We present the first measurements of the Probability Distribution Function ( PDF ) of galaxy fluctuations in the four-passes , first-epoch VIMOS-VLT Deep Survey ( VVDS ) cone , covering 0.4 x 0.4 deg between 0.4 < z < 1.5 .
We show that the PDF of density contrasts of the VVDS galaxies is an unbiased tracer of the underlying parent distribution up to redshift z =1.5 , on scales R=8 and 10 h ^ { -1 } Mpc .
The second moment of the PDF , i.e. , the rms fluctuations of the galaxy density field , is with good approximation constant over the full redshift baseline investigated : we find that , in redshift space , \sigma _ { 8 } for galaxies brighter than \mathcal { M } _ { B } ^ { c } = -20 + 5 \log h has a mean value of 0.94 \pm 0.07 in the redshift interval 0.7 < z < 1.5 .
The third moment , i.e. , the skewness , increases with cosmic time : we find that the probability of having underdense regions is greater at z \sim 0.7 than it was at z \sim 1.5 .
By comparing the PDF of galaxy density contrasts with the theoretically predicted PDF of mass fluctuations we infer the redshift- , density- , and scale-dependence of the biasing function b ( z, \delta,R ) between galaxy and matter overdensities up to redshift z =1.5 .
Our results can be summarized as follows : i ) the galaxy bias is an increasing function of redshift : evolution is marginal up to z \sim 0.8 and more pronounced for z \mathrel { \hbox { \hbox to 0.0 pt { \hbox { \lower 4.0 pt \hbox { $ \sim$ } } } \hbox { $ > $ } } } 0.8 ; ii ) the formation of bright galaxies is inhibited below a characteristic mass-overdensity threshold whose amplitude increases with redshift and luminosity ; iii ) the biasing function is non linear in all the redshift bins investigated with non-linear effects of the order of a few to \sim 10 \% on scales > 5 h ^ { -1 } Mpc .
By subdividing the sample according to galaxy luminosity and colors , we also show that : iv ) brighter galaxies are more strongly biased than less luminous ones at every redshift and the dependence of biasing on luminosity at z \sim 0.8 is in good agreement with what is observed in the local Universe ; v ) red objects are systematically more biased than blue objects at all cosmic epochs investigated , but the relative bias between red and blue objects is constant as a function of redshift in the interval 0.7 < z < 1.5 , and its value ( b ^ { rel } \sim 1.4 ) is similar to what is found at z \sim 0 .