SL / 2013 - Jean

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
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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)
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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 ?
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0
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11 - Signatures :
Correspondant chargé du suivi de la thèse au CEA
Date :
/_._/_._/_._._._/
Signature :
Marc SAUVAGE
Date :
Directeur de Thèse (lorsqu'il est identifié)
Martin Kilbinger
/_._/_._/_._._._/
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 :
/_._/_._/_._._._/
Signature :
Avis :
Favorable
Défavorable
Avis circonstancié :
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