The search and investigation of the Large Groups of Quasars

Recently, it was suggested that large concentrations or groups of quasars may trace sites of enhanced matter density at medium and high redshifts analogous to how galaxy clusters trace them in nearby space. We have checked existing quasar data for the presence of such groups. Large Quasar Groups (LQGs) were identified using a well-known cluster analysis technique. Our final list consists of 12 groups, including one reported previously. Most of the quasars in these groups come from deep homogeneous surveys. We have found that: (i) the probability that the detected groups are random is rather small (generally a few per cent); (ii) their sizes range from $\sim 70$ to $\sim 160h^{-1} Mpc$, which is comparable to sizes of nearby rich superclusters; (iii) the detected groups all have redshifts $0.5<z\leq 2$; (iv) the abundance of the LQGs is comparable with the abundance of large superclusters at $z\sim 0$, which is consistent with the idea that quasar groups and superclusters can be evolutionarily related. We argue that quasar groups could be a common feature of the spatial distribution of medium redshift quasars, and that the quasars in groups may belong to concentrations of young galaxy clusters and groups (distant superclusters) and hence be biased tracers of large-scale structure of matter distribution in the early Universe. Theoretical implications as well as other observations needed to test this point are discussed.