Lensing Convergence Maps Research Articles

WEAK LENSING MEASUREMENT OF GALAXY CLUSTERS IN THE CFHTLS-WIDE SURVEY

We present the first weak gravitational lensing analysis of the completed Canada-France-Hawaii Telescope Legacy Survey (CFHTLS). We study the 64 square degrees W1 field, the largest of the CFHTLS-Wide survey fields, and present the largest contiguous weak lensing convergence "mass map" yet made. 2.66 million galaxy shapes are measured, using a KSB pipeline verified against high-resolution Hubble Space Telescope imaging that covers part of the CFHTLS. Our i'-band measurements are also consistent with an analysis of independent r'-band imaging. The reconstructed lensing convergence map contains 301 peaks with signal-to-noise ratio {\nu}>3.5, consistent with predictions of a {\Lambda}CDM model. Of these peaks, 126 lie within 3.0' of a BCG identified from multicolor optical imaging in an independent, red sequence survey. We also identify 7 counterparts for massive clusters previously seen in X-ray emission within 6 square degrees XMM-LSS survey. With photometric redshift estimates for the source galaxies, we use a tomographic lensing method to fit the redshift and mass of each convergence peak. Matching these to the optical observations, we confirm 85 groups/clusters with \chi^2 reduced < 3.0, at a mean redshift <z_c> = 0.36 and velocity dispersion <\sigma_c> = 658.8 km/s. Future surveys, such as DES, LSST, KDUST and EUCLID, will be able to apply these same techniques to map clusters in much larger volumes and thus tightly constrain cosmological models.

From weak lensing to non-Gaussianity via Minkowski functionals

We present a new harmonic-domain approach for extracting morphological information, in the form of Minkowski Functionals (MFs), from weak lensing (WL) convergence maps. Using a perturbative expansion of the MFs, which is expected to be valid for the range of angular scales probed by most current weak-lensing surveys, we show that the study of three generalized skewness parameters is equivalent to the study of the three MFs defined in two dimensions. We then extend these skewness parameters to three associated skew-spectra which carry more information about the convergence bispectrum than their one-point counterparts. We discuss various issues such as noise and incomplete sky coverage in the context of estimation of these skew-spectra from realistic data. Our technique provides an alternative to the pixel-space approaches typically used in the estimation of MFs, and it can be particularly useful in the presence of masks with non-trivial topology. Analytical modeling of weak lensing statistics relies on an accurate modeling of the statistics of underlying density distribution. We apply three different formalisms to model the underlying dark-matter bispectrum: the hierarchical ansatz, halo model and a fitting function based on numerical simulations; MFs resulting from each of these formalisms are computed and compared. We investigate the extent to witch late-time gravity-induced non-Gaussianity (to which weak lensing is primarily sensitive) can be separated from primordial non-Gaussianity and how this separation depends on source redshift and angular scale.

Comparison of halo detection from noisy weak lensing convergence maps with Gaussian smoothing and MRLens treatment

Taking into account the noise from intrinsic ellipticities of source galaxies, we study the efficiency and completeness of halo detections from weak lensing convergence maps. Particularly, with numerical simulations, we compare the Gaussian filter with the so called MRLens treatment based on the modification of the Maximum Entropy Method. For a pure noise field without lensing signals, a Gaussian smoothing results in a residual noise field that is approximately Gaussian in terms of statistics if a large enough number of galaxies are included in the smoothing window. On the other hand, the noise field after the MRLens treatment is significantly non-Gaussian, resulting in complications in characterizing the noise effects. Considering weak-lensing cluster detections, although the MRLens treatment effectively deletes false peaks arising from noise, it removes the real peaks heavily due to its inability to distinguish real signals with relatively low amplitudes from noise in its restoration process. The higher the noise level is, the larger the removal effects are for the real peaks. For a survey with a source density ng ∼ 30 arcmin−2, the number of peaks found in an area of 3 × 3 deg2 after MRLens filtering is only ∼ 50 for the detection threshold κ = 0.02, while the number of halos with M > 5 × 1013 M⊙ and with redshift z ⩽ 2 in the same area is expected to be ∼ 530. For the Gaussian smoothing treatment, the number of detections is ∼ 260, much larger than that of the MRLens. The Gaussianity of the noise statistics in the Gaussian smoothing case adds further advantages for this method to circumvent the problem of the relatively low efficiency in weak-lensing cluster detections. Therefore, in studies aiming to construct large cluster samples from weak-lensing surveys, the Gaussian smoothing method performs significantly better than the MRLens treatment.

A Subaru Weak‐Lensing Survey. I. Cluster Candidates and Spectroscopic Verification

We present the results of an ongoing weak-lensing survey conducted with the Subaru Telescope whose initial goal is to locate and study the distribution of shear-selected structures, or halos. Using a Suprime-Cam imaging survey spanning 21.82 deg2, we present a catalog of 100 candidate halos located from lensing-convergence maps. Our sample is reliably drawn from that subset of our survey area (totaling 16.72 deg2) uncontaminated by bright stars and edge effects and is limited at a convergence signal-to-noise ratio of 3.69. To validate the sample, detailed spectroscopic measures have been made for 26 candidates using the Subaru multiobject spectrograph, FOCAS. All are confirmed as clusters of galaxies, but two arise as the superposition of multiple clusters viewed along the line of sight. Including data available in the literature and an ongoing Keck spectroscopic campaign, a total of 41 halos now have reliable redshifts. For one of our survey fields, the XMM-LSS field, we compare our lensing-selected halo catalog with its X-ray equivalent. Of 15 halos detected in the XMM-LSS field, 10 match published X-ray-selected clusters, and a further two are newly detected and spectroscopically confirmed in this work. Although three halos have not yet been confirmed, the high success rate within the XMM-LSS field (12/15) confirms that weak lensing provides a reliable method for constructing cluster catalogs, irrespective of the nature of the constituent galaxies or the intracluster medium.

Intrinsic Alignments of Galaxies and Their Effects on Weak‐Lensing Detections of Mass Concentrations

In this paper we investigate the influence of the intrinsic alignment of background galaxies on weak lensing detections of mass concentrations. Specifically, we analyze the number counts of false peaks resulting from intrinsic ellipticities in lensing convergence maps. Including the alignment of source galaxies, the full noise variance from intrinsic ellipticites in convergence $\kappa$-maps can be written as $\sigma^2_0=\sigma^2_{0ran}+\sigma^2_{0corr}$, where $\sigma^2_{0ran}$ is the noise contributed from randomly oriented source galaxies and $\sigma^2_{0corr}$ denotes the additional noise from intrinsic alignments. However, it is observationally difficult to measure $\sigma^2_{0corr}$ and usually only $\sigma^2_{0ran}$ can be estimated in weak lensing observations. Thus the observational signal-to-noise ratio is often defined with respect to $\sigma_{0ran}$, which is denoted as $\nu_{ran}$ in this paper. The true signal-to-noise ratio $\nu$ in terms of $\sigma_0$ is then $\nu=\nu_{ran}/(1+\sigma^2_{0corr}/\sigma^2_{0ran})^{1/2}$. Given a detection threshold on $\nu_{ran}$, a larger value of $\sigma^2_{0corr}/\sigma^2_{0ran}$ leads to a lower threshold on $\nu$ and therefore a larger expected number of false peaks. With $\sigma^2_{0corr}/\sigma^2_{0ran}\sim 10%$, the average number of false peaks with $\nu_{ran}\ge 3.5$ nearly doubles compared to that without considering the alignment, and for $\nu_{ran}\ge 5$, the number is tripled. As a result, the efficiency of weak lensing cluster detection degrades significantly. (Abridged)