The Vimos VLT deep survey. Global properties of 20,000 galaxies in the IAB < 22.5 WIDE survey
Résumé
The VVDS-Wide survey has been designed to trace the large-scale distribution of galaxies at z ~ 1 on comoving scales reaching ~100~h-1 Mpc, while providing a good control of cosmic variance over areas as large as a few square degrees. This is achieved by measuring redshifts with VIMOS at the ESO VLT to a limiting magnitude IAB = 22.5, targeting four independent fields with sizes of up to 4 deg2 each. We discuss the survey strategy which covers 8.6 deg2 and present the general properties of the current redshift sample. This includes 32 734 spectra in the four regions, covering a total area of 6.1 deg2 with a sampling rate of 22 to 24%. This paper accompanies the public release of the first 18 143 redshifts of the VVDS-Wide survey from the 4 deg2 contiguous area of the F22 field at RA = 22^h. We have devised and tested an objective method to assess the quality of each spectrum, providing a compact figure-of-merit. This is particularly effective in the case of long-lasting spectroscopic surveys with varying observing conditions. Our figure of merit is a measure of the robustness of the redshift measurement and, most importantly, can be used to select galaxies with uniform high-quality spectra to carry out reliable measurements of spectral features. We also use the data available over the four independent regions to directly measure the variance in galaxy counts. We compare it with general predictions from the observed galaxy two-point correlation function at different redshifts and with that measured in mock galaxy surveys built from the Millennium simulation. The purely magnitude-limited VVDS Wide sample includes 19 977 galaxies, 304 type I AGNs, and 9913 stars. The redshift success rate is above 90% independent of magnitude. A cone diagram of the galaxy spatial distribution provides us with the current largest overview of large-scale structure up to z ~ 1, showing a rich texture of over- and under-dense regions. We give the mean N(z) distribution averaged over 6.1 deg2 for a sample limited in magnitude to IAB = 22.5. Comparing galaxy densities from the four fields shows that in a redshift bin Δz = 0.1 at z ~ 1 one still has factor-of-two variations over areas as large as ~ 0.25 deg2. This level of cosmic variance agrees with that obtained by integrating the galaxy two-point correlation function estimated from the F22 field alone. It is also in fairly good statistical agreement with that predicted by the Millennium simulations. The VVDS WIDE survey currently provides the largest area coverage among redshift surveys reaching z ~ 1. The variance estimated over the survey fields shows explicitly how clustering results from deep surveys of even 1 deg2 size should be interpreted with caution. The survey data represent a rich data base to select complete sub-samples of high-quality spectra and to study galaxy ensemble properties and galaxy clustering over unprecedented scales at these redshifts. The redshift catalog of the 4 deg2 F22 field is publicly available at http://cencosw.oamp.fr.