Evidence for overdensity around z_em > 4 quasars from the proximity effect

We study the density field around z_em > 4 quasars using high quality medium spectral resolution ESI-Keck spectra (R~4300, SNR > 25) of 45 high-redshift quasars selected from a total of 95 spectra. This large sample considerably increases the statistics compared to previous studies. The redshift evolution of the mean photo-ionization rate and the median optical depth of the intergalactic medium (IGM) are derived statistically from the observed transmitted flux and the pixel optical depth probability distribution function respectively. This is used to study the so-called proximity effect, that is, the observed decrease of the median optical depth of the IGM in the vicinity of the quasar caused by enhanced photo-ionization rate due to photons emitted by the quasar. We show that the proximity effect is correlated with the luminosity of the quasars, as expected. By comparing the observed decrease of the median optical depth with the theoretical expectation we find that the optical depth does not decrease as rapidly as expected when approaching the quasar if the gas in its vicinity is part of the standard IGM. We interpret this effect as revealing gaseous overdensities on scales as large as ~15 Mpc/h. The mean overdensity is of the order of two and five within, respectively, 10 and 3 Mpc/h. If true, this would indicate that high redshift quasars are located in the center of overdense regions that could evolve with time into massive clusters of galaxies. The overdensity is correlated with luminosity: brighter quasars show higher overdensities.