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<p>Stars form in clusters amidst complex and coupled physical phenomena. Among the most important of these is radiative feedback, which heats the surrounding gas to suppress the formation of many low-mass stars. In simulations of star formation, pre-main-sequence modeling has often been neglected and stars are assumed to have the radii and luminosities of zero-age main sequence stars. We challenge this approach by developing and integrating a one-zone protostellar evolution model for FLASH and using it to regulate the radiation output of forming stars. The impact of accurate pre-main-sequence models is less ionizing radiation and less heating during the early stages of star formation. For stars modeled in isolation, the effect of protostellar modeling resulted in ultracompact HII regions that formed slower than in the ZAMS case, but also responded to transitions in the star itself. The HII region was seen to collapse and subsequently be rebuilt as the star underwent a swelling of its radius in response to changes in stellar structure and nuclear burning. This is an important effect that has been missed in previous simulations. It implies that observed variations in HII regions may signal changes in the stars themselves, if these variation can be disentangled from other environmental effects seen in the chaotic cluster environment.</p> / Master of Science (MSc)

New radio telescopes under development, will significantly enhance the capabilities of radio astronomy in the Southern Hemisphere. South Africa, in particular, is actively involved in the development of a new array (MeerKAT) as well as in the expansion of existing very long baseline interferometer arrays in the south. Participation in these new developments demands a thorough understanding of radio astronomy techniques, and data analysis, and this thesis focusses on two projects with the aim of gaining such experience. The Southern Hemisphere very long baselines array is not well served with calibrator sources and there are significant gaps in the present calibrator distribution on the sky. An adequately dense, well distributed, set of strong, compact calibrator or reference sources is needed. With this in mind, observations using the Southern Hemisphere long baseline array were conducted to investigate a sample of candidate calibrator sources. The compactness of the sources was investigated and new potential calibrators have been identified. Single antenna radio spectroscopy of OH masers has identified sources of 1720 MHz emission associated with supernova remnants at the shock interface between the expanding supernova remnant and a molecular cloud. Models indicate that these masers are shock excited and can only be produced under tight physical constraints. Out ows from newly-formed stars create nebulous regions known as Herbig-Haro objects when they interact with the surrounding medium, and these regions are potentially similar to those seen in supernova remnants. If conditions behind the shock fronts of Herbig-Haro objects are able to support 1720-MHz OH masers they could be a useful diagnostic tool for star formation. A survey toward Herbig-Haro objects using a single-dish radio telescope did detect 1720-MHz OH lines in emission, but neither their spectral signature nor follow-up observations with the Very Large Array showed evidence of maser emission. / Mathematical Sciences / Ph.D. (Astronomy)

Cette thèse présente la mesure des redshifts photométriques et des paramètres physiques dans le cadre des sondages de galaxies à grande échelle, ainsi que la contrainte qui peut en être extraite sur l'évolution des galaxies. Je montre notamment dans cette thèse dans quelle mesure la calibration photométrique affecte la précision des redshifts photométriques, afin de contraindre la stratégie photométrique à développer pour la mission Euclid.Afin de prendre en compte les problèmes inhérents à l'observation, les analyses ont été effectuée sur la base de données observées dont la configuration est proche de celle qui est attendue pour Euclid. Ces données combinent de nouvelles observations en proche-infrarouges conduites pour couvrir le sondage spectroscopique VIPERS et la photométrie du CFHTLS. Sur la base des conclusions tirées de cette analyse, j'ai produit le nouveau catalogue photométrique de VIPERS, ainsi que le catalogue de redshifts photométriques associé. J'ai finalement utilisée la même photométrie pour dériver les paramètres physiques d'environ 760 000 galaxies, réparties sur plus de 22 degrés carrés à une magnitude limite Ks(AB) < 22. J'ai ainsi pu étudier l'évolution de la fonction de masse stellaire entre les redshifts z = 0.2 et z = 1.5. Ceci a permis de montrer que les galaxies dont la masse stellaire est d'environ log(M/Msol) = 10.66 voient généralement leur formation stellaire stoppée en 2 à 4 milliards d'années, alors que les galaxies de faible masse (log(M/Msol) < 9.5) ne formant plus d'étoiles ont vu leur formation stellaire être arrêtée 5 à 10 fois plus rapidement (en environ 0.4 milliard d'années). / This thesis presents the measurement of the photometric redshifts and physical parameters in the framework of large scale surveys, and their constraint on galaxy evolution. The photometric redshift measurement allows us to study the entire photometric sample. For this reason, the weak lensing signal measurement used in the Euclid mission as a primary cosmological probe will rely on photometric redshift measurements. However, the method is strongly affected by the quality of the photometry. In particular, I show in this thesis how the photometric calibration impacts the photometric redshift precison, in order to constrain the photometric strategy to use in the Euclid mission.Aiming to take into account for observationnal problems, the analysis is done with observationnal data whose photometric configuration is close to the expected Euclid one. These data combine new near-infrared observations conduected to cover the VIPERS spectroscopic survey and the CFHTLS photometry.Using the conclusions of this analysis, I have producted the new photometric catalogue for VIPERS and the associated photometric redshift calalogue.Finally, I used the same photometry to compute the stellar masses of 760,000 galaxies covering 22 square degrees at the limiting magnitude Ks(AB) < 22. This enabled me to study the evolution of the stellar mass function between redshifts z= 0.2 and z = 1.5. We have then shown that the star formation of galaxies with stellar masses around log(M/Msol) = 10.66 is stopped in 2-4 Gyr, while in quiescent low-mass (log(M/Msol) < 9.5) galaxies, the star formation has been stopped 5-10 times faster (approximatelly in 0.4 Gyr).

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Les radio galaxies sont les candidats préférentiels pour comprendre la formation et l'évolution des galaxies sur une grande échelle de temps. Observées jusqu'à z>5 en raison de leur brillance, elles sont abritées par des galaxies elliptiques géantes. L'émission radio révèle la présence d'un trou noir supermassif. Un tore de poussière entourant le noyau actif de galaxie (AGN) agit comme un coronographe naturel permettant alors l'étude de la galaxie hôte. L'objectif de cette thèse est de déterminer l'évolution de la composante stellaire en présence d'un AGN. La décomposition est faite à partir de la distribution spectrale d'énergie (SED) de l'UV au submillimétrique en utilisant le code d'évolution de galaxies PEGASE.3 et un code d' AGN, les deux modélisant l'émission de la poussière par transfert radiatif.En premier lieu, nous présentons le projet HeRGE, 70 radio galaxies observées avec Herschel, qui permet de mesurer leurs luminosités totales infrarouges, comparables à celles des ULIRG. Une décomposition de la luminosité infrarouge entre l'émission AGN et un modèle de starburst est proposée pour l'ensemble de l'échantillon. Ces luminosités élevées sont interprétées en termes de taux d'accrétion et de formation d'étoiles, favorisant la croissance du trou noir par rapport à la galaxie hôte.En second lieu, l'orientation du jet par rapport au tore est contrainte à partir de l'infrarouge moyen et du rapport des émissions radio des lobes (isotrope, 500MHz) et du coeur (anisotrope, 20GHz). Ces observations en accord avec le modèle d'unification permettent d'évaluer le facteur d'absorption Av, l'inclinaison du tore et de contraindre le facteur de Lorentz.Une sélection de 12 radio galaxies observées de l'UV au sub-mm est analysée avec PEGASE.3 et un modèle d'AGN. Une seule composante stellaire est insuffisante. Seules deux composantes (une évoluée et massive, et une jeune issue d'un starburst) permettent un ajustement significatif de la SED complète. La composante évoluée est très massive (environ 10^12 msun) formée sur une courte période de temps (<10^9 ans). La composante jeune (<4.10^7 ans), moins massive (environ 10^11 msun), confirme un processus épisodique de croissance par sursauts. Ces résultats sont des contraintes fortes pour les modèles de formation de galaxies. La relation avec le noyau actif reste encore à préciser. Les projets d'observations complémentaires, optique et mm, permettront de confirmer ces résultats. / Powerful radio galaxies are excellent candidates for investigating and ultimately understanding the formation and evolution of galaxies. These beacons are now observed out to z>5 and are commonly associated with the massive early-type galaxies observed in the local universe. While the radio emission reveals the presence of a supermassive black hole, a dusty parsec-scale torus acts like a natural coronograph, making it easier to study the properties of the host galaxy. The aim of this PhD thesis is to characterise the nature and evolution of the stellar population and the relationship between the stellar population and the active galactic nucleus (AGN). To reach our scientific goals, we use the galaxy evolution code, PEGASE, combined with a AGN model which both consider the radiative transfer of the UV, optical, and IR photons through dust. To begin, we present the HeRGE project consisting of 70 radio galaxies which have been observed with Herschel. These IR observations allow us to calculate the total infrared luminosities and reveal that our sample belongs to the ULIRG regime. We decompose the infrared SED into an AGN and starburst components using observational templates. Converted into accretion and star formation rate, their relative luminosities indicate that the black holes are growing proportionally faster than are the host galaxies.In addition, we constrain the configuration of the jet and torus by combining the results from mid-infrared spectral energy distribution (SED), and the radio emission from the lobes (isotropic at 500MHz) and the core (anisotropic at 20GHz). In agreement with the unified scheme, these observations allow us to estimate the absorption Av, the inclination of the torus, and provides a constraint on the Lorentz factor for the radio jet.A subsample of 12 radio galaxies observed from the UV to sub-mm is also analysed with PEGASE.3 and an AGN torus model. While one stellar component is clearly insufficient to fit the observations, two stellar components are necessary to successfully reproduce the SED (one evolved and massive, about 10^12 msun, formed over a reasonably short time, <1Gyr at high redshift; and a much younger component, <40Myr, that is also less massive, about 10^11 msun. Such a star formation history suggests rapid growth at high redshift of longer duration followed much by another period of rapid, stochastic growth.These results put strong constraints on galaxy formation models. Unfortunately, the crudeness of some of our data and theoretical understanding the IR emission from AGN, means that the relation of the galaxy to its AGN is still not well constrained. Additional observations at optical through millimeter wavelengths are needed to extend our findings.

New radio telescopes under development, will significantly enhance the capabilities of radio astronomy in the Southern Hemisphere. South Africa, in particular, is actively involved in the development of a new array (MeerKAT) as well as in the expansion of existing very long baseline interferometer arrays in the south. Participation in these new developments demands a thorough understanding of radio astronomy techniques, and data analysis, and this thesis focusses on two projects with the aim of gaining such experience. The Southern Hemisphere very long baselines array is not well served with calibrator sources and there are significant gaps in the present calibrator distribution on the sky. An adequately dense, well distributed, set of strong, compact calibrator or reference sources is needed. With this in mind, observations using the Southern Hemisphere long baseline array were conducted to investigate a sample of candidate calibrator sources. The compactness of the sources was investigated and new potential calibrators have been identified. Single antenna radio spectroscopy of OH masers has identified sources of 1720 MHz emission associated with supernova remnants at the shock interface between the expanding supernova remnant and a molecular cloud. Models indicate that these masers are shock excited and can only be produced under tight physical constraints. Out ows from newly-formed stars create nebulous regions known as Herbig-Haro objects when they interact with the surrounding medium, and these regions are potentially similar to those seen in supernova remnants. If conditions behind the shock fronts of Herbig-Haro objects are able to support 1720-MHz OH masers they could be a useful diagnostic tool for star formation. A survey toward Herbig-Haro objects using a single-dish radio telescope did detect 1720-MHz OH lines in emission, but neither their spectral signature nor follow-up observations with the Very Large Array showed evidence of maser emission. / Mathematical Sciences / Ph.D. (Astronomy)

Galaxy clusters are the largest gravitationally bound structures in the Universe. Their masses are dominated by dark matter ($\sim$85\% of the mass) with stars representing 1-4\% of their masses. A hot, X-ray emitting gas called the intracluster medium makes most of their baryonic mass. The presence of this gas and of numerous neighbouring galaxies prematurely stop the star formation in clusters. In other terms, more galaxies in clusters are passive than in the general population of galaxies. This effect is mass and position-dependant: high-mass galaxies are more likely to be passive than less massive ones; galaxies inhabiting the cluster core are also less likely to form stars than those in the outskirts. The fraction of passive galaxies is greater in local clusters than in high-redshift ones, because they had more time to evolve. Much is unknown about the cessation of star formation, called quenching, in clusters. Thus, although many examples of infalling galaxies being stripped of their gas have been reported for low-mass galaxies, it is unclear if the most massive members became quenched before or after they become cluster members. The relationship between quenching and the cluster mass is also poorly understood. Despite the variety of methods devised to find clusters of galaxies, most of what we know about quenching in $z\gtrsim 1$ clusters was discovered with optically/infrared-selected cluster samples (clusters found as overdensities of galaxies), or samples of mixed origin. Yet, there is tentative evidence that optically/infrared-selected samples are biased toward having more passive galaxies than those that were X-ray selected. In the present dissertation, quenching is explored in X-ray selected cluster samples. A sample of high-redshift, low-mass galaxy clusters is built by finding galaxy overdensities coincident with sources of extended X-ray emission. A photometry-based analysis reveals that the fraction of quenched galaxies in these clusters is very variable. Moreover, the brightest cluster galaxies are also diverse. Yet, for all the information that photometry can provide, this sample candidate clusters need to be confirmed with spectroscopy. Spectroscopic observations obtained for four candidate clusters are reduced and analysed. The results show that three of them are clusters, the fourth candidate being a superposition of structures. Member spectra are examined to infer their star formation history, and the results shows the existence of an intermediary population of galaxies, where an old stellar population coexists with weak star formation. Finally, the galaxies of a $z=1.98$ X-ray selected cluster, XLSSC 122 are investigated in detail. Photometric data in 12 bands are organized to perform spectral energy distribution fittings, a technique that allows a simplified reconstitution of the history of the star formation. Results show that the members were formed at diverse epochs, the oldest being about 2.5 Gyrs old. Simulations drawn from the Multi Dark Planck 2 are used to infer the mass-scale of the cluster when the oldest galaxies were formed, something that has never been done before. The oldest galaxies were probably formed when XLSSC 122 had accreted $<$10\% of its $z=1.98$ mass, i.e. the mass-scale of a galaxy group. / Graduate

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