Tracing high redshift cosmic web with quasar systems. Invited talk at
  IAU Symposium 308
  We study the cosmic web at redshifts 1.0 <= z <= 1.8 using quasar systems
based on quasar data from the SDSS DR7 QSO catalogue. Quasar systems were
determined with a friend-of-friend (FoF) algorithm at a series of linking
lengths. At the linking lengths l <= 30 Mpc/h the diameters of quasar systems
are smaller than the diameters of random systems, and are comparable to the
sizes of galaxy superclusters in the local Universe. The mean space density of
quasar systems is close to the mean space density of local rich superclusters.
At larger linking lengths the diameters of quasar systems are comparable with
the sizes of supercluster complexes in our cosmic neighbourhood. The richest
quasar systems have diameters exceeding 500 Mpc/h. Very rich systems can be
found also in random distribution but the percolating system which penetrate
the whole sample volume appears in quasar sample at smaller linking length than
in random samples showing that the large-scale distribution of quasar systems
differs from random distribution. Quasar system catalogues at our web pages
(http://www.aai.ee/~maret/QSOsystems.html) serve as a database to search for
superclusters of galaxies and to trace the cosmic web at high redshifts.
