The great observatories origins deep survey VLT/VIMOS spectroscopy in the GOODS-south field

Aims. We present the first results from the VIsible Multiobject Spectrograph (VIMOS) ESO/GOODS program of spectroscopy for faint galaxies in the Chandra Deep Field South (CDF-S). This program complements the FORS2 ESO/GOODS campaign. Methods. All 3312 spectra were obtained in service mode with VIMOS at the ESO/VLT UT3. The VIMOS LR-Blue and MR grisms were used to cover different redshift ranges. Galaxies at 1.8 < z < 3.5 were observed in the GOODS VIMOS-LR-Blue campaign. Galaxies at z 1 and Lyman Break Galaxies at z > 3.5 were observed in the VIMOS MR survey. Results. Here we report results for the first 12 masks ( out of 20 total). We extracted 2344 from 6 LR-Blue masks and 968 from 6 MR masks. A large percentage, 33% of the LR-Blue and 18% of the MR spectra, are serendipitous observations. We obtained 1481 and 656 redshifts in the LR-Blue and MR campaign, respectively, for a total success rate of 70% and 75%, respectively, which decrease to 63% and 68% when also the serendipitous targets are considered. The typical redshift accuracy is sigma(z) = 0.001. The reliability of the redshift estimate varies with the quality flag. The LR-Blue quality flag A redshifts are reliable at similar to 95% confidence level, flag B redshifs at similar to 70% and quality C et similar to 40%. The MR redshift reliability is somewhat higher: 100% for quality flag A, similar to 90% for quality flag B and similar to 70% for flag C. By complementing our VIMOS spectroscopic catalog with all existing spectroscopic redshifts publicly available in the CDF-S, we created a redshift master catalog. By comparing this redshift compilation with different photometric redshift catalogs we estimate the completeness level of the CDF-S spectroscopic coverage in several redshift bins. Conclusions. The completeness level is very high, > 60%, at z 3.5, and it is very uncertain at higher redshift. The master catalog was used also to estimate completeness and contamination levels of different galaxy photometric selection techniques. The BzK selection method leads to a similar to 86% complete sample of z > 1.4 galaxies at i(AB) 25 mag and with a contamination similar to 23% of lower redshift objects. The so-called sub-U-dropout and the U-dropout methods lead to an 80% complete galaxy sample at z > 1.4 and iAB < 25 mag, with similar to 24% low redshift contaminants.