The search for and investigation of large quasar groups

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 the way in which galaxy clusters trace them in nearby space. We checked existing quasar data for the presence of such groups. Large quasar groups (LQGs) were identified using a well-known cluster analysis technique and the following selection criteria: (i) an LQG must contain at least 10 quasars; (ii) the number density of quasars 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 redshifts. 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 distribution of quasars in these surveys shows that: (i) the probability that the detected groups are random is rather small (generally a few per cent); (ii) their sizes range from ~70 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<=2 (iv) the abundance of the LQGs is comparable to the abundance of large superclusters at z~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 the large-scale structure of matter distribution in the early Universe. Theoretical implications, as well as other observations needed to test this point, are discussed.