THE SEARCH FOR AND INVESTIGATION OF LARGE QUASAR GROUPS

Recently, it was suggested that large concentrations or groups of quas ars may trace sites of enhanced matter density at medium and high reds hifts, analogous to the way in which galaxy clusters trace them in nea rby space. We checked existing quasar data for the presence of such gr oups, Large quasar groups (LQGs) were identified using a well-known cl uster analysis technique and the following selection criteria: (i) an LQG must contain at least 10 quasars; (ii) the number density of quasa rs in a group must exceed that of the background by at least a factor of 2; (iii) the majority of quasars in a group must have reliable reds hifts. Our final list contains 12 such groups, including one reported previously. It was found that most of the quasars in these groups come from deep homogeneous surveys. Further analysis of the spatial distri bution of quasars in these surveys shows that: (i) the probability tha t the detected groups are random is rather small (generally a few per cent); (ii) their sizes range from similar to 70 to similar to 160 h(- 1) Mpc, which is comparable to the sizes of nearby rich superclusters; (iii) the detected groups all have redshifts 0.5<z less than or equal to 2; (iv) the abundance of the LQGs is comparable to the abundance o f large superclusters at z similar to 0, which is consistent with the idea that quasar groups and superclusters can be evolutionarily relate d. We argue that quasar groups could be a common feature of the spatia l distribution of medium-redshift quasars, and that the quasars in gro ups may belong to concentrations of young galaxy clusters and groups ( distant superclusters) and hence be biased tracers of the large-scale structure of matter distribution in the early Universe. Theoretical im plications, as well as other observations needed to test this point, a re discussed.