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Gravitationally Lensed Quasars: Light Curves, Observational Constraints, Modeling and the Hubble Constant

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Gravitationally Lensed Quasars: Light Curves, Observational Constraints, Modeling and the Hubble Constant Universit´ede Li`ege Facult´edes Sciences D´epartement d’Astrophysique, de G´eophysique et d’Oc´eanographie Gravitationally lensed quasars: light curves, observational constraints, modeling and the Hubble constant Artist view of gravitational distortions caused by a hypothetical black hole in front of the Large Magellanic Cloud. Credit: this file is licensed under the Creative Commons Attribution ShareAlike 2.5 license. Virginie Chantry Research Fellow, Belgian National Fund for Scientific Research (FNRS) - December 2009 - Astrophysique et Dissertation realized for the Traitement de l’image acquisition of the grade of Prof. Pierre Magain Doctor of Philosophy in Space Science Supervisor: Pr. dr. Pierre Magain Members of the thesis commitee: Pr. dr. Pierre Magain Dr. Fr´ed´eric Courbin Pr. dr. Hans Van Winckel Jury composed of: Pr. dr. Jean-Claude G´erard as president Pr. dr. Pierre Magain Dr. Fr´ed´eric Courbin Pr. dr. Hans Van Winckel Pr. dr. Jean Surdej Dr. Damien Hutsem´ekers Dr. C´ecile Faure Dr. G´eraldine Letawe Copyright © 2009 V. Chantry To dreamers. May their wish come true. We should do astronomy because it is beautiful and because it is fun. We should do it because people want to know. We want to know our place in the universe and how things happen. John N. Bahcall (1934 - 2005) Abstract The central topic of this thesis is gravitational lensing, a phenomenon that occurs when light rays from a background source pass near a massive object located on the line of sight and are deflected. It is one of the most wonderful observational fact in favour of the General Theory of Relativity (Einstein, 1916). This phenomenon constitutes a powerful tool to probe different areas in astrophysics, including cosmology, which is our main interest. In particular we study gravitationally lensed quasars. Refsdal (1964) was the first to state that time delays between different lensed images of the same object, if this one is intrisically variable, can lead to the measurement of the Hubble constant H0, which is related the actual expansion rate of the Universe. Up to now, only a few lensed quasars have led to H0 and the precision on it has never reached the one obtained with other methods as the ones based on the Cosmic Distance Ladder. That is why some scientists from around the globe decided to unite their force to measure H0 from about thirty lensed quasars. To reach that goal, these objects are being monitored with some mid-sized ground-based telescopes located in both hemispheres. This thesis is realised in the framework of this collaboration called COSMOGRAIL for COSmological MOnitoring of GRAvItational Lenses. This work focuses on image processing and on several steps mandatory to obtain a measurement of H0 from lensed quasars: the acquisiton of the light curves from which it is possible to extract the time delays and the acquisition of the observational constraints necessary to model the gravitational potential responsible for the observed configura- tion. The central technique of this work is the image deconvolution with the MCS algorithm (Magain, Courbin & Sohy, 1998). The main principle of this algorithm is the non-violation of the sampling theorem in trying to obtain a better resolution in the deconvolved frame instead of an infinite one. The final resolution in the deconvolved frame is chosen by the user and as it is known, every image is decomposed in a contri- bution from the point sources and another one from all the extended structures such as arcs, rings and galaxies. To obtain good light curves from data coming from several telescopes, good reduction procedures are needed. That is why Vuissoz (2008) developed a semi-automated reduc- tion pipeline including deconvolution with the MCS algorithm. In the framework of the i ii Abstract present thesis, we adapt it to one of the telescopes used by the collaboration whose data were never used before, i.e. the Mercator telescope. We also bring some modifications to this pipeline, e.g. concerning the estimation of the error on the magnitudes of the light curves. We apply this revised version of the reduction pipeline to HE 0435-1223, a quadruply imaged quasar with already measured time delays (Kochanek et al., 2006). Another object, the quad WFI J2026-4536, is then investigated: we obtain light curves for each of the four lensed images. Thanks to the CASTLES project (Cfa-Arizona Space Telescope LEns Survey1), many lensed quasars have been observed with the camera 2 of NICMOS (Near Infrared Camera and Multi-Object Spectrometer) on board the Hubble Space Telescope. With these high resolution images, we can obtain very accurate constraints on the geometry of the lensed systems. But most of the time no star is available in the field of view to obtain a good Point Spread Function (PSF). That is why we develop an iterative strategy combined with the MCS algorithm: we call it ISMCS. This technique allows to use the lensed images themselves to improve the PSFs step by step while simultaneously deconvolving the frame to obtain better estimations of the extended structures in the image. We first test this strategy on a quadruply imaged quasar, the Cloverleaf gravitational lens (H1413+117), and obtain relative positions precise to 1 milliarcsecond (mas). We then apply ISMCS to the quadruply imaged quasar WFI J2033-4723 in order to con- tribute to the estimate of the Hubble constant, as this object was monitored by our team. We then study a sample of seven lensed systems currently monitored by COSMOGRAIL and for which time delays have never been obtained. Here again, we obtain positional constraints with an accuracy of around 1 to 2 mas thanks to the application of ISMCS. We then model these systems with simple mass profiles for the main lens galaxy and obtain an estimation of the values of the time delays. Finally we apply ISMCS to a sample of eleven lensed quasars which already have measured time delays. When the delays have been remeasured by our team, in four cases until now, we also model the potential of the lens with simple mass profiles to estimate H0. 1http://www.cfa.harvard.edu/castles Pour tirer le meilleur parti des connaissances acquises, pour en ex- traire toute la richesse, il importe de ne pas s’y habituer trop vite, de se laisser le temps de la surprise et de l’´etonnement. Hubert Reeves (1932 - ) R´esum´e Le sujet central de cette th`ese est le ph´enom`ene de lentille gravitationnelle qui se pro- duit lorsque des rayons lumineux ´emis par une source d’arri`ere-plan passent `aproximit´e d’un objet massif situ´esur la ligne de vis´ee et sont d´evi´es. Il s’agit d’un des faits obser- vationnels les plus parlants en faveur de la th´eorie de la Relativit´eG´en´erale (Einstein, 1916). Ce ph´enom`ene constitue un outil puissant pour investiguer diff´erents domaines de l’astrophysique comme la Cosmologie, sujet qui nous int´eresse ici. En particulier nous ´etudions le ph´enom`ene de lentille gravitationnelle appliqu´eaux quasars. Refsdal (1964) fut le premier `a´enoncer et prouver que les d´elais temporels entre diff´erentes images- lentilles du mˆeme objet, si ce dernier est intrins`equement variable, peuvent mener `a la mesure de la constante de Hubble H0, param`etre reli´eau taux actuel d’expansion de l’Univers. Jusqu’`amaintenant, seulement quelques quasars subissant le ph´enom`ene de lentille gravitationnelle ont men´e`a H0 et la pr´ecision obtenue n’a jamais atteint celle d’autres m´ethodes bas´ees par exemple sur l’´echelle Cosmique des distances. C’est pourquoi certains scientifiques originaires de diff´erents pays ont d´ecid´ed’unir leurs forces pour mesurer H0 `apartir d’une trentaine de quasars subissant le ph´enom`ene de lentille gravitationnelle. Pour atteindre cet objectif, ces quasars sont actuellement l’objet d’un suivi photom´etrique grˆace `aplusieurs t´elescopes de taille moyenne situ´es dans les deux h´emisph`eres. Cette th`ese est r´ealis´ee dans le cadre de cette collaboration qui se nomme COSMOGRAIL (COSmological MOnitoring of GRAvItational Lenses). Ce travail se concentre sur les techniques de traitement d’images et sur plusieurs ´etapes obligatoires pour la mesure de la constante de Hubble `apartir des quasars subissant le ph´enom`ene de lentille gravitationnelle: l’obtention des courbes de lumi`ere desquelles il est possible d’extraire les d´elais temporels et l’obtention de contraintes g´eom´etriques n´ecessaires pour mod´eliser le potentiel gravitationnel responsable de la configuration observ´ee. La technique au centre de ce travail est la d´econvolution d’images avec l’algorithme MCS (Magain et al., 1998). Le principe de cet algorithme est la non-violation du th´eor`eme de l’´echantillonnage en essayant d’obtenir une meilleure r´esolution dans l’image d´econvolu´ee et non une r´esolution infinie. La r´esolution finale dans l’image d´econvolu´ee est choisie par l’utilisateur et, comme elle est connue, chaque image est d´ecompos´ee en une contribution des sources ponctuelles et une autre des iii iv R´esum´e structures ´etendues comme les arcs, anneaux et autres galaxies lentilles. Pour obtenir de bonnes courbes de lumi`ere `apartir de donn´ees provenant de diff´erents t´elescopes, de bonnes m´ethodes de r´eduction sont n´ecessaires. C’est pourquoi Vuissoz (2008) a d´evelopp´eun pipeline de r´eduction semi-automatique incluant la d´econvolution avec l’algorithme MCS. Nous l’adaptons, dans le cadre de cette th`ese, `al’un des t´elescopes utilis´es par la collaboration, le t´elescope Mercator, dont les donn´ees n’ont encore jamais ´et´eutilis´ees auparavant.
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