The density structure around quasars inferred from optical depth statistics

We present a method for studying the proximity effect and use it to investigate the density structure around QSOs. It is based on the pixel optical depth probability distribution and its redshift evolution. We validate the method using mock spectra obtained from hydrodynamical simulations, and then apply it to a sample of 12 bright quasars at redshifts 2-3 observed with UVES at the VLT-UT2 Kueyen ESO telescope. These quasars do not show signatures of associated absorption and have a mean monochromatic luminosity of 5.4 × 10 h ergs/s/Hz at the Lyman limit. The distribution of optical depths changes considerably when the proper distance to the QSO is less than 10hMpc. The size of this proximity region is small given that these QSOs are very bright, which suggests that the quasars are located in regions that are overdense by factors 2-10 on scales < ∼ 10hMpc.