Submitted by Renata Lopes (renatasil82@gmail.com) on 2016-06-07T13:34:38Z No. of bitstreams: 1 samyragomesfurtado.pdf: 18227826 bytes, checksum: a36abfd9b85d16b4d61e026b578565bc (MD5) / Approved for entry into archive by Adriana Oliveira (adriana.oliveira@ufjf.edu.br) on 2016-07-13T13:14:30Z (GMT) No. of bitstreams: 1 samyragomesfurtado.pdf: 18227826 bytes, checksum: a36abfd9b85d16b4d61e026b578565bc (MD5) / Made available in DSpace on 2016-07-13T13:14:30Z (GMT). No. of bitstreams: 1 samyragomesfurtado.pdf: 18227826 bytes, checksum: a36abfd9b85d16b4d61e026b578565bc (MD5) Previous issue date: 2016-02-26 / CAPES - Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / A vegetação montanhosa abriga grande diversidade de plantas por combinar fatores como o isolamento, geodiversidade e variação climática a pequenas distâncias, causada pela elevação, a qual altera a comunidade vegetal ao longo do gradiente. As epífitas são particularmente ricas em florestas nebulares da Região Neotropical, mas no Brasil ainda são poucos os estudos nessa vegetação. O Parque Estadual do Ibitipoca (PEIB), localizado na Serra da Mantiqueira, possui relevo caracterizado por escarpas e colinas, em altitudes que variam de 1000 a ca. 1800 m que abrigam, entremeadas aos campos, fragmentos de floresta e nanofloresta nebulares, sendo a última dividida em quatro subfisionomias. Essas formações florestais se destacam por reterem a umidade da neblina possibilitando alta abundância de indivíduos epifíticos. Para conhecer a composição e estrutura da comunidade das epífitas vasculares nessas formações foram utilizados exemplares provenientes das coletas da flora do Parque ao longo de mais de 40 anos depositados em herbários, além de novas expedições realizadas entre julho de 2014 e julho de 2015 para observação, registro fotográfico e coleta das plantas que foram depositadas no herbário CESJ da Universidade Federal de Juiz de Fora. Os resultados são apresentados em dois capítulos. O primeiro capítulo apresenta uma listagem das epífitas vasculares ocorrentes nas florestas nebulares do PEIB, composta por 223 espécies distribuídas em 81 gêneros, sendo Pleurothallis s.l. R.Br. o mais rico (13 spp.), e 22 famílias, das quais Orchidaceae é a mais rica (86 spp.), correspondendo a 28% da flora vascular dessa formação. Tanto o número absoluto de espécies, quanto o quociente epifítico observado correspondem a uma das maiores diversidades já amostradas em estudos desta natureza na Floresta Atlântica brasileira, fato ainda mais relevante se levado em consideração que a fisionomia estudada representa pouco menos de 300 ha ou ca. 20% da área total do PEIB (ca. 1488 ha). Esta relevância é também reforçada pela presença de 13 e 23 espécies ameaçadas, respectivamente, em nível nacional e estadual em categorias variadas. O segundo capítulo apresenta os resultados da avaliação da comunidade de epífitas vasculares ao longo dos gradientes altitudinal e ambiental, representado por três das quatro subfisionomias da nanofloresta nebular. Para isto foram estabelecidas 24 parcelas de 10 x 20 m, divididas em oito conjuntos formados, cada um, por nanoflorestas tenuissólica humífera ripícola, crassissólica arenosa de encosta e tenuissólica arenosa de encosta. O estabelecimento dos conjuntos teve como base as cavernas cuja vegetação do entorno é composta pela primeira formação supracitada em altitudes distintas (Ponte de Pedra, Coelhos, Bromélias, Monjolinho, Cruzeiro, Pião, Viajantes e Três Arcos), estando as demais formações o mais próximo e com altitude mais similar possível. Em vii cada parcela foram analisados todos os forófitos com diâmetro a altura do peito acima de 10 cm. Para avaliar uma potencial influência da luminosidade sobre a ocorrência das epífitas, foram tiradas fotografias do dossel, para estimar a sua porcentagem de cobertura empregando o programa CanopyDigi. Para análise da estrutura da comunidade, foram calculados frequências absoluta e relativa, índices de diversidade de Shannon (H’), de uniformidade de Pielou (J) e de diversidade taxonômica [distinção taxonômica média (K+) e variação na distinção taxonômica (M+)] para cada uma das três subdivisões e para a comunidade como um todo. Foram amostrados 568 forófitos (com o mais rico abrigando 29 espécies), resultando em 3771 ocorrências de epífitas, distribuídas em 146 espécies, das quais 20 foram acidentais. Orchidaceae é a mais rica das 22 famílias encontradas, com 53 espécies (36%) e, junto com Polypodiaceae (19 spp.) e Bromeliaceae (16 spp.), somam 60% do total das espécies. Não houve correlação da cobertura do dossel com a riqueza observada de epífitas (r = 0,26, p = 0,23). Os valores de H’ e J para a comunidade foram, respectivamente, 3,97 e 0,82. A diversidade foi significativamente diferente entre as fisionomias da nanofloresta e ao longo do gradiente altitudinal, se tornando maior com o aumento da altitude. O valor de K+ para a Gruta dos Coelhos foi significativamente menor do que o esperado, representando um provável impacto causado pela maciça visitação de turistas, resultando em coletas de plantas ornamentais ou degradação ambiental, uma vez que é a gruta de acesso mais fácil no PEIB. Os resultados obtidos evidenciaram a diversidade da região e a contribuição importante das epífitas, bem como sua sensibilidade às diferentes fisionomias vegetais e as variações climáticas causadas pela altitude, que levaram a um gradiente atípico de diversidade, além da importância de se levar em consideração o impacto antrópico, mesmo em uma unidade de conservação. / The montane vegetation harbors great plant diversity due to the combination of features like the isolation, geodiversity and climatic variation in short distances, caused by elevation, which alter the plant community along the gradient. Epiphytes are especially rich in cloud forests of the Neotropical Region, but in Brazil there are only few studies in such vegetation. The Parque Estadual do Ibitipoca (PEIB), located at Serra da Mantiqueira, has a relief characterized by hills and cliffs, in altitudes between 1000 and around 1800 m that presents, interspersed to field vegetation, fragments of cloud forest and dwarfforest, being the latter divided in four physiognomies. These forest formations stand out for retaining moisture from the fog allowing high abundance of epiphytic individuals. In order to know the composition and structure of the community of vascular epiphytes in such formations were used specimens collected in the Park along more than 40 years deposited in herbaria, besides new expeditions conducted between July 2014 and July 2015 for observation, photographic record, and collection of the plants, which were deposited in the herbarium CESJ of Universidade Federal de Juiz de Fora. Results are presented in two chapters. The first chapter presents the checklist of vascular epiphytes in the cloud forests of PEIB, composed by 223 species distributed in 81 genera, being Pleurothallis s.l. R.Br. the richest (13 spp.), and 22 families, of which Orchidaceae is the richest (86 spp.), corresponding to 28% of the vascular flora of this formation. Both the absolute number of species and the epiphytic quotient represent one of the largest diversities ever found in such studies in the Brazilian Atlantic Forest, which is of great relevance if taking in account that the studied physiognomy represents less than 300 ha or around 20% of the total area of PEIB (around 1488 ha). This relevance is reinforced by the presence of 13 and 23 species threatened with extinction respectively in national and state levels in distinct categories. The second chapter presents the results of evaluation of the community of vascular epiphytes along the elevation and environmental gradients, represented by three of four subphysiognomies of dwar-forest. For this purpose were established 24 plots of 10 x 20 m, divided in eight sets each composed by riverine dwarf-forest with humiferous shallow soil, slope dwarf-forest with sandy deep soil, and slope dwarf-forest with sandy shallow soil. The establishment of the sets was based on the caves whose surrounding vegetation comprises the first aforementioned formation at different altitudes (Ponte de Pedra, Coelhos, Bromélias, Monjolinho, Cruzeiro, Pião, Viajantes e Três Arcos), being the other formations nearest as possible and with the most similar elevation possible. All phorophytes with diameter at breast height larger than 10 cm were analyzed in each plot. In order to evaluate the influence of luminosity on the ix occurrence of epiphytes, photographs of the canopy were taken, to estimate the cover percentage using the software CanopyDigi. The community structure was analyzed through calculation of absolute and relative frequencies, diversity index of Shannon (H’), uniformity index of Pielou (J), and taxonomic diversity indices [average taxonomic distinctness (K+) and variation in taxonomic distinctness (M+)], for each subdivision of the dwarf-forest and for the community as a whole. It were sampled 568 phorophytes (the richest having 29 species), resulting in 3771 occurrences of epiphytes, distributed in 146 species, of which 20 were accidental. Orchidaceae is the richest of the 22 families found, with 53 species (36%) and, together with Polypodiaceae (19 spp.), and Bromeliaceae (16 spp.), have 60% of the total. There was no correlation between the canopy cover and the observed richness of epiphytes (r = 0.26, p = 0.23). The values of H’ and J for the community were respectively 3.97 and 0.82. The diversity was different between the physiognomies of dwarf-forest and along the elevation gradient, being greater with enhancement of the elevation. The value of K+ to Gruta dos Coelhos was lower than expected, and represents a potential impact caused by the massive visitation of tourists, resulting in collection of ornamental plants or environmental degradation, once this is the cave of easiest access in PEIB. The obtained results highlight the diversity of this region and the important contribution of the epiphytes, as well as its sensibility to the different vegetation physiognomies and the climatic variations caused by the elevation, which conducted to an atypical diversity gradient, besides the importance of taking in account the anthropic impact, even in a conservation unity.

In this thesis we test and expand our current knowledge of Star Formation Laws (SF laws) in the extreme environment of dwarf irregular galaxies. We focus on the SF characteristics of our 18 galaxies sample, extending current investigations of the Schmidt-Kennicutt law to the low luminosity, low metallicity regime. The Hi data used in this project have been observed, calibrated and imaged according to the LITTLE THINGS Survey prescription to which I brought my own contribution as a member of the team. Apart from high resolution, VLA data in B, C and D array configurations, this project makes use of an extensive set of multi- wavelength data (H , FUV, 24 m, 3.6 m, V-band and K-band). Molecular gas in dwarfs is very difficult to observe, mainly because due to the low metallicity environment, we lose our only molecular tracer, the CO which becomes under luminous. Therefore the gas distribution is represented by Hi gas only. We create our Star Formation Rate (SFR) maps mainly based on FUV maps because our analysis shows that FUV is the SF tracer that allows us the most extensive sampling of the SFR surface density (SFRD) and Hi surface density relation. The main results of our study are: Whereas in spiral galaxies Bigiel et al. (2008) have found a one to one relation between star formation rate and molecular gas and no relation between the SFR and the neutral gas, in a small sample of dwarfs as well as in the outskirts of spiral galaxies Bigiel et al. (2010b) has found that SFRD does correlate with Hi surface density. We confirm the existence of the SFRD vs. Hi surface density relation in dwarf irregular galaxies and a linear fitting through all our data (all 18 galaxies combined) yields a power law relation ΣSFR ∝ Σ1.87±0.3/HI . We find that the interiors of Hi shells, at 400 pc scales, become resolved and show up in SFRD versus Hi surface density plots although within the shell interior we have SFRD values but no Hi surface density related to them. Thus, the points originating from those regions contribute significantly to the increase of the scatter in the plot. We show that by excluding those points the correlation between SFRD and Hi surface density improves between 10% and 20%. Eight of the 18 galaxies in our sample have Hi maxima higher than the 10M pc-2 value found by Bigiel et al. (2008) for spiral galaxies. Krumholz et al. (2011) predicted that the 10M pc-2 threshold is metallicity dependent in galaxies with sub-solar metallicity, however the theoretically predicted values for our galaxies only match the observed Hi maxima in one case (DDO168). We find that metallicity cannot be the only factor setting the Hi to H2 transition. In fact, we find evidence that the higher the interstellar radiation field (ISRF), the higher the Hi maximum is, hence we suggest that the ISRF should also be taken into consideration in predicting the Hi to H2 transition threshold. We find that even tighter than the SFRD vs. Hi surface density relation is the SFRD vs. V-band surface density relation. Unlike the SFRD vs. Hi surface density relation the SFRD vs. V-band surface density relation follows a power law and can be written as follows: ΣSFR ∝ (10^μv)^-0.43±0.03. The SFRD vs. V-band surface density relation suggests that the existing stars also play a role in the formation of the next generation of stars. Within our sample of dwarf galaxies the average pressure per resolution element and the SFRD are in a 1:1 linear relation: ΣSFR ∝ P_h^1.02±0.05. A similar relation has been found by Blitz & Rosolowsky (2006) for the low-pressure regimes of spiral galaxies. In conclusion we find that in the extreme environments of dwarf galaxies the metal deficiency and the lack of the classic SF stimulators (spiral arms, shear motions) do not impede the star forming process. In these galaxies, dust-shielding becomes predominantly self-shielding and there is plenty of Hi available to achieve this additional task. Existing stars assume the role of pressure enhancers, which in turn will stimulate SF without the need of spiral arms or shear motion.

L’Univers est dominé par une composante invisible appelée Matière Noire (MN), de nature inconnue mais dont les effets gravitationnels sur la matière visible sont clairement observés. Il a été proposé que la MN soit constituée de particules massives et interagissant faiblement avec la matière, permettant ainsi de concilier théorie, observations et simulations. L’annihilation de ces particules dans les régions où la MN est fortement concentrée pourrait produire des rayons γ de très haute énergie dont les signatures spectrales peuvent être détectées par le réseau de télescopes H.E.S.S. Un excès à ~3σ est observé dans la direction de la Galaxie Naine du Sagittaire, avec la méthode standard d’analyse ON-OFF. Plus de données sont nécessaires pour conclure quant à son origine. La seconde partie du travail concerne la recherche de raies spectrales en γ dans la région du Centre Galactique. Une méthode de Maximum de Vraisemblance Complète a été développée, étalonnée et appliquée à une fraction d’un ensemble de 20h de données prises en 2014. Aucun excès de γ n’étant observé, des limites sur la section efficace d’annihilation de la MN sont produites pour des masses de 100 GeV à 2 TeV, la sensibilité de H.E.S.S. à basse énergie étant obtenue par l’ajout d’un 5ème télescope depuis 2012. Ces limites complètent efficacement les précédents résultats de Fermi-LAT et H.E.S.S. D’autre part l’analyse finale devrait permettre d’exclure un potentiel signal à 130 GeV observé dans les données de Fermi-LAT en 2012 et ce avec plus de 95% CL, et de proposer les limites les plus solides à ce jour sur les modèles d’émission de raies spectrales en γ dans le domaine d’énergie couvert par H.E.S.S. / The Universe is full of gravitational evidence of a dominant invisible Dark Matter (DM) component at the Galactic and cosmological scales. Although its nature is still one of the major puzzles of the 21st Century, Weakly Interacting Massive Particles (WIMPs) are an excellent scenario for matching theoretical predictions with observations and simulations. In particular, their self-annihilations would give rise to characteristic spectral signatures in γ-rays, detectable at Very High Energies (VHE) with the H.E.S.S. telescope array in regions such as the Galactic Centre (GC) and Dwarf Spheroidal Galaxies (dSphs). The standard ON-OFF analysis method is applied in the observation of the Sagittarius dSph where a ~3σ hotspot is observed above 300 GeV, although more statistics is required to conclude on its potential DM origin. The second part of the work is focused on the search for monochromatic γ-ray line signatures in the GC region. A Full Likelihood method has been developed, calibrated with Monte-Carlo simulations and applied to a sub-sample of a 20h dataset acquired in 2014. No excess signal is found, thus leading to limits on the DM annihilation cross-section down to a 100 GeV mass range, the sensitivity at the lowest energies being achieved by the 5th H.E.S.S. telescope added in 2012. These limits efficiently fill the gap in mass between results from Fermi-LAT and the first phase of H.E.S.S. On the other side the analysis of the complete dataset is expected to exclude the 130 GeV line-like feature recently reported in the Fermi-LAT data, with more than 95% CL, and to provide the most constraining DM limits so far on γ-ray line emission in the VHE range.

The topic of this thesis is the study of the vertical structure of the disk galaxies and their dark matter halos through theoretical modeling and numerical calculations. The basic theoretical model of the galactic disk used involves gravitationally-coupled stars and gas under the force-field of a dark matter halo; the disk is rotationally-supported in the plane and pressure-supported perpendicular to the plane of the galaxy. The first part of the thesis involves evaluating the vertical structure of stars and gas in normal as well as dwarf spiral galaxies. The second part of the thesis deals with probing the dark matter halo density profiles of disk galaxies using both the observed rotation curve and the H i scale height data. Following is the layout of the thesis. Chapter 1 gives a general introduction to the topic of vertical structure of spiral galaxies and their dark matter halos, followed by a broad overview of the theoretical development of the topic and ends with highlighting the motivation and challenges met in this thesis. Chapters 2 & 3 deal with the vertical structure of stars and gas in galaxies, Chapters 4-6 focus on obtaining the dark matter halo density profiles of disk galaxies from the observed rotation curve and the H i scale height data whereas Chapter 7 is devoted to the summary of results and future research plans. Vertical structure of stars and gas in galaxies The vertical thickness of the stars and the gas, namely atomic hydrogen (H i) and molecular hydrogen (H2) in a spiral galaxy, is crucial in regulating the disk dynamics close to the mid-plane, especially in the inner galaxy. However, measuring it observationally is not in general practicable due to the limitations of astronomical observations, and often impossible as in the case of face-on galaxies. Therefore, it is imperative to develop a theoretical model of the galaxy which can predict the thickness of the disk components by using as input parameters the physical quantities, which are more observationally-amenable compared to the disk thickness. The vertical thickness of the disk components is determined by a trade-off between the upward kinetic pressure and the net downward gravitational pull of the galaxy. The fraction of the disk mass due to the stars is an order of magnitude higher than that of the gas in ordinary spiral galaxies, and therefore the gas contribution to the disk gravity is ignored in general. We have developed a multi-component model of gravitationally-coupled stars, HI and H2 subjected to the force-field of an external dark matter halo, and conclusively demonstrated the importance of the inclusion of gas gravity in explaining the steep vertical stellar distribution observed in galaxies. These apart, this model does not implicitly assume a flat rotation curve for the galaxy and therefore is applicable in general to obtain the thickness of stars and gas in dwarfs (with linearly rising rotation curves) as well as in ordinary spirals. In Chapter 2, we investigate the origin of the steep vertical stellar distribution in the Galactic disk. One of the direct fall outs of our above model of the galaxy, which incor¬porates the self-gravity of the gas unlike the earlier theoretical models, lies in explaining the long-standing puzzle of the steep vertical stellar density distribution of the disk galax¬ies near the mid-plane. Over the past two decades, observations revealed that the vertical density distribution of stars in galaxies near the mid-plane is substantially steeper than the sech2 function that is expected for a self-gravitating system of stars under isothermal ap¬proximation. However, the physical origin for this has not been explained so far. We have clearly demonstrated that the inclusion of the self-gravity of the gas in the dynamical model of the Galaxy solves the problem even under the purview of isothermal approximation for the disk components. Being a low dispersion component, the gas resides closer to the mid¬plane compared to the stars, and forms a thin, compact layer near the mid-plane, thereby strongly governing the local disk dynamics. This novel idea, highlighting the significance of gas gravity has produced substantial impact on the field and triggered research activities by other groups in related areas of disk dynamics. The strong effect of the gas gravity on the vertical density profile of the stellar disk indicates that it should also bear its imprint on the Milky way thick disk, as the epoch of its formation 109 years ago is marked by a value of gas fraction, almost an order of magnitude higher than its present day value. Interest-ingly, the findings of the upcoming Gaia mission can be harnessed to verify this theoretical prediction. It may also hold the clue as to the reason behind the absence of thick disk in superthin galaxies. In Chapter 3, we use the same model to theoretically determine the H i vertical scale heights in the dwarf galaxies: DDO 154, Ho II, IC 2574 & NGC 2366 for which most of the necessary input parameters are available from observations. We stress the fact that the observational determination of the gas thickness in these dwarf irregulars is not viable. Nevertheless, it is important to estimate it theoretically as it plays a crucial role in calculating the star-formation activities and other related phenomena. However, two vital aspects have to be taken care of while modeling these dwarf galaxies. Firstly, the mass fraction in gas in these galaxies is comparable to that of the stars, and hence the gas gravity cannot be ignored on any account unlike in the case of large spirals. Secondly, dwarf galaxies have a rising rotation curve over most of the disk unlike the flat rotation curves of ordinary spirals. Both these factors have been considered in developing our model of the dwarf galaxies. We find that three out of the four galaxies studied show a flaring of their H i disks with increasing radius, by a factor of a few within several disk scale lengths. The fourth galaxy (Ho II) has a thick H1 disk throughout. A comparison of the size distribution of H1 holes in the four sample galaxies reveals that of the 20 type 3 holes, all have radii that are in agreement with them being still fully contained within the gas layer. Probing the dark matter halo profiles of disk galaxies The next part of the thesis involves the dynamical study of the shapes and density profiles of galactic dark matter halos using observational constraints on our theoretical model of a spiral galaxy. The density distribution of the dark matter halo is generally modeled using the observed rotation curve of the spiral galaxies. The rotational velocity at any radius is determined by the radial component of the net gravitational force of the galaxy, which, however, is weakly dependent on the shape of the dark matter halo. Therefore, one cannot trace the dark matter halo shape by the observed rotation curve alone. The vertical thickness of the stars and gas, on the other hand, is strongly dependent on the flattening of the dark matter halo, and therefore the observed gas thickness can be used as a diagnostic to probe the halo shape. In this thesis, we have used the double constraints of the rotation curve and the H i thickness data to obtain the best-fit values of the core density, core radius and the vertical-to-planar axis ratio (or flattening) of the dark matter halos of our largest nearby galaxy Andromeda (or M31), a low-surface brightness (LSB) superthin galaxy UGC 7321 and to study the dark matter halo shape of our Galaxy. In Chapter 4, we study the dark matter halo of M31 or Andromeda, the largest nearby galaxy to the Milky Way. We find that M31 has a highly flattened isothermal dark matter halo with the vertical-to-horizontal axis ratio equal to 0.4, which interestingly lies at the most oblate end of the halo shapes found in cosmological simulations. This indicates that either M31 is a unusual galaxy, or the simulations need to include additional physics, such as the effect of the baryons, that can affect the shape of the halo. This is quite a remarkable result as it challenges the popular practice of assuming a spherical dark matter halo in the dynamical modeling of the galaxy In Chapter 5, we have applied this technique to the superthin galaxy UGC 7321. Su¬perthins are somewhat the “extreme” objects in the local Universe because of their high gas fraction and absence of a thick disk component. It is interesting to analyze their so-called extreme characteristics in the light of the physical mechanisms which determined them to understand better the properties of ordinary spirals. We find that UGC 7321 has a spher¬ical isothermal halo, with a core radius almost equal to the disk scale length. This reveals that the dark matter dominates the dynamics of this galaxy at all radii, including the inner parts of the galaxy. This is unlike the case for the large spiral galaxies, where the core radius is typically about 3-4 disk scale lengths. Interestingly, the best-fit halo core density and the core radius are consistent, with deviations of a few percent, with the dark matter fundamental plane correlations, which depict the systematic properties of the dark matter halo in late-type and dwarf spheroidal galaxies. This apart, a high value of the gas velocity dispersion is required to get a better fit to the H i scale height data, although the superthin nature of the stellar disk implies a dynamically cold dynamic galactic disk. However, it explains the low star-formation rates in these galaxies since the Toomre Q criterion (Q < 1) for instability is less likely to be satisfied, and hence the disk is liable to be more stable to star formation. In Chapter 6, we investigate the shape of the dark matter halo in the outer Galaxy. We find that the halo is prolate, with the vertical-to-planar axis ratio monotonically increasing to 2.0 at 24 kpc, or 8 radial disk scale lengths. The resulting prolate-shaped halo can explain several long-standing puzzles in galactic dynamics, for example, it permits long-lived warps thus explaining their ubiquitous nature. It also imposes novel constraints on the galaxy formation models. Finally, in Chapter 7, the thesis is concluded with a summary of the main results and a brief discussion of the scope for future work.

WD 1145+017 est une étoile naine blanche polluée par des métaux avec un astéroïde en décomposition autour d’elle. Ce système est le premier à montrer la phase de décomposition active de l’objet polluant, et permet d’en apprendre sur cette phase du phénomène d’accrétion. Les différentes observations montrent un système très complexe qui est composé de plusieurs morceaux de l’objet rocheux, d’un disque de poussière et d’un disque de gaz, tous en orbite autour de la naine blanche polluée. Nous présentons un modèle de disque de gaz excentrique en précession conçu pour l’étude des zones d’absorption circumstellaire variables détectées pour WD 1145+017. Ce modèle, inspiré de celui récemment présenté par Cauley et al., calcule explicitement l’opacité du gaz pour toutes conditions physiques du disque prédéterminées et prédit la force et la forme de toutes les zones d’absorption, de l’ultraviolet au visible, à n’importe quelle phase du cycle de précession. Les réussites et échecs de ce modèle simple fournissent de l’information précieuse concernant les caractéristiques physiques du gaz qui entoure l’étoile, entre autres sa composition chimique, sa température et sa densité. Le modèle de disque excentrique met aussi en évidence le besoin de composantes supplémentaires, probablement des anneaux circulaires, pour expliquer la présence d’absorption à décalage de vitesse nul ainsi que celle de raies de Si hautement ionisé. Nous trouvons qu’une période de précession de 4.6±0.3 ans peut reproduire avec succès la forme et le profil de vitesse observé pour la majorité des époques d’observation d’avril 2015 à janvier 2018, bien que des différences mineures à certains moments indiquent que la configuration géométrique supposée n’est probablement pas encore optimale. Finalement, nous montrons que notre modèle peut expliquer quantitativement le changement morphologique des zones d’absorption durant les transits de l’objet en orbite autour de l’étoile. / WD 1145+017 is a metal polluted white dwarf with an actively disintegrating asteroid orbiting around it. This system is the first to show the active decomposition phase of the accretion process. The different observed data show a complex system composed of many pieces of the rocky objets, a dust disk and a gaseous disk, all orbiting the polluted white dwarf. We present an eccentric precessing gas disk model designed to study the variable circumstellar absorption features detected for WD 1145+017. This model, inspired by one recently proposed by Cauley et al., calculates explicitly the gas opacity for any predetermined physical conditions in the disk, predicting the strength and shape of all absorption features, from the UV to the optical, at any given phase of the precession cycle. The successes and failures of this simple model provide valuable insight on the physical characteristics of the gas surrounding the star, notably its composition, temperature and density. This eccentric disk model also highlights the need for supplementary components, most likely circular rings, in order to explain the presence of zero velocity absorption as well as highly ionized Si lines. We find that a precession period of 4.6±0.3 yrs can successfully reproduce the shape of the velocity profile observed at most epochs from April 2015 to January 2018, although minor discrepancies at certain times indicate that the assumed geometric configuration may not be optimal yet. Finally, we show that our model can quantitatively explain the change in morphology of the circumstellar feature during transiting events.

No description available.

A spectroscopic analysis of 10 intergalactic star-forming objects (ISFOs) and a photometric analysis of 67 ISFOs in a sample of 14 interacting systems is presented. The majority of the ISFOs have relative polycyclic aromatic hydrocarbon (PAH) band strengths similar to those of nearby spiral and starburst galaxies. In contrast to what is observed in blue compact dwarfs (BCDs) and local giant H II regions in the Milky Way (NGC 3603) and the Magellanic Clouds (30 Doradus and N 66), the relative PAH band strengths in ISFOs correspond to models with a significant PAH ion fraction (<50%) and bright emission from large PAHs (∼100 carbon atoms). The [Ne III]/[Ne II] and [S IV]/[S III] line flux ratios indicate moderate levels of excitation with an interstellar radiation field that is harder than the majority of the Spitzer Infrared Nearby Galaxies Survey and starburst galaxies, but softer than BCDs and local giant H II regions. The ISFO neon line flux ratios are consistent with a burst of star formation ∼6 million years ago. Most of the ISFOs have ∼106 M ⊙ of warm H2 with a likely origin in photo-dissociation regions (PDRs). Infrared Array Camera photometry shows the ISFOs to be bright at 8 μm, with one-third having [4.5]-[8.0] > 3.7, i.e., enhanced non-stellar emission, most likely due to PAHs, relative to normal spirals, dwarf irregulars, and BCD galaxies. The relative strength of the 8 μm emission compared to that at 3.6 μm or 24 μm separates ISFOs from dwarf galaxies in Spitzer two-color diagrams. The infrared power in two-thirds of the ISFOs is dominated by emission from grains in a diffuse interstellar medium. One in six ISFOs have significant emission from PDRs, contributing ∼30%-60% of the total power. ISFOs are young knots of intense star formation.

The physics pertaining to the asymptotic giant branch (AGB) phase of stellar evolution has been studied for many years. However, the mechanics behind many characteristics displayed at this stage are still not fully understood. As a member of the Long Period Variable class of stars, AGB stars are invaluable in creating three-dimensional maps of the Milky Way, the Magellanic System and other galaxies with resolved stellar populations. Variable stars can be used to determine radial distances from Earth using their periodic luminosity variations. As this type of star has unknown qualities, models of AGB populations need to be calibrated with observed data. Previous research has derived a best-fitting model using the TRILEGAL code (a TRIdimensional modeL of thE GALaxy). This model was calibrated against single-epoch luminosity functions (LFs) calculated from resolved stellar populations in the Small Magellanic Cloud (SMC). With multi-epoch data now available from the VISTA survey of the Magellanic Clouds (VMC), this best-fitting model can now be compared with the LFs as they vary with time. Firstly, statistical tests are completed to measure the extent of the LF variation between epochs and from the mean LF for both the full VMC AGB catalogue and for the oxygen-rich, carbon-rich and extreme AGB classes. Statistical tests are then performed to measure the similarity between the LFs from different epochs and the simulated LFs, again for the entire sample and the three classes. This investigation shows that, while the current best-fitting model is a good approximation of many individual epochs’ AGB LFs in the SMC to within 3σ, inclusion of multi-epoch data would make for a more robust analysis. In order to do this, it would be desirable to have more epochs with deeper and regular observations that could cover full lightcurves of some of the sources. There also seems to be a statistical difference between the inner and outer areas of the SMC, perhaps due to tidal disruptions. It would be interesting to see the results of a similar study using the LMC, which is less affected by the gravitational influence of its smaller companion. / <p>This thesis was written under the supervision of Maria-Rosa Cioni at the Leibniz Institute for Astrophysics in Potsdam. The presentation was held online due to the COVID-19 pandemic.</p>

L’expérience H.E.S.S. (High Energy Spectroscopic System) composée de cinq télescopes Tcherenkov observe le ciel en rayons gamma au-delà d'une centaine de GeV jusqu'à plusieurs dizaines de TeV. Les rayons gamma sont produits par des phénomènes non-thermiques parmi les plus violents dans l'univers au voisinage d'objets astrophysique comme les pulsars, supernovae ou trous noirs, mais pourraient être également produits par l'annihilation de particules de matière noire.De nombreuses sondes cosmologiques et astrophysiques suggèrent que 85% de la matière dans l'Univers est d'origine inconnue. Cette matière appelée matière noire, de nature non baryonique, serait constituée de particules non encore découvertes dont les candidats privilégiés seraient des particules massives interagissant faiblement (WIMPs) avec la matière ordinaire, particules prédites au-delà du Modèle Standard de la physique des particules.Des particules de matière noire peuvent s'annihiler en particules du Modèle Standard dans les régions denses de l'Univers. Parmi les produits d'annihilations se trouvent les photons dont la détection à hautes énergies par des télescopes au sol à effet Tcherenkov pourrait apporter des informations uniques sur la nature de la matière noire.H.E.S.S. observe des régions du ciel dense en matière noire comme le Centre Galactique et des galaxies naines satellites de la Voie Lactée.Une interprétation d'un excès de rayons gamma détecté au Centre Galactique par H.E.S.S. en termes d’accélération de protons par une population de pulsars millisecondes est présenté.10 ans d'observations du Centre Galactique avec le réseau H.E.S.S. I de quatre télescopes, cinq ans de prise de données vers la région du Centre Galactique avec le réseau complet H.E.S.S. II, et un jeu de deux ans de données vers des galaxies naines découvertes récemment sont analysés. Les recherches de signaux d'annihilation de matière noire vers ces cibles ont produit les limites plus fortes à présent sur la section efficace d'annihilation de matière noire dans la plage en masse du TeV. Le potentiel de détection de matière noire avec le futur réseau de télescopes CTA (Cherenkov Telescope Array) vers la région central du halo Galactique est étudiés. / The H.E.S.S. (High Energy Spectroscopic System) experiment is an array of five Cherenkov telescopes that observe the sky in gamma-rays from about 100 GeV up to several ten TeV.Gamma rays are produced in violent non-thermal phenomena in the Universe in the neighborhood of pulsars, supernovae, black holes, ..., and could also be produced by the annihilation of dark matter particles.Numerous cosmological and astrophysical probes suggest that 85% of the total matter budget in the Universe is of unknown origin. This component of matter known as dark matter is non baryonic and could consist of yet undiscovered particles which privileged candidates are arguably massive particles with electroweak couplings with ordinary matter (WIMPs).Dark matter particles may annihilate into Standard Model particles in dense regions of the Universe. Among the annihilation products are photons which detection at high energy with ground-based Cherenkov telescopes could bring unique information on the nature of the dark matter.H.E.S.S. observes dark-matter-dense regions of the sky such as the Galactic Center and dwarf galaxy satellites of the Milky Way. A study on the interpretation of an excess of gamma-rays detected by H.E.S.S. at the Galactic Center in terms of acceleration of protons by a population of unresolved millisecond pulsars is performed.10 years of observations of the Galactic Center with the four-telescope H.E.S.S.-I array, five years of data taking towards the Galactic Center region with the full H.E.S.S.-II array and a two-years dataset towards newly discovered dwarf spheroidal galaxies are analyzed. The search for dark matter annihilation signals towards these targets provided the strongest limits so far on dark matter annihilation cross section in gamma rays of TeV energies. The potential of dark matter detection with the upcoming Cherenkov Telescope Array (CTA) towards the inner Galactic halo are studied. They may annihilate into Standard Model particles in dense regions of the Universe. Among the annihilation products are high energy photons. The detection of these photons with ground-based Cherenkov telescopes may reveal the nature of the dark matter. H.E.S.S. have observed some dark-matter-dense regions of the sky likethe Galactic Center and dwarf galaxies satellites of the Milky Way. In this work 10 years of observations of the Galactic Center with the four-telescopes H.E.S.S.-I array, five years of data taking towards the Galactic Center region with the full H.E.S.S.-II array and a two-years dataset towards newly discovered dwarf spheroidal galaxies are analyzed. The searches for dark matter annihilation signals towards these targets produced the strongest limits so far on dark matter annihilation cross section in gamma rays of TeV energies.Perspectives of dark matter detection with the future array CTA (Cherenkov Telescope Array) towards the inner Galactic halo are also discussed. A study on the interpretation of an excess of gamma-rays detected by H.E.S.S. at the Galactic Center in terms of acceleration of protons by a population of unresolved millisecond pulsars complements the dark matter searches.

In this thesis I test the modified Newtonian dynamics as an alternative to the cold dark matter hypothesis. In the Milky Way, I show that the dynamics of the dwarf galaxies are well described by the paradigm and I confirm its distant low surface brightness globular clusters provide a strong test, for which I make predictions. Through analysis of a sample of 26 X-ray bright galaxy groups and clusters I demonstrate that the three active neutrinos and their anti-particles are insufficient to reconcile modified Newtonian dynamics with the observed temperatures of the X-ray emitting gas, nor with weak-lensing measurements, in particular for the bullet cluster. To this end, I propose an 11eV sterile neutrino to serendipitously resolve the residual mass problem in X-ray bright groups and clusters, as well as matching the angular power spectrum of the Cosmic Microwave Background. With this in mind, I show that the large collision velocity of the bullet cluster and the high number of colliding clusters is more naturally reproduced in MOND than in standard dynamics.