Terapix Star Formation Region IC 1396, © 2002 CFHT
DESCART : Weak Lensing and Cosmology
Last content update March 9th, 2007

Dark matter from Ellipticity Sources CARTography

The DESCART project aims at using gravitational lensing effects to describe the properties of large scale structure in the universe and the cosmic history of structure formation. The weak gravitational shear produced by the dark matter distribution on cosmological scales generates a distortion field that manifests by an increase of galaxy ellipticities. Galaxies that experience the same gravitational distortion then tend to get similar shape and orientation, producing coherent pattern of projected galaxy ellipticities. The statistical analysis of this "cosmological weak shear" signal (cosmic shear) can then reveal the dark matter and dark energy properties in the universe as well as its global geometrical properties.

The amplitude of cosmological weak lensing signal is so tiny that its detection relies on accurate measurement of millions of galaxy shapes that only best high image quality wide field imaging surveys can provide. The DESCART project uses several imaging surveys carried out at ESO with the VLT (the VLT-Descart survey) and with the future VST (The KIDS survey), at CFHT with the CFH12k (The VIRMOS-Decart survey) and with Megacam (The CFHTLS Cosmic Shear Legacy Survey) and with the HST (the COSMOS survey). These projects are parts of international collaboration with other French, European, Canadian and American Institutes. The main goals are the following:

1. - The detection and statistical analysis of the weak distortion field produced by foreground mass distribution on background (lensed) galaxies as function of angular scale.

The primary scientific goals are the understanding of the growth mechanisms of large-scale structure, the properties of the dark matter power spectrum and its evolution with redshift. These observations should provide details on the amount of dark matter and dark energy in the universe as well as the equation of state of dark energy.

2. - The properties of the dark matter power spectrum at very small scale, in the non-linear regime, and the properties of small dark halos.

3. - The correlation between the mass maps and the galaxy distribution (light) to derive the bias properties and its evolution with redshift.

4. - At small angular scales galaxy/galaxy lensing effects will put constraints on the shape and size of galaxy halos.

5. - Finally, the mass maps derived from weak lensing can be correlated with the CMB temperature and polarization maps. The detection of such a correlation would put in particular interesting constraints on the dark energy equation of state.

Gravitational distortion maps over very large fields permit to explore the large-scale distribution of the (dark) matter in the universe without regard on the light (galaxy and gas) distribution. By using this technique, it is possible to study the cosmic mass fluctuations.

The gravitational shear, produced by the cumulative light deflexion of matter along line of sights, can be recovered from the gravitational distortion which increases the ellipticity of the galaxies. From these distortion patterns we can construct mass maps of the projected dark matter distribution, recover the power spectrum of the mass density fluctuations, put constrains on the cosmological parameters Omega and Lambda, and derive the evolution of the biasing factor with redshift and with angular scale. These goals can be achieved provided noises and systematics produced by the intrinsic ellipticity distribution of the galaxies can be minimized. The PSF anisotropy produced by the atmosphere and the instruments, the cosmic variance and the Poisson noise are the primary noise and systematic sources that cosmic shear surveys have to control carefully.

One of the main scientific goals of DESCART is a big cosmic shear survey with MegaPrime at CFHT. The MegaPrime project consists in a refurbishing of the CFHT prime-focus and the building of the world largest wide field CCD camera, MEGACAM. The camera is under the responsibility of DAPNIA/SAp (CEA Saclay). The image processing will be done at the TERAPIX center, located at IAP.

This project is part of a global investigation of the large-scale structures of the universe that includes several surveys, namely the VIRMOS instrument on the VLT and the XMM satellite for X-ray mapping clusters anf groups of galaxies.

The project develops the following tools in order to produce reliable results and to handle wide field surveys:
-  Pipelines for processing images from the WIFI, UH8K, CFH12K, MEGACAM and OMEGACAM wide field CCD cameras (TERAPIX)
-  Pipelines for gravitational shear analysis (object detection, shape measurement, PSF corrections, lensing and cosmological interpretation of distortion signal). Several DESCART and TERAPIX participants are part of the STEP initiative
-  Sets of simulations of cosmological large-scale structure formation that includes gravitational lensing effects and realistics wide fields images.

Subsections
section Gravitational lensing at IAP
section Cosmic shear surveys
section Slides : general , useful for any talks
section Lectures on lensing or cosmology
section Conference announcement and link
section Public conferences

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