THE NONLINEAR BIASING OF THE zCOSMOS GALAXIES UP TO z ∼ 1 FROM THE 10k SAMPLE

We use the zCOSMOS galaxy overdensity field to study the biasing of galaxies in the COSMOS field. By comparing the probability distribution function of the galaxy density contrast delta(g) to the lognormal approximation of the mass density contrast delta, we obtain the mean biasing function b(delta, z, R) between the galaxy and matter overdensity fields and its second moments (b) over cap and (b) over tilde. Over the redshift interval 0.4 < z < 1 the conditional mean function = b(delta, z, R)delta is of a characteristic shape, requiring nonlinear biasing in the most overdense and underdense regions. Taking into account the uncertainties due to cosmic variance, we do not detect any significant evolution in the function, but we do detect a significant redshift evolution in the linear biasing parameter (b) over cap from 1.23 +/- 0.11 at z similar to 0.55 to 1.62 +/- 0.14 at z similar to 0.75, for a luminosity-complete sample of M-B < -20 - z galaxies. The (b) over cap parameter does not change significantly with smoothing scale between 8 and 12 h(-1) Mpc, but increases systematically with luminosity (at 2 sigma-3 sigma significance between the M-B < -20.5 - z and M-B < -20 - z samples). The nonlinearity parameter (b) over tilde/(b) over cap is offset from unity by at most 2%, with an uncertainty of the same order. The (b) over tilde/(b) over cap parameter does not show any significant redshift evolution, dependence on the smoothing scale or on the luminosity. By matching the linear bias of galaxies to the halo bias, we infer that the M-B < -20 - z galaxies reside in dark matter halos with a characteristic mass of about (2.6-5.6) x 10(12) M-circle dot with a small dependence on the adopted bias-mass relation. Our detailed error analysis and comparison with previous studies lead us to conclude that cosmic variance is the main contributor to the differences in the linear bias measured from different surveys. While our results support the general picture of biased galaxy formation up to z similar to 1, the fine-tuning of the galaxy formation models is still limited by the restrictions of the current spectroscopic surveys at these redshifts.