Tracing high redshift cosmic web with quasar systems

Abstract

Context. To understand the formation, evolution, and present-day properties of the cosmic web we need to study it at low and high redshifts. Aims. We trace the cosmic web at redshifts 1.0 ≤ z ≤ 1.8 using the quasar (QSO) data from the SDSS DR7 QSO catalogue (Schneider et al. 2010). Methods. We apply a friend-of-friend (FoF) algorithm to the quasar and random catalogues to determine systems at a series of linking lengths, and analyse richness and sizes of these systems. Results. At the linking lengths l ≤ 30 h −1 Mpc the number of quasar systems is larger than the number of systems detected in random catalogues, and systems themselves have smaller diameters than random systems. The diameters of quasar systems are comparable to the sizes of poor galaxy superclusters in the local Universe, the richest quasar systems have four members. The mean space density of quasar systems, ≈ 10 −7 (h −1 Mpc) −3 , is close to the mean space density of local rich superclusters. At intermediate linking lengths (40 ≤ l ≤ 70 h −1 Mpc) the richness and length of quasar systems are similar to those derived from random catalogues. Quasar system diameters are similar to the sizes of rich superclusters and supercluster chains in the local Universe. The percolating system which penetrate the whole sample volume appears in quasar sample at smaller linking length than in random samples (85 h −1 Mpc). At the linking length 70 h −1 Mpc the richest systems of quasars have diameters exceeding 500 h −1 Mpc, three systems in our catalogue are the same as described in Clowes et al. (2012Clowes et al. ( , 2013. Quasar luminosities in systems are not correlated with the system richness. Conclusions. Quasar system catalogues at our web pages serve as a database to search for superclusters of galaxies and to trace the cosmic web at high redshifts.

Large SDSS Quasar Groups and Their Statistical Significance

“…4, that contains 95 members and the maximum extent of 712 h −1 Mpc. This group was not found by previous studies because its center is located at z = 0.366, which is outside the redshift limit of 1.0 < z < 1.8 adopted by Clowes et al, Nadathur (2013), and Einasto et al (2014). This group is much richer and has a size larger than U1.27.…”

“…This terminology can give a misleading idea that they are compact and physically interacting internally as in galaxy groups. Einasto et al (2014) used 'quasar systems' to avoid the confusion. The name 'quasar association' may also be an appropriate name for these loose quasar groups since the name 'stellar association' has been already used in astronomy for loose groups of stars that are not gravitationally bound.…”

“…It is also much larger in the sky area (5758 deg 2 versus 3725 deg 2 ) and in redshift range (0.1 ≤ z ≤ 1.8 versus 1.0 ≤ z ≤ 1.8). It makes our sample contain 1.72 times more quasars than that used by Clowes et al (2013), Nadathur (2013), and Einasto et al (2014).…”

“…This is because they represent density fluctuations on linear scales. It is then very important to rigorously study the physical properties and cosmological meaning of observed superstructures of the universe (Park 1990;Park et al 2012;Einasto et al 2011Einasto et al , 2014.…”

“…Even though it is cosmologically interesting to search for associations of quasars 2 (Webster 1982;Clowes & Campusano 1991;Komberg et al 1996;Williger et al 2002;Clowes et al 2012Clowes et al , 2013Einasto et al 2014), one needs to be careful in interpreting the quasar groups identified with specific criteria. This is because the size and richness of LSS depend sensitively on how they are identified.…”