----------------------------------------------------------------------- Proposal for a join MEGACAM/XMM/VIRMOS/ARCHEOPS survey on Weak Lensing ----------------------------------------------------------------------- PROPOSERS: PI: Yannick MELLIER (IAP, mellier@iap.fr) Co-I: CEA/SPhT Saclay: F. Bernardeau CEA/SAp Saclay: P. Didelon IAP Paris: E. Bertin F. Bouchet S. Colombi B. Fort R. Maoli Y. Mellier A. Thion IAS Orsay: N. Aghanim J.-L. Puget OMP Toulouse: J.-P. Kneib J.-F. Leborgne R. Pello Observatoire de Paris (DEMIRM) Fabienne Casoli Mireille Dantel-Fort LAS Marseille O. Le Fevre MPA Garching: T. Erben P. Schneider CITA Toronto: L. van Waerbeke CONTEXT: Joint observations of a wide field common to the VIRMOS, the XMM, the ARCHEOPS and the MEGACAM surveys will provide a unique dataset where the galaxies, the hot gas, and the dark matter could be analyzed simultaneously in a broad cosmological context. In this large programme, we propose a weak lensing survey over 100 square degrees, centered on common areas of the XMM (see Pierre's proposal) and VIRMOS (see Le Fevre's proposal) surveys. Since the ARCHEOPS balloon will cover 25% of the sky we will select areas that will also be observed by this experiment. From the weak lensing point of view, our goal is to provide constraints on Omega, Lambda, and the power spectrum of density fluctuations, on the estimate of galaxy biasing, on the evolution of the baryon fraction as well as on the dark matter halos, using the analysis of the galaxy/galaxy lensing effect. Furthermore, we will use the survey to search for dark (or low light-to-mass ratio) clusters, and thus obtain constraints on the fraction of dark structures in the universe. FULL SCIENTIFIC AND TECHNICAL INFORMATION ABOUT THIS PROJECT AS WELL AS DETAILED SIMULATIONS CAN BE FOUND AT: http://terapix.iap.fr/Descart/ and in the papers Bernardeau, van Waerbeke and Mellier (1997, A&A 322, 1), van Waerbeke, Bernardeau and Mellier (1999, A&A 342, 15), Schneider, van Waerbeke, Jain, Kruse (1998, MNRAS 296, 873), van Waerbeke (1998, A&A 334, 1), Schneider, Rix (1997, ApJ 474, 25), Natarayan, Kneib, Smail, Ellis (1998, ApJ 499, 600), Natarayan, Kneib (1997, MNRAS 287, 833), Erben, van Waerbeke, Mellier, Schneider, Cuillandre, Castander, Dantel-Fort (1999, astro-ph/ 9907134), Gautret, Fort, Mellier (1998, astro-ph/9812388), Mellier (1999, ARAA Vol. 37 in press, astro-ph 9812172), Pello , Kneib, Bolzonella, Miralles (1999, astro-ph/9907054). See also: http://www-crtbt.polycnrs-gre.fr/archeops/DESCRIPTION PROPOSED MEGACAM SURVEY: The gravitational lensing effect can be used to probe directly the (dark) matter, regardless of its geometry and its physical state. In particular, it can map the projected mass density of the large- scale structure of the universe, thus providing important clues on the cosmological parameters and the cosmic history of structure formation. In principle, Omega and lambda, as well as the normalization of the dark matter power spectrum, can be recovered from the variance and the skewness of the gravitational convergence or shear. van Waerbeke et al (1999) have shown that a weak lensing analysis of 100 square degrees, having a galaxy number density of 30 gal/square arc-minutes, can distinguish between an Omega=0.3-Universe and an Omega=1-Universe at a 10-sigma confidence level. So the proposed survey will allow us to disentangle less extreme models of the universes. Furthermore, the shape of the power spectrum can be reconstructed up to scales of 1 degree directly, i.e., without referring to the uncertain relation between luminous tracers (such as galaxies) and the underlying dark matter distribution. We propose to observe 100 square degrees. The goal is the production of a shear limited sample of lensed galaxies. The goal is the measurement of image shapes of galaxies down to a very faint limiting magnitude, thereby obtaining a huge sample of about 10 million faint background galaxies with measured image elipticities. We will use these ellipticities to compute the gravitational shear and reconstruct the projected mass density. From the statistical analysis of the mass maps, the statistics of peaks, and the investigation of galaxy-galaxy lensing, we propose to focus on these scientific objectives: (1) the measurement of the power spectrum P(k) of mass density fluctuations, from one arcminute-scale up to about 1 degree, in order to put strong constraints on the history of structure formation in the universe. The history of baryonic and non-baryonic components can then be analyzed from the cross correlation of P(k) with the SZ, X-ray and galaxy data. This will permit to understand the role of each component (hot gas, stellar component in galaxies, dark mater) in the cosmic scenario of structure formation, (2) the measurement of the cosmological parameters Omega and Lambda from the skewness and the variance of the convergence or shear. The weak lensing analysis will provide constrains which are perpendicular in parameter space to those coming from supernovae, (3) the production of a unique shear-limited catalog of galaxy clusters in order to define targets for XMM and ARCHEOPS for a joint X-ray and SZ analysis, (4) the search for giant arcs and the analysis of fraction of clusters producing giant arcs, as well as the mass reconstruction of the inner region of a subsample of individual clusters from a strong lensing analysis. This study will permit us to have detailed mass reconstructions of the central regions and also to use these clusters to obtain constrains on the cosmological parameters from the triplet statistics, (6) the size and velocity dispersion of galaxy halos from the statistical analysis of galaxy-galaxy lensing with a unique sample of about 10 million galaxies, (7) the analysis of the dust content in the cluster medium using further deep observations in U of the cluster sample. This test is important also for possible lensing correction of the magnification bias around clusters. Similarly, as a by-product of the galaxy-galaxy lensing work, we will study the possible dust absorption (occultation) in close environment of spiral galaxies from a statistical point of view. Although extremely tenuous effects are so far expected, the galaxy/galaxy dust occultation and its cosmic evolution can be an important issue for many observations of distant objects. The broad expertise we have among the co-Is allows us to cover the project as a whole. Our strategy is the following: following the quantitative analysis done by van Waerbeke et al (1999), we propose to cover 100 square degrees and to reach a galaxy number density of 30 galaxies/sq-armin. Owing to boundary effects in the mass reconstruction procedure, we prefer to observe a compact squared area. For this topology, the fraction of the survey which will be lost at the periphery is less than 1.5% of the total area. However, we can compromise the strategy in order to better profit of the night length. The minimum area we would accept to loose is 10%, which corresponds to 100 square degrees spread over 4 fields of 5x5 square degrees each. In such a topology we would request the four fields along a strip of 35 degrees (4x 5degresx 5degrees) , each 5x5 square degrees targets separated by 5 degrees from each other along the strip. This permits spreading the observation of the fields over the night and to analyze structures on very large scales. For instance, 6 degrees on the sky correspond to about 100 Mpc at z=0.3 (Omega=0.3, Lambda=0, H=65), a very interesting physical scale for the analysis of the power spectrum. A strip of 35 degrees would sample 7 such physical sizes, which is good enough for exploring the power spectrum on that scale. We plan to use the VIRMOS optical and NIR photometric data set as well as the spectroscopic survey to constrain the redshift distribution of the galaxies. However, in order decouple the background and foreground galaxies used in galaxy-galaxy lensing and in the efficiency function of the cosmic shear, we require observation with three filters. This will allow us to approximately separate galaxies into z<0.4 and z>0.4 subsamples. Preliminary investigations of the efficiency of photometruc redshift from the Pello et al (1999) work show that the optimal choices are the VRI filters. 1) Location We have selected an equatorial region and high galactic latitude areas, which are the best suited for ARCHEOPS, and the join VIRMOS/XMM survey in order to avoid high galactic contamination. Our top priority target is the one selected by VIRMOS And XMM: around RA = 2h20 Dec=-5 deg. 2) Sensitivity AB magnitude Megacam exposure time S/N =5 for extended object 3arc-second aperture. I 24.8 1h R 25.6 1h V 25.4 1h30 That is about 45 nights. Seeing requested: <0.7" 3) Time scales We plan to schedule the observations over two years. 4) Processing The data processing will be done at the TERAPIX data center at IAP.