zCOSMOS-bright catalog

Abstract

zCOSMOS is a large-redshift survey that is being undertaken in the COSMOS field using 600hr of observation with the VIMOS spectrograph on the 8m VLT. The survey is designed to characterize the environments of COSMOS galaxies from the 100kpc scales of galaxy groups up to the 100Mpc scale of the cosmic web and to produce diagnostic information on galaxies and active galactic nuclei. The zCOSMOS survey consists of two parts: (1) zCOSMOS-bright, a magnitude-limited I-band IAB<22.5 sample of about 20000 galaxies with 0.1<z<1.2 covering the whole 1.7deg2 COSMOS ACS field, for which the survey parameters at z∼0.7 are designed to be directly comparable to those of the 2dFGRS at z∼0.1; and (2) zCOSMOS-deep, a survey of approximately 10000 galaxies selected through color-selection criteria to have 1.4<z<3.0, within the central 1deg2. This paper describes the survey design and the construction of the target catalogs and briefly outlines the observational program and the data pipeline. In the first observing season, spectra of 1303 zCOSMOS-bright targets and 977 zCOSMOS-deep targets have been obtained. A total of 10643 spectra could be extracted from the VIMOS observations and are presented in this release. Throughout the zCOSMOS-bright survey, 1arcsec wide slits have been used with a wavelength range of approximately 5500 to 9700Å sampled at roughly 2.5Å/pixel. The primary input catalogue for slit-mask design was generated using SExtractor (Bertin et al., 1996A&AS..117..393B) applied to the COSMOS F814W HST/ACS images sampled at 0.03arcsec/pixel (Koekemoer et al., 2007ApJS..172..196K, Leauthaud et al., 2007ApJS..172..219L) in a "hot and cold" two-pass process to first identify bright objects. This substantially reduced the tendency of the HST-based catalogue to "over-resolve" extended galaxies into multiple components. This initial SExtractor catalogue was then "cleaned" by carrying out a detailed comparison with one extracted from a stack of i* images obtained with MEGACAM on the 3.6m Canada-France-Hawaii telescope and processed at the TERAPIX data reduction center in Paris. This catalogue was also used to supplement the ACS catalogue for regions where the ACS images were unavailable or unusable. The zCOSMOS-bright target catalogue is intended to be simply defined as having an ACS/HST SExtractor "magauto" brightness in the range 15.00<IAB(814)<22.50. Generally, objects to be inserted into the slit mask are chosen randomly from the target catalogue. However, a few percent of targets (generally X-ray sources) are designated as "compulsory" targets and inserted into the masks with first priority. As a result, they are over-represented in the spectroscopic catalogue, by a factor which happens to be close to 2.0. Objects strongly suspected of being stars on the basis of morphology and spectral energy distribution are not included in the masks as targets and are classified as "forbidden". These are about 15% of the I<22.5 sample. However, the criteria for this exclusion are deliberately quite conservative and about 4% of the spectroscopic targets turn out to be stars. For each slit there is a primary target. Naturally, sometimes other targets happen to fall in the same slit. These "secondary targets" are indicated by preceding the redshift confidence class (see below) by a "2". Sometimes, these secondary objects were forbidden, but their spectra were anyway reduced and included in the catalogue. Given the inevitable complexity of the sample, users who require statistically complete samples are strongly encouraged to contact the zCOSMOS team for guidance.