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 report on the measurement of the physical properties (rest-frame K-band luminosity and total stellar mass) of the hosts of 89 broad-line (type-1) active galactic nuclei (AGNs) detected in the zCOSMOS survey in the redshift range 1 < z < 2.2. The unprecedented multi-wavelength coverage of the survey field allows us to disentangle the emission of the host galaxy from that of the nuclear black hole in their spectral energy distributions (SEDs). We derive an estimate of black hole masses through the analysis of the broad Mg ii emission lines observed in the mediumresolution spectra taken with VIMOS/VLT as part of the zCOSMOS project. We found that, as compared to the local value, the average black hole to host-galaxy mass ratio appears to evolve positively with redshift, with a best-fit evolution of the form (1 + z) 0.68±0.12 +0.6 −0.3 , where the large asymmetric systematic errors stem from the uncertainties in the choice of initial mass function, in the calibration of the virial relation used to estimate BH masses and in the mean QSO SED adopted. On the other hand, if we consider the observed rest-frame K-band luminosity, objects tend to be brighter, for a given black hole mass, than those on the local M BH -M K relation. This fact, together with more indirect evidence from the SED fitting itself, suggests that the AGN hosts are likely actively star-forming galaxies. A thorough analysis of observational biases induced by intrinsic scatter in the scaling relations reinforces the conclusion that an evolution of the M BH -M * relation must ensue for actively growing black holes at early times: either its overall normalization, or its intrinsic scatter (or both) appear to increase with redshift. This can be interpreted as signature of either a more rapid growth of supermassive black holes at high redshift, a change of structural properties of AGN hosts at earlier times, or a significant mismatch between the typical growth times of nuclear black holes and host galaxies. In any case, our results provide important clues on the nature of the early co-evolution of black holes and galaxies and challenging tests for models of AGN feedback and self-regulated growth of structures.

We make use of the deep Karl G. Jansky Very Large Array (VLA) COSMOS radio observations at 3 GHz to infer radio luminosity functions of star-forming galaxies up to redshifts of z ∼ 5 based on approximately 6 000 detections with reliable optical counterparts. This is currently the largest radio-selected sample available out to z ∼ 5 across an area of 2 square degrees with a sensitivity of rms ≈ 2.3 µJy beam −1 . By fixing the faint and bright end shape of the radio luminosity function to the local values, we find a strong redshift trend that can be fitted with a pure luminosity evolution L 1.4 GHz ∝ (1 + z) (3.16±0.2)−(0.32±0.07)z . We estimate star formation rates (SFRs) from our radio luminosities using an infrared (IR)-radio correlation that is redshift dependent. By integrating the parametric fits of the evolved luminosity function we calculate the cosmic SFR density (SFRD) history since z ∼ 5. Our data suggest that the SFRD history peaks between 2 < z < 3 and that the ultraluminous infrared galaxies (ULIRGs; 100 M ⊙ yr −1 < SFR < 1000 M ⊙ yr −1 ) contribute up to ∼25% to the total SFRD in the same redshift range. Hyperluminous infrared galaxies (HyLIRGs; SFR > 1000 M ⊙ yr −1 ) contribute an additional 2% in the entire observed redshift range. We find evidence of a potential underestimation of SFRD based on ultraviolet (UV) rest-frame observations of Lyman break galaxies (LBGs) at high redshifts (z 4) on the order of 15-20%, owing to appreciable star formation in highly dust-obscured galaxies, which might remain undetected in such UV observations.