Ludovic Van Waerbeke


Relevant Degree Programs


Graduate Student Supervision

Doctoral Student Supervision (Jan 2008 - April 2022)
Probing the universe with multiple large-scale structure tracers (2021)

Different large-scale structure(LSS) tracers bear rich information about our Universe. In this dissertation, I present my studies on galaxy clusters and multi-tracer cross-correlations to highlight the potential and importance of combining multiple LSS tracers in studying our Universe. I use simulated clusters from the BAHAMAS simulation to study the off-centring effect. I define seven observational-motivated centroids from stars, as well as X-ray and thermal Sunyaev-Zeldovich (tSZ) effect data of these clusters. I find that stacked, mis-centred density profiles yield highly biased shape and size parameters. I also quantify and model the offset distributions between these centroids and the `true' centre of these clusters. The fitting is useful for future measurements of stacked density profiles.With the same set of simulated clusters, I evaluate the ability of Convolutional Neural Networks (CNNs) to measure galaxy cluster masses from cluster images. I independently train four separate networks with images of the four tracers mentioned above. I also train a `multi-channel' CNN that predicts mass from all these four tracers. For the clusters that have masses in the range $10^{13.25}\mathrm{M}_{\odot}
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Weak gravitational lensing cross-correlations (2017)

The matter content of the Universe is dominated by dark matter. Beyond its abundance and its lack of non-gravitational interactions with standard model matter, little is known about the nature of dark matter. This thesis attempts to illuminate different aspects of dark matter by using gravitational lensing in conjunction with other cosmological probes. Gravitational lensing describes the deflection of light by gravitational potentials and is a direct and unbiased probe of the matter distribution in the Universe.We investigate the weakly interacting massive particle (WIMP) model of dark matter by performing a tomographic and spectral cross-correlation between weak gravitational lensing from the Canada-France-Hawaii Telescope Lensing Survey (CFHTLenS), Red Cluster Sequence Lensing Survey (RCSLenS), and Kilo-Degree Survey (KiDS), and gamma rays from Fermi-LAT. The non-detection of a correlation allows us to constrain the allowed masses, annihilation cross-sections, and decay rates of WIMP dark matter.Even though most matter in the Universe is dark matter, about 16% is baryonic matter. To make precision measurements of the dark matter distribution with gravitational lensing, it is therefore necessary to understand the behaviour and distribution of baryonic matter. We measure and analyse the cross-correlation between weak gravitational lensing from RCSLenS and the thermal Sunyaev-Zeldovich (tSZ) effect from the Planck satellite to constrain the effect of baryon physics on the matter distribution.Models of gravitational lensing often make use of a range of approximations. We study the effect of dropping these approximations on the cross-correlation between gravitational lensing and tSZ by performing a detailed calculation up to fourth order in the gravitational potential. We find that the common approximations are sufficiently accurate even for future surveys.Finally, we explore the growth of structure and the effect of residual weak lensing systematics in a tomographic cross-correlation between weak gravitational lensing from KiDS and lensing of the cosmic microwave background (CMB) from Planck.

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Galaxy cluster studies with weak lensing magnification & shear (2015)

Clusters of galaxies offer a unique window for studying the Universe on the largest scales. As the most massive gravitationally bound systems to have formed, they serve as probes of the large-scale distributions of dark matter, the underlying cosmology, and the complicated intracluster physics that characterizes the evolution of these massive systems. Gravitational lensing is the deflection of light coming from distant sources, by gravitational potentials along its path. Being sensitive to all mass regardless of type or dynamical state, lensing is a valuable tool for studying dark matter and characterizing galaxy clusters. In the weak lensing regime, the very slight apparent distortion of galaxy shapes is referred to as the shear, while the focusing and amplification of light is referred to as the magnification. The former has become a well-developed and robust technique in astronomy over the past decade, but the latter has been largely overlooked until now. The work embodied in this thesis includes the first-ever significant detection of magnification by galaxy groups, and the first comparison between masses measured with weak lensing magnification and shear (Chapter 2). This is followed by an application to an enormous sample of galaxy clusters, yielding ground-breaking signal-to-noise for magnification and an analysis of redshift-dependent systematic effects. This project also provides measurements of the cluster mass-richness scaling relation, and is a milestone in moving from magnification detection to useful science (Chapter 3). Finally, a comprehensive gravitational lensing shear analysis is performed on the previous cluster sample, allowing for a critical comparison between cluster masses measured with the independent techniques, as a function of both richness and redshift. These shear measurements also allow for important constraints on a new sample of galaxy clusters, including the distribution of cluster centroid offsets, the mass-richness relation, and cluster redshift evolution (Chapter 4). This thesis details unprecedented measurements using a new technique -- weak lensing magnification -- and comparisons with the well-studied shear approach. The final product exemplifies the promise of the new method for measuring galaxy cluster masses, and also points to likely issues that will need to be addressed in future experiments.

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A new approach to photometric redshift contamination, providing critical insight for weak lensing cosmology (2013)

Light travelling through the Universe is deflected by the presence of mass, this effect will distort the size and shape of observed galaxies. Weak gravitational lensing measures the amount of mass in the Universe by observing these subtle changes in the shapes of distant galaxies. In order to properly interpret the observed shapes of galaxies their distances must be accurately known, this information is encoded in the redshift distribution.A detailed spectroscopic observation is the most reliable way to measure the redshift of a galaxy. Unfortunately this is a time-intensive process and weak lensing surveys are composed of millions of galaxies many of which are too faint for spectroscopic observation. For this reason photometric redshifts are used. Photometric redshifts are less accurate than spectroscopic redshifts but are easier to obtain since they rely on only a few measurements over large ranges of wavelength.Thorough knowledge of uncertainties in the photometric redshifts is vital to weak lensing because photometric redshifts provide the distances necessary to understand the weak lensing signal. In this thesis we present a new technique to measure the reliability of photometric redshifts with the goal of improving the estimated redshift distribution for use in weak lensing studies. Mock observational surveys are used to test the technique before applying it to two surveys: the Deep component of the Canada-France-Hawaii Telescope Legacy Survey (CFHTLS-Deep) and the CFHT Lensing Survey (CFHTLenS). We demonstrate our ability to construct both the true redshift distribution and the true average redshift of galaxies in a given photometric redshift range. Furthermore, we show that the photometric redshiftprobability distribution function can be used as an accurate measure of the true redshift distributionwhen summed for an ensemble of galaxies.Using our tested redshift distribution we present cosmological constraints for CFHTLenS from a weak lensing analysis. We present constraints on cosmological parameters for a model of the Universewith dark energy and cold (non-interacting) dark matter (ΛCDM). We find that our weak lensing analy-sis, combined with other cosmological probes, improves the precision of these measurements by a factor of 1.5 to 2.

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Alternative Estimators for Weak Gravitational Lensing (2012)

Weak gravitational lensing provides a means to measure the total mass in the Universe. The incoming light from distant galaxies is disturbed by the inhomogeneity of the dark matter distribution along the line of sight.The correlations of shape in an observed galaxy population can be used to probe the total mass density fluctuations in the Universe. Studies of correlations between galaxy shapes have been the basis of weak lensing research.In this thesis we investigate various non-conventional weak lensing statistics that are complementary to the traditional two-point shear correlation functions. The goal is to constrain the matter density Ωm and normalization of matter power spectrum σ₈ parameters.These higher order statistics have long been advocated as a powerful tool to break measured degeneracies between cosmological parameters.Using ray-tracing simulations, which incorporate important survey features such as a realistic depth-dependent redshift distribution, we find that joint two- and three-point correlation function analysis is a much stronger probe of cosmology than the two-point analysis alone.We apply the higher order statistics technique to the 160 deg² of the Canada-France-Hawaii-Telescope Legacy Survey (CFHTLS) and show preliminary results from the joint two- and three-point likelihood analysis.We reveal the possibilities that lie in the projected mass probability distribution function to discriminate models with different values of the matter density parameter.In the process we develop a hybrid data set based on the simulations and the CFHTLenS data for systematics testing and covariance matrix estimations. Our error analysis includes all non-Gaussian terms, finding that the coupling between cosmic variance and shot noise is a non-negligible contribution.

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Optical galaxy cluster detection and weak gravitational lensing derived galaxy cluster mass functions: the development of a new algorithm and its application to the Millennium Simulation, CFHTLS-Deep, and CFHTLS-Wide surveys (2011)

Hierarchical structure formation and the λCDM model of the Universe reveal that large structures, such as galaxy clusters, form from the aggregation and merging of smaller structures over time. Exploring the morphologies and distribution of galaxy clusters enables the identification of processes that affect this growth, including the significant role of dark matter in structure formation. The particle constituting dark matter remains unknown and current experimental searches for it abound. Weak gravitational lensing (WL) is an observational astronomy technique that can successfully measure dark matter and thus is an important tool in its characterisation, and furthermore, in distinguishing between competing gravity theories.Using WL to measure galaxy cluster mass, including both baryonic matter and dark matter, across a range of redshifts, exposes details of cluster assembly processes. It is imperative to obtain a complete, pure and understood galaxy cluster sample for statistical, accurate investigations of the properties and evolution of all types of galaxy clusters, from which to draw unbiased conclusions about the evolution of the Universe. Cognisant of these factors, this thesis presents a new algorithm, 3D-MF, a three-dimensional matched-filter method, that enables searches for galaxy clusters in large, optical wavelength astronomical surveys. 3D-MF is used herein to successfully discover thousands of galaxy clusters in the Canada-France-Hawaii Telescope Legacy Survey (CFHTLS). Subsequently, theoretical mass profiles are used to characterise cluster candidate WL signals, providing mass or velocity dispersions.The CFHTLS-Deep and Wide galaxy cluster catalogues in this thesis represent a new and abundant source of galaxy cluster information, containing ~170 and ~92 galaxy clusters per square degree respectively (with σ > 3.5, and 0.2 ≤ z ≤ 1.0). WL shear catalogues were used to analyse > 15,000 CFHTLS-Wide 3D-MF galaxy clusters, and the average cluster stacks (for clusters 0.2 ≤ z ≤ 0.5) were found to cover a mass range of 10¹³M๏≲ M₂₀₀ ≲ 10¹⁴M๏, having 1D velocity dispersions of σν 200-500 km/s. 3D-MF finds higher redshift clusters down to a lower mass range than other methods; its successes have already led to its implementation in other surveys, as we progress toward a deeper understanding of the Universe.

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