Gravitational Lensing

While being a graduate student, since 1995 I was involved in the then largest statistically complete gravitational lens survey, the Cosmic Lens All-Sky Survey (CLASS; 22 lenses), which ended in 2003. Among many things, I monitored radio lenses with the VLA and WSRT to measure their time-delays, as well as develop lens-modelling codes.

Around 2001, I and Tommaso Treu started the Lenses Structure & Dynamics (LSD) survey, while at Caltech, for the first time systematically combining gravitational lensing with stellar kinematic constraints to measure the mass distribution in massive galaxies.  Later, while back at the Kapteyn Institute, this resulted in the first and still only self-consistent lensing & dynamics modelling code ‘CAULDRON’, which we then applied to 2D kinematic data of lens galaxies from a  VIMOS Large Program on the VLT.

In 2003, I co-founded the Sloan Lens ACS Survey (SLACS), which is still the largest statistically complete lens survey (100+ lenses) with HST data, lens and source redshifts, as well as the lens velocity dispersions, known for all lenses. This survey set a benchmark for many current galaxy-scale lens surveys, yielding the first systematic studies of the internal dark-matter mass distribution in massive early-type galaxies beyond the local Universe.

In 2005 I developed a new method to ‘gravitationally image’ the internal mass distribution of strong lenses, in particular focussing on CDM substructure in lens galaxies; this method was further developed in to a fully Bayesian adaptive method, and forms the basis for many CDM substructure studies. This method recently spawned many new ‘free-form’ codes.

In parallel, we started the XLENS survey combining lensing, dynamics with stellar population studies, with as recent spin-off a novel grid-based Bayesian Composite Stellar Population IMF modelling code.

In parallel in 2010 I was founding member of the SHARP survey to follow-up know lenses with Keck Adaptive Optics and VLBI with very high spatial resolution. This survey is now starting to yield its first exciting results.

Most recently I’ve been involved in the Euclid Strong Lens Science Working Group, being a founding members, developing Neural Network method to find new lenses, and also develop new modelling codes, statistical methodologies to model lens perturbations.

I am currentl PI of the VST-KiDS strong lensing survey that has recently yielded its first lens systems.