My research is mainly based on the zCOSMOS survey, a large 600 hr spectroscopic survey on the ESO VLT. The zCOSMOS-bright part of the survey is flux limited at I(AB) < 22.5, and will ultimately obtain over 20,000 spectra. At this flux limit, the redshifts of the majority of the observed galaxies fall in the range 0< z< 1.4. A higher redshift range, 1.4< z< 3 is targeted in the zCOSMOS-deep programme. Our current research is based on the first ~10,000 spectra from the zCOSMOS-bright, which we refer to as the ``10k sample''.

We use the current sample of ~10,000 zCOSMOS spectra of sources selected with I(AB) < 22.5 to define the density field out to z~1, with much greater resolution in the radial dimension than has been possible with either photometric redshifts or weak lensing. We apply new algorithms that we have developed (ZADE) to incorporate objects not yet observed spectroscopically by modifying their photometric redshift probability distributions using the spectroscopic redshifts of nearby galaxies. This strategy allows us to probe a broader range of galaxy environments and reduce the Poisson noise in the density field. The reconstructed overdensity field of the 10k zCOSMOS galaxies consists of cluster-like patterns surrounded by void-like regions, extending up to z~1. Some of these structures are very large, spanning the ~50 Mpc/h transverse direction of the COSMOS field and extending up to Delta z~0.05 in redshift. We present the three dimensional overdensity maps and compare the reconstructed overdensity field to the independently identified virialised groups of galaxies and clusters detected in the visible and in X-rays. The distribution of the overdense structures is in general well traced by these virialised structures. A comparison of the large scale structures in the zCOSMOS data and in the mock catalogues reveals an excellent agreement between the fractions of the volume enclosed in structures of all sizes above a given overdensity between the data and the mocks in 0.2< z< 1.

Distribution of the structures in the 10k zCOSMOS overdensity field in 0.1< z<1 reconstructed using the ZADE-algorithm. The structures are presented as isosurfaces enclosing regions 1.5, 3, 5 and 10 larger than mean (projected) density from the left to the right, respectively. All three axis, distance from the survey centre in right ascension (left axis) and in declination (right axis), and distance (corresponding to redshift; vertical axis) are expressed in comoving Mpc/h.

We use the overdensity field reconstructed in the volume of the COSMOS area to study the nonlinear biasing of the zCOSMOS galaxies. The galaxy overdensity field is reconstructed using the current sample of ~8500 accurate zCOSMOS redshifts at I(AB)<22.5 out to z~1 on scales R from 8 to 12 Mpc/h. By comparing the probability distribution function (PDF) of galaxy density contrast delta_g to the lognormal approximation of the PDF of the mass density contrast delta, we obtain the mean biasing function b(delta,z,R) between the galaxy and matter overdensity field and its second moments b(hat) and b(tilde) up to z~1. Over the redshift interval 0.4 = b(delta,z,R) delta is of the following characteristic shape. The function vanishes in the most underdense regions and then sharply rises in a nonlinear way towards the mean densities. is almost a linear tracer of the matter in the overdense regions, up to the most overdense regions in which it is nonlinear again and the local effective slope of vs. delta is smaller than unity. The function is evolving only slightly over the redshift interval 0.4

We study the evolution of galaxies inside and outside of the group environment since z=1 using a large well defined set of groups and galaxies from the zCOSMOS-bright redshift survey in the COSMOS field. The fraction of galaxies with early-type morphologies increases monotonically with M_B luminosity and stellar mass and with cosmic epoch. It is higher in the groups than elsewhere, especially at later epochs. The emerging environmental effect is superposed on a strong global mass-driven evolution, and at z~0.5 and log(M*/Msol)~10.2, the "effect" of group environment is equivalent to (only) about 0.2 dex in stellar mass or 2 Gyr in time. The stellar mass function of galaxies in groups is enriched in massive galaxies. We directly determine the transformation rates from late to early morphologies, and for transformations involving colour and star formation indicators. The transformation rates are systematically about twice as high in the groups as outside, or up to 3-4 times higher correcting for infall and the appearance of new groups. The rates reach values, for masses around the crossing mass 10^10.5 Msol, as high as (0.3-0.7)/Gyr in the groups, implying transformation timescales of 1.4-3 Gyr, compared with less than 0.2/Gyr, i.e. timescales >5 Gyr, outside of groups. All three transformation rates decrease at higher stellar masses, and must decrease also at the lower masses below 10^10 Msol which we cannot well probe. The rates involving colour and star formation are consistently higher than those for morphology, by a factor of about 50%. Our conclusion is that the transformations which drive the evolution of the overall galaxy population since z~1 must occur at a rate 2-4 times higher in groups than outside of them.

Derived transformation rates for morphology (late to early), colour (blue to red) and star formation activity (active to passive) in the group environment (left) and in the non-group environment (right). The solid symbols correspond to the final rates, which are corrected in the case of the group galaxies for the infall of new members onto existing groups and the appearance of new groups above the selection threshold. It should be noted that this is a ``worst-case'' correction which assumes that the new group members are representative of the previous non-group population. Using this same assumption, the rates in the non-group population need not to be corrected for the loss of these galaxies. The open symbols in the left hand plot also correspond to the rates in the groups with no infall correction applied for the group galaxies. The morphological, colour and star formation activity rates are shown with (red) circles, (green) triangles and (violet) squares, respectively.

• Kovac, K.; Porciani, C.; Lilly, S. J. and zCOSMOS team: "The nonlinear biasing of the 10k zCOSMOS galaxies up to z~1", arXiv:0910.0004
• Kovac, K.; Lilly, S. J.; Knobel, C. and zCOSMOS team: "The 10k zCOSMOS: morphological transformation of galaxies in the group environment since z~1", arXiv:0909.2032
• Iovino, A.; Cucciati, O.; Scodeggio, M.; Knobel, C.; Kovac, K. and zCOSMOS team: " The zCOSMOS Redshift Survey: How group environment alters global downsizing trends", arXiv:0909.1951
• Tasca, L. A. M.; Kneib, J.-P.; Iovino, A.; Le Fevre, O.; Kovač, K. and zCOSMOS team: "The zCOSMOS redshift survey: the role of environment and stellar mass in shaping the rise of the morphology-density relation from z ~ 1", 2009, A&A, 503, 379
• Bolzonella, M.; Kovac, K.; Pozzetti, L. and zCOSMOS team: "Tracking the impact of environment on the Galaxy Stellar Mass Function up to z~1 in the 10k zCOSMOS sample", arXiv:0907.0013
• Knobel, C.; Lilly, S. J.; Iovino, A.; Porciani, C.; Kovač, K. and zCOSMOS team: "An Optical Group Catalog to z = 1 from the zCOSMOS 10 k Sample", 2009, ApJ, 697, 1842
• Kovac, K.; Oosterloo, T. A.; van der Hulst, J. M.: "A blind HI survey in the Canes Venatici region", arXiv:0904.2775, MNRAS in press
• Silverman, J. D.; Kovač, K.; Knobel, C, and zCOSMOS team: "The Environments of Active Galactic Nuclei within the zCOSMOS Density Field", 2009, ApJ, 695, 17
• Kovac, K.; Lilly, S. J.; Cucciati, O. and zCOSMOS team: "The density field of the 10k zCOSMOS galaxies", arXiv:0903.3409, accepted to ApJ
• Caputi, K. I.; Kovač, K.; Bolzonella, M. and zCOSMOS team: "The Close Environment of 24 micron Galaxies at 0.6 < z < 1.0 in the Cosmos Field", 2009, ApJ, 691, 91
• Kovač, Katarina; Somerville, Rachel S.; Rhoads, James E.; Malhotra, Sangeeta; Wang, JunXian: "Clustering of Lyman alpha emitters at z ~ 4.5", 2007, ApJ, 668, 15