The Cosmic Evolution Survey (COSMOS) is designed to probe the correlated evolution of galaxies, star formation, active galactic nuclei (AGNs), and dark matter (DM) with large-scale structure (LSS) over the redshift range z > 0:5Y 6. The survey includes multiwavelength imaging and spectroscopy from X-rayYtoYradio wavelengths covering a 2 deg 2 area, including HST imaging. Given the very high sensitivity and resolution of these data sets, COSMOS also provides unprecedented samples of objects at high redshift with greatly reduced cosmic variance, compared to earlier surveys. Here we provide a brief overview of the survey strategy, the characteristics of the major COSMOS data sets, and a summary the science goals.

We describe the details of the Hubble Space Telescope (HST ) Advanced Camera for Surveys (ACS) Wide Field Channel ( WFC) observations of the COSMOS field, including the data calibration and processing procedures. We obtained a total of 583 orbits of HST ACS/ WFC imaging in the F814W filter, covering a field that is 1.64 deg 2 in area, the largest contiguous field ever imaged with HST. The median exposure depth across the field is 2028 s (one HST orbit), achieving a limiting point-source depth AB( F814W ) ¼ 27:2 (5 ). We also present details of the astrometric image registration and distortion removal and image combination using MultiDrizzle, motivating the choice of our final pixel scale (30 mas pixel À1 ), based on the requirements for weak-lensing science. The final set of images are publicly available through the archive sites at IPAC and STScI, along with further documentation on how they were produced.

zCOSMOS is a large redshift survey that is being undertaken in the COSMOS field using 600 hours of observation with the VIMOS spectrograph on the 8-m VLT. The survey is designed to characterise the environments of COSMOS galaxies from the 100 kpc scales of galaxy groups up to the 100 Mpc scale of the cosmic web and to produce diagnostic information on galaxies and active galactic nuclei. The zCOSMOS survey consists of two parts: (a) zCOSMOS-bright, a magnitude-limited I-band I AB < 22.5 sample of about 20,000 galaxies with 0.1 < z < 1.2 covering the whole 1.7 deg 2 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 (b) zCOSMOS-deep, a survey of approximately 10,000 galaxies selected through colourselection criteria to have 1.4 < z < 3.0, within the central 1 deg 2 . This paper describes the survey design and the construction of the target catalogues, and briefly outlines the observational program and the data pipeline. In the first observing season, spectra of 1303 zCOSMOS-bright targets and of 977 zCOSMOS-deep targets have been obtained. These are briefly analysed to demonstrate the characteristics that may be expected from zCOSMOS, and particularly zCOSMOS-bright, when it is finally completed between 2008-2009. The power of combining spectroscopic and photometric redshifts is demonstrated, especially in correctly identifying the emission line in single-line spectra and in determining which of the less reliable spectroscopic redshifts are correct and which are incorrect. These techniques bring the overall success rate in the zCOSMOS-bright so far to almost 90% and to above 97% in the 0.5 < z < 0.8 redshift range. Our zCOSMOS-deep spectra demonstrate the power of our selection techniques to isolate high redshift galaxies at 1.4 < z < 3.0 and of VIMOS to measure their redshifts using ultraviolet absorption lines.

We present spectroscopic redshifts of a large sample of galaxies with I AB < 22.5 in the COSMOS field, measured from spectra of 10,644 objects that have been obtained in the first two years of observations in the zCOSMOSbright redshift survey. These include a statistically complete subset of 10,109 objects. The average accuracy of individual redshifts is 110 km s −1 , independent of redshift. The reliability of individual redshifts is described by a Confidence Class that has been empirically calibrated through repeat spectroscopic observations of over 600 galaxies. There is very good agreement between spectroscopic and photometric redshifts for the most secure Confidence Classes. For the less secure Confidence Classes, there is a good correspondence between the fraction of objects with a consistent photometric redshift and the spectroscopic repeatability, suggesting that the photometric redshifts can be used to indicate which of the less secure spectroscopic redshifts are likely right and which are probably wrong, and to give an indication of the nature of objects for which we failed to determine a redshift. Using this approach, we can construct a spectroscopic sample that is 99% reliable and which is 88% complete in the sample as a whole, and 95% complete in the redshift range 0.5 < z < 0.8. The luminosity and mass completeness levels of the zCOSMOS-bright sample of galaxies is also discussed.

The text of this paper has today been replaced with a version which contains all the figures and tables, including the galaxy redshifts.Unfortunately, I and my group, being small (because I have received no federal funding for this research for several years) have not finished two final pieces of analysis: (1) a paper on the evolution in the galaxy luminosity function and (2) a paper on the relationships between broadband colors, morphologies, and narrow spectral features (lines and breaks). I therefore request that the redshifts not be used for projects which are substantially similar to either of these two projects until I have submitted such papers to astro-ph. I expect this to happen by 2000 October. Thank you very much.