Clusters Featured Project: Shear Measurement Challenge

Gravitational Lensing from a Simulated Cluster of Galaxies
Caption: 

Gravitational Lensing from a Simulated Cluster of Galaxies

(Authors: Ian Dell'Antonio, Douglas Clowe)

The evolution of the mass function of clusters of galaxies is a powerful probe of dark energy. Use of this probe requires accurate and unbiased mass measurements for a large sample of clusters at each redshift to overcome the statistical uncertainty in the mass measurements. LSST will provide the necessary sample; however, we are already in the regime where systematic uncertainties limit the cosmological constraints from clusters (Applegate et al. 2012). One of the two largest uncertainties arises from the weak lensing mass calibration for these clusters, and specifically on the method for converting the measurement of the galaxy shapes into a gravitational lensing measurement of the galaxy mass. Traditionally, weak gravitational measurements have used the galaxy ellipticities to estimate the shear; in galaxy images this fails because the shear is not completely described by the ellipticity. Over the past decade, great progress has been made in validating codes for extracting the shear signal in the weak gravitational lensing limit, via the STEP and GREAT challenges (Heymans et al. 2006, Massey et al. 2007, Bridle et al. 2009). These tests, however, have only probed the weak (reduced shear g<<0.05) limit of weak lensing. Up to now, an equally systematic treatment of the cluster regime has been missing.

To remedy this shortfall, we have produced a set of simulations that serve as a backbone to a data challenge to test cluster analysis codes from research groups seeking to use cluster mass measurements as a cosmological probe. In the spirit of the GREAT challenges for weak lensing, we aim to produce a sequence of tests that break down the complex lensing signal from clusters, allowing us to determine where the sources of bias in the codes are. 

For the first iteration of the simulations (with release date of Aptil 8th, 2014):

  1.    Weak lensing effects (shear, flexion, etc) are created using sub-pixel deflection and amplification across the source galaxy.
  2.    Source galaxies are taken from high-resolution HST images, providing more realistic background galaxies than smooth exponential disks or Sersic profiles.
  3.    All background galaxies have 90 degree rotated images (pre-lensing), allowing for the cancellation of most of the intrinsic shape noise in shear measurements.
  4.    All source galaxies are placed in the same redshift plane.
  5.    No cluster galaxy or foreground population is included.
  6.    All clusters are modeled as circular NFW halos without substructure or line-of-sight projected structures.
  7.    All galaxies are convolved with the expected PSF from LSST (modeled by PhoSim), as well as actual PSFs from the Dark Energy Camera on the Blanco telescope and Suprimecam on the Subaru telescope.

In later iterations of the simulations, we will spread the background galaxies over a range of redshifts and use clusters with substructure and line-of-sight projections based on cosmological N-body simulations.

This first simulation contains two independent datasets:  

  • The first dataset consists of 200,000 galaxies arranged onto a grid, similar to the GREAT set of challenges.  In each image, all the galaxies have an identical reduced shear applied, although the applied shear is allowed to vary between images.  
  • The second dataset consists of lensing in a 2 Mpc x 2 Mpc region around 160 different clusters, resulting in a large range of shears within each image.  Galaxy densities are reduced in order to minimize problems with galaxy blending, but some judgement on which galaxies to reject due to blending will still be needed.

The images, as well as more information and instructions on how to enter your analysis codes into the competition are available at http://www.het.brown.edu/people/ian/ClustersChallenge/index.html