, d.h. die Wahrscheinlichkeit, dass ein Stern von RAVE beobachtet wurde, musste, in Abhängigkeit von den Eigenschaften des Sterns, bestimmt werden. Der Hauptteil der Dissertation gliedert sich in drei weitgehend unabhängige Studien. Im ersten Teil wird die oben erwähnte Auswahlfunktion benutzt, um voraus zu sagen, was das RAVE Projekt beobachtet hätte, falls bestimmte theoretische Modelle unserer Milchstraße zu träfen. Auf diese Art und Weise umgehe ich das problematische Unterfangen, die Beobachtungsdaten zu einem physikalischen Modell zu verallgemeinern. Die Problematik hierbei liegt darin, dass astronomische Beobachtungen nicht direkt physikalisch relevante Größen, wie Massen oder Alter der Sterne, liefern, sondern scheinbare Helligkeiten oder Winkelpositionen. In dieser Studie wird der umgekehrte Weg beschritten und synthetische Beobachtungen aus den Modellen generiert. Untersucht wurden dabei sowohl klassische analytische Modelle als auch Modelle, die aus numerischen Simulationen resultieren. Letztere wurden zu ersten Mal überhaupt auf diese Art und Weise getestet und es zeigt sich, dass solche Modelle den klassischen in bestimmten Aspekten, die mit der Entwicklungsgeschichte der Milchstraße verknüpft sind, überlegen sind. Im zweiten Teil der Arbeit werden die RAVE-Daten benutzt um die Masse der Milchstraße, bzw. die Masse der in ihr enthaltenen dunklen Materie, ab zu schätzen. Zur Eichung der Analysemethode wird dabei wieder auf Ergebnisse von Simulationen zurück gegriffen, die die Entwicklung von ähnlichen Galaxien wie der Milchstraße verfolgt haben. Zuerst wird die lokale Entweichgeschwindigkeit, d.h. die Mindestgeschwindigkeit, die ein Körper benötigt, um unsere Galaxie zu verlassen, bestimmt. Die beste Abschätzung beträgt 533 (+54/-41) km/s. Anhand dieser Schätzung kann, in Kombination mit vereinfachten analytischen Modellen der Materieverteilung in unserer Galaxie, die Masse der Milchstraße innerhalb von 180 kpc auf 1,3 (+0,4/-0,3) x 10^12 Sonnenmassen bestimmt werden. Dieses Ergebnis bestätigt frühere unabhängige Massenschätzungen, die auf anderen Beobachtungsdaten und anderen Analysestrategien basieren. Im letzten Teil der Arbeit wird eine spezielle Population von Sternen im Außenbereich unserer Galaxie untersucht, sogenannte Hyperschnellläufersterne (HSS). Diese wurde in einer weiteren Himmelsdurchmusterung, dem Sloan Digital Sky Survey (SDSS), gefunden. Die Besonderheit dieser Sterne besteht in ihren extrem hohen Geschwindigkeiten oberhalb der Entweichgeschwindigkeit. Allgemein wird angenommen, dass die Sterne ihre Geschwindigkeiten im Zuge der Spaltung eines Doppelsternsystems durch Gezeitenkräfte nahe des supermassereichen Schwarzen Lochs im Zentrum der Milchstraße erreichen. Vor Kurzem wurde jedoch ein alternatives Szenario vorgeschlagen. Nach diesem können solche Sterne auch während des Einfalls einer Satellitengalaxie auf die Milchstraße entstehen. Diese Hypothese wird anhand von numerischen Simulationen, die diese Situation nachbilden, getestet. Es zeigt sich, dass HSS auf diese Weise entstehen können, aber dass die beobachtete Population höchstwahrscheinlich einen anderen Ursprung hat.

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No description available.

Nesta tese é apresentado um catálogo fotométrico no infravermelho próximo de 35 Regiões HII, todas pertencentes ao disco Galáctico. Esta faixa espectral é útil uma vez que os comprimentos de onda são grandes o suficiente para se ter uma baixa extinção interestelar comparada ao visível, e são pequenos o suficiente para diagnosticar as fotosferas estelares. Foram obtidas imagens nas bandas J, H e K e imagens do Spitzer nos canais de 3,5, 5,8 e 8,0 m. Após a fotometria nas imagens JHK, foi possível construir diagramas cor-cor e cor-magnitude. Foram utilizadas imagens coloridas, compostas de uma combinação RGB das imagens nas três bandas, tanto para as imagens JHK quanto para as imagens do Spitzer. Estas imagens, junto com os diagramas, foram utilizadas para levantar candidatos a fontes ionizantes das regiões HII, assim como objetos ainda em estágios primordiais de evolução (CTTs e MYSOs). Estes dados também foram utilizados para associar à cada região HII um estágio evolutivo (de A até D, da região mais jovem à mais evoluída). Baseado na posição da Sequência Principal em diagramas cor-magnitude, foi possível comparar as distâncias cinemáticas com nossos dados. Além disso, quando possível, foram utilizadas distâncias de regiões HII determinadas por paralaxe espectrofotométrica (disponíveis na literatura) e utilizando duas leis de extinção interestelar extremas mostrou-se que estas distâncias são menores que suas contrapartidas cinemáticas, e estão em acordo com distâncias determinadas por outros métodos, como por paralaxe trigonométrica. Sabendo que estas regiões de formação estelar seguem a dinâmica do gás, o mapeamento da distribuição destas regiões permite checar a estrutura espiral da Via Láctea. / In this work, a near infrared photometric catalog of 35 HII regions that belongs to the Galactic plane is presented. This spectral range is useful since the wavelengths are long enough to have less influence of the interstellar extinction compared to the visible domain, and they are small enough to still show stellar photospheric features. Images of these HII regions in the J, H and K-band together with IRAC-Spitzer images (channels 4.5, 5.8 and 8.0 m) were used. After the photometry in the JHK images, color-color and color-magnitude diagrams were constructed. These two group of images (JHK and 4.5, 5.8 and 8.0 m) colored in a RGB combination were used, together with the diagrams, to identify the ionizing sources candidates, as well as objects still embedded in their natal cocoon (CTTs and MYSOs). An evolutionary stage to these regions (from A to D, from the younger region to the more evolved) was inferred based on the images and diagrams. These diagrams were also used to infer if the kinematic distance is correct, based on the Main Sequence location. Non-kinematic distances to several HII regions, when it was possible, were collected from the literature. Using two extreme interstellar extinction laws, it was possible to compare these distances with the kinematic results. These non-kinematic distances are lower than that from kinematic techniques. Also, these distance discrepancies are in agreement with distances derived by others methodologies, as trigonometric parallax. Since these star forming regions follow the gas dynamics, mapping their distribution along the Galaxy allows to check the spiral pattern of the Milky Way.

Nesta tese é apresentado um catálogo fotométrico no infravermelho próximo de 35 Regiões HII, todas pertencentes ao disco Galáctico. Esta faixa espectral é útil uma vez que os comprimentos de onda são grandes o suficiente para se ter uma baixa extinção interestelar comparada ao visível, e são pequenos o suficiente para diagnosticar as fotosferas estelares. Foram obtidas imagens nas bandas J, H e K e imagens do Spitzer nos canais de 3,5, 5,8 e 8,0 m. Após a fotometria nas imagens JHK, foi possível construir diagramas cor-cor e cor-magnitude. Foram utilizadas imagens coloridas, compostas de uma combinação RGB das imagens nas três bandas, tanto para as imagens JHK quanto para as imagens do Spitzer. Estas imagens, junto com os diagramas, foram utilizadas para levantar candidatos a fontes ionizantes das regiões HII, assim como objetos ainda em estágios primordiais de evolução (CTTs e MYSOs). Estes dados também foram utilizados para associar à cada região HII um estágio evolutivo (de A até D, da região mais jovem à mais evoluída). Baseado na posição da Sequência Principal em diagramas cor-magnitude, foi possível comparar as distâncias cinemáticas com nossos dados. Além disso, quando possível, foram utilizadas distâncias de regiões HII determinadas por paralaxe espectrofotométrica (disponíveis na literatura) e utilizando duas leis de extinção interestelar extremas mostrou-se que estas distâncias são menores que suas contrapartidas cinemáticas, e estão em acordo com distâncias determinadas por outros métodos, como por paralaxe trigonométrica. Sabendo que estas regiões de formação estelar seguem a dinâmica do gás, o mapeamento da distribuição destas regiões permite checar a estrutura espiral da Via Láctea. / In this work, a near infrared photometric catalog of 35 HII regions that belongs to the Galactic plane is presented. This spectral range is useful since the wavelengths are long enough to have less influence of the interstellar extinction compared to the visible domain, and they are small enough to still show stellar photospheric features. Images of these HII regions in the J, H and K-band together with IRAC-Spitzer images (channels 4.5, 5.8 and 8.0 m) were used. After the photometry in the JHK images, color-color and color-magnitude diagrams were constructed. These two group of images (JHK and 4.5, 5.8 and 8.0 m) colored in a RGB combination were used, together with the diagrams, to identify the ionizing sources candidates, as well as objects still embedded in their natal cocoon (CTTs and MYSOs). An evolutionary stage to these regions (from A to D, from the younger region to the more evolved) was inferred based on the images and diagrams. These diagrams were also used to infer if the kinematic distance is correct, based on the Main Sequence location. Non-kinematic distances to several HII regions, when it was possible, were collected from the literature. Using two extreme interstellar extinction laws, it was possible to compare these distances with the kinematic results. These non-kinematic distances are lower than that from kinematic techniques. Also, these distance discrepancies are in agreement with distances derived by others methodologies, as trigonometric parallax. Since these star forming regions follow the gas dynamics, mapping their distribution along the Galaxy allows to check the spiral pattern of the Milky Way.

Comment les étoiles se forment elles ?. Cette vaste question fait appel à des connaissances dans plusieurs domaines dont deux majeurs, la Formation Stellaire et le Milieu Interstellaire. C’est dans ce cadre générale que s’inscrit ma thèse. Notre galaxie est un vaste laboratoire d’études de cette formation et je me suis donc intéressé aux premières étapes de la formation des étoiles, allant du nuage moléculaire à la proto-étoile. J’ai principalement utilisé des données provenant du télescope Herschel qui nous fournit des images et des données dans l’infrarouge lointain et le domaine sub-milimétrique à une résolution inégalée. J’ai d’abord construit un catalogue de sources à l’aide d’un algorithme d’identification croisée, SPECFIND, puis appliqué un algorithme de clustering, MST, sur près de 100 000 sources afin de construire le premier catalogue d’amas d’objets stellaires jeunes à l’échelle galactique. Ceci m’a conduit à étudier les propriétés de ces amas et des sources les constituant. / How stars form? This broad question uses knowledges in several areas, including two majors, the Star Formation and the Interstellar Medium. My thesis is a part of this overall framework. Our galaxy is a laboratory complex for the study of this formation. I became interested in the first stages of the star formations, from Molecular Clouds to protostars. I mainly used data from the Herschel telescope which provides us with images and data in the far infrared and sub-millimiter at an unparalleled resolution. First of all, I built a catalogue of young clumps using SPECFIND, an algorithm of cross-identification. Then I applied an algorithm of clustering, MST, over 100 000 young clumps to find over-densities in order to release the first catalogue of young stellar clusters in a galactic scale. Finally, I studied the physical properties of these clusters and their young clumps.