Henk Hoekstra

Research Summary

Henk Hoekstra studies the distribution of dark matter in the universe through its weak gravitational lensing effect. The lensing signal induced by large scale structure can be used to constrain cosmological parameters, whereas the signal around galaxies provides a unique way to study their dark matter halos. Hoekstra uses data from various large surveys, such as the Red-Sequence Cluster Survey, VIRMOS-DESCART survey and the CFHT Legacy Survey. Recently, Hoekstra has started a project to search for binary stars in wide field imaging data.

Research Projects:
(September 2002 - August 2003)

Weak lensing constraints on galaxy dark matter halos

Henk Hoekstra, Howard Yee (Toronto), and Mike Gladders (Toronto) continued their weak lensing analysis of the imaging data from the Red-Sequence Cluster Survey (RCS). A maximum likelihood deconvolution of the galaxy-mass cross-correlation function allowed them to constrain the sizes of dark matter halos around galaxies. In addition, they were able to measure the (projected) shapes of the halos. These measurements also provide strong constraints on alternative theories of gravity (without dark matter): modified newtonian dynamics (MOND) is inconsistent with these findings.

Effects of Large Scale Structure on Cluster Mass Profiles

Hoekstra continued work on the effect of distant large scale structure on weak lensing measurements of galaxy clusters. In particular he focussed on the constraints that can be placed on the cluster mass profiles. Hoekstra found that the large scale structure increases the uncertainties in the measurements by about a factor two.

Systematics in cosmic shear surveys

Hoekstra also estimated the effect of imperfections in the modeling of the spatial variation in the PSF anisotropy on cosmic shear measurements. He found that a good parameterisation, based on dense star fields can significantly reduce systematics. These findings turned out to be very relevant for the VIRMOS-DESCART survey.

Reanalysis of the VIRMOS-DESCART survey

With Ludo van Waerbeke, Hoekstra reanalysed the latter data set, which resulted in a significant improvement over previously published results. The new measurements appear free of systematics, which is a major advance in this area of cosmology.

Combined weak lensing and CMB analysis

With Carlo Contaldi and Antony Lewis, Hoekstra combined CMB measurements from WMAP with the RCS lensing results, to obtain some of the best constraints on cosmological parameters to date.

Statistics of giant arcs

With Gladders, Yee, Hall and Barrientos, Hoekstra studies the incidence of giant arcs around galaxy clusters. On the basis of their data they conclude that a subset of clusters is responsible for most of the observed lensing.

Algorithm to detect blended stars

With Yanqin Wu, Hoekstra started a project to examine whether some OGLE planet transit candidates might be blends of bright stars with a fainter eclipsing binary system. The developed technique can also be used to find binary stars in the field.

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