The XMM-LSS survey I. Scientic motivations, design and rst results?

We have designed a medium deep large area X-ray survey with XMM { the XMM Large Scale Structure survey, XMM-LSS { with the scope of extending the cosmological tests attempted using ROSAT cluster samples to two redshift bins between 0 < z < 1 while maintaining the precision of earlier studies. Two main goals have constrained the survey design: the evolutionary study of the cluster-cluster correlation function and of the cluster number density. The adopted XMM observing conguration consists of an equatorial mosaic of 10 ks pointings, separated by 200 and covering 8 8 , giving a sensitivity of 3 10 15 erg cm2 s 1 in the (0.5-2) keV band. This will yield more than 800 clusters of galaxies and a sample of X-ray AGN with a space density of about 250 deg 2. We present the expected cosmological implications of the survey in the general context of CDM models and cluster evolution. The XMM-LSS survey is associated to several other major surveys, ranging from the UV to the radio wavebands which will provide the necessary resources for X-ray source identication and further statistical studies. In particular, the associated CFHTLS weak lensing and AMiBA Sunyaev-Zel'dovich surveys over the entire XMM-LSS area, will provide for the rst time a coherent study of the mass distribution and of cluster physics in the universe on scales of a few hundreds of Mpc. We describe the main characteristics of our wavelet based X-ray pipeline and source identication procedures, including the classication of the cluster candidates by means of a photometric redshift analysis. This permits the selection of suitable targets for spectroscopic follow-up. We present the preliminary results pertaining to the rst 15 XMM-LSS pointings : X-ray source properties, optical counterparts,highlights from the rst Magellan and VLT/FORS2 spectroscopic runs as well as preliminary results from the NIR search for z > 1 clusters. The results are promising and, so far, in accordance with our predictions as to the survey sensitivity and cluster number density. The feasibility of the programme is demonstrated and further X-ray coverage is awaited in order to proceed with a truly signican t statistical analysis.