S.L / 2013 Coordination de la Formation par la Recherche
Sujet de Thèse CEA "SUJET-LABO 2013" Référence du dossier : Pôle :
DSM
N° :
SL-DSM-13-0021
1 - Laboratoire d'accueil au CEA Centre :Saclay Département/Service :IRFU / Service d'Astrophysique
Nom du laboratoire :LCEG/Laboratoire de Cosmologie et d'Evolution des Galaxies
2 - Titre du sujet de thèse L'effet de lentille gravitationelle faible par des structures massive et non-Gaussiennes
3 - Thématique de Recherche Physique corpusculaire et cosmos / Astrophysique 4 - Pièce jointe Y a t-il une pièce jointe associée ?
Non
Intitulé de la pièce jointe :
Date d'édition
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5 - Résumé L'effet de lentille gravitationnelle faible (weak gravitational lensing) est une des sondes cosmologiques les plus importantes pour élucider l'origine de l'expansion accélérée de l'Univers et la nature de la matière noire. Jusqu'à aujourd'hui, la plupart des observations ont été faites avec des statistiques Gaussiennes, d'ordre deux. Toutefois, il y a beaucoup d'information cosmologique dans des observables non-Gaussiennes. On peut accéder à cette information en regardant les maximums (peaks) dans les cartes de weak-lensing, qui sont créés par des structures massives comme des halos. Des prédictions théoriques pour le nombre et la distribution des peaks n'ont pas encore été faites avec succès. Cette thèse aura pour but de prédire le signal de peaks dans les données de weak lensing. Des simulations rapides vont être créées pour effectuer cette prédiction. Elles vont être comparées avec des simulations à N corps. En utilisant des outils statistiques avancées, on étudiera des méthodes optimisées pour extraire le les peaks des données lensing, pour contraindre des paramètres cosmologiques. La méthode finale va être appliquée à des observations de CFHTLenS. Avec une couverture du ciel de 154 degrés carré et un redshift moyen de 0.8, ce grand relevé représente le volume cosmique le plus large utilisé en weak lensing.
6 - Exposé du sujet As predicted by Einstein’s Theory of General Relativity, light is influenced by the gravitational field of massive objects. This also happens on cosmic scales: Light emitted by distant galaxies propagating through the Universe towards us is being deflected continuously along its path, due to the inhomogeneous large-scale structure (LSS) or cosmic web. These deflections cause the images of galaxies to be distorted. These distortions are very small, hence weak lensing, and cannot be detected from individual galaxies. However, because the light from galaxies that are nearby on the sky travels through the same structures, it experiences a coherent deflection, causing the observed shapes of these galaxies to be correlated. These shape correlations can be measured statistically, by averaging over a large number of galaxies. The strength of this correlation is larger the more inhomogeneous and clumpy the cosmic web is. By studying this correlation in detail, we can infer many properties about the cosmic web. Over the last few years, weak cosmological lensing has been very successful to constrain cosmological parameters (e.g. Fu et al. 2008, Schrabback et al. 2008, Kilbinger et al. 2012). Up to now, weak-lensing observations have mainly used second-order statistics, based on pair-wise correlations of galaxy shapes. The use of second-order functions only is a severe limitation for thecosmological exploitation of lensing: There is a wealth of informationcontained in observables beyond second-order, in particular about the non-Gaussianity of the LSS. Together with second-order statistics, non-Gaussian quantities can break the degeneracy between cosmological parameters, thereby increasing the constraining power of weak lensing by a factor of many (Kilbinger & Schneider 2005). Moreover, qualitatively different physics is probed by higher-order statistics, in particular, primordial non-Gaussianity. One very promising probe of the non-Gaussianity of the cosmic web are weak-lensing peak counts. Projected overdensities in the cosmic web manifest themselves by a tangential alignment pattern of lensed background galaxies. Peaks in weak-lensing data can therefore be found by averaging over galaxy Date d'édition
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shapes in circular regions on the sky. Peaks trace the high-density regions and are a very effective measure of the non-Gaussianity of the cosmic web. Peak counts are a first-order measure, avoiding the measurement of correlations between noisy galaxy shapes. Despite this observational simplicity, weak-lensing peak counts have the potential to be a powerful cosmological probe (Pires, Leonhard & Starck 2012). Up to now, no theoretical model or prediction of weak-lensing peak counts has been accomplished, which can account the non-linear, high-density structures. This thesis will tackle this outstanding problem of weak lensing. Simple and fast numerical simulations, informed by analytical models of massive structures such as halos, will be created to provide predictions of weak-lensing peaks counts. halos are simply drawn from a theoretical halo mass function to generate a sample of halos with the correct distribution of masses and redshifts. For given survey characteristics the weak gravitational shear is calculated for each simulated background galaxy by adding up the contribution of all halos along the line of sight, assuming a halo density profile, e.g. NFW. The resulting peak counts can be directly compared to observations. These simulations will be very fast, since they do not rely on time-consuming N-body calculations. The can be generated on the fly during a Monte-Carlo sampling process for each probed cosmology. First, these simulations have to be compared to full N-body simulations, to verify that randomly positioned halos produce the same number of peaks than the full cosmic web. If this is not the case, those simulations have to be refined, for example by taking into account the clustering of halos. Large N-body simulations are available within the CosmoStat lab at SAp, and from the XXL collaboration (PI: Marguerite Pierre), of which the thesis director is a member. Next, the extraction of peaks from weak-lensing data will be optimized. Statistical methods involving for example wavelet filtering and de-noising will be employed, using the expertise in this area of members of the CosmoStat lab (Jean-Luc Starck, Sandrine Pires, Florent Sureau). With Monte-Carlo sampling techniques, the prediction can be compared to observations, and cosmological parameters be constrained. The thesis director is collaborating on this and similar subjects with scientists at Ile-de-France (Karim Benabed, IAP; Christian Robert, Dauphine; Gersende Fort, ParisTech). Finally, the method will be applied to data from CFHTLenS (Canada-France-Hawaii Lensing Survey, Heymans et al. 2012). This survey spans 154 square degrees in five optical bands. It provides state-of-the-art measurements of shapes and photometric redshifts for over 4 million galaxies. The thesis director is a founding member of CFHTLenS. The data is reduced, the lensing catalogs are available and ready to use. The weak-lensing peaks can be compared to other tracers of massive structures, such as matched filters using galaxy overdensities (Milkeraitis et al. 2010) or X-ray observations from XXL. References: Fu et al., 2008, A&A, 479, 9 Kilbinger et al., 2012, MNRAS submitted Kilbinger, Schneider, 2005, A&A, 442, 69 Milkeraitis et al., 2010, MNRAS, 406, 673 Pires, Leonard & Starck, 2010, MNRAS, 423, 983 Schrabback et al., 2010, A&A, 516, A63
7 - Collaborations (éventuelles) prévues 8 - Partenariat(s) industriels prévu(s) (éventuellement) Date d'édition
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9 - Correspondant chargé du suivi de la thèse au CEA Nom: SAUVAGE
Prénom:
Marc
Adresse : Orme des Merisiers Batiment 709 CEA/SAp F-91191 Gif-sur-Yvette Téléphone
01 69 08 62 99
@mail:
[email protected]
Habilitation à diriger des recherches :
Oui
Organisme de rattachement : CEA Combien de thèses avez-vous déjà
3
Combien de doctorants encadrerez-vous durant l'année universitaire 2013/2014 ? 0
10 - Directeur de thèse Nom: Kilbinger
Prénom:
Martin
Adresse : Orme des Merisiers Bât 709 CEA/SAp 91191 Gif-sur-Yvette Téléphone:
01 69 08 17 53
@mail:
[email protected]
Habilitation à diriger des recherches :
En cours
Organisme de rattachement : CEA Combien de thèses avez-vous déjà encadrées
0
Combien de doctorants encadrerez-vous durant l'année universitaire 2013/2014 ?
Date d'édition
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0
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11 - Signatures : Correspondant chargé du suivi de la thèse au CEA
Date :
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Signature :
Marc SAUVAGE
Date :
Directeur de Thèse (lorsqu'il est identifié) Martin Kilbinger
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Signature :
Chef de Département CEA (ou son représentant)
Date :
/_._/_._/_._._._/
Signature :
Philippe CHOMAZ Directeur du Pôle CEA (ou son représentant)
Date :
/_._/_._/_._._._/
Signature :
Gabriele FIONI 12 - Avis du Responsable de l'Ecole Doctorale :
Astronomie et Astrophysique d'Île de France - Observatoire Paris -
Date :
Nom du Responsable :
/_._/_._/_._._._/
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Avis :
Favorable
Défavorable
Avis circonstancié :
Date d'édition
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