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.