Large SDSS Quasar Groups and Their Statistical Significance

Abstract

Abstract:We use a volume-limited sample of quasars in the Sloan Digital Sky Survey (SDSS) DR7 quasar catalog to identify quasar groups and address their statistical significance. This quasar sample has a uniform selection function on the sky and nearly a maximum possible contiguous volume that can be drawn from the DR7 catalog. Quasar groups are identified by using the Friend-of-Friend algorithm with a set of fixed comoving linking lengths. We find that the richness distribution of the richest 100 quasar groups or the size distribution of the largest 100 groups are statistically equivalent with those of randomly-distributed points with the same number density and sky coverage when groups are identified with the linking length of 70 h −1 Mpc. It is shown that the large-scale structures like the huge Large Quasar Group (U1.27) reported by Clowes et al. (2013) can be found with high probability even if quasars have no physical clustering, and does not challenge the initially homogeneous cosmological models. Our results are statistically more reliable than those of Nadathur (2013), where the test was made only for the largest quasar group. It is shown that the linking length should be smaller than 50 h −1 Mpc in order for the quasar groups identified in the DR7 catalog not to be dominated by associations of quasars grouped by chance. We present 20 richest quasar groups identified with the linking length of 70 h −1 Mpc for further analyses.

Quasars as a Tracer of Large-Scale Structures in the Distant Universe

“…Nadathur (2013), Einasto et al (2014), and Park et al (2015) pointed out that the cosmological interpretation by Clowes et al (2013) of large quasar groups, which questions the validity of the cosmological assumption of homogeneity and isotropy, is misleading. They stressed the importance of a statistically precise analysis in order to draw conclusions on cosmological implications from the observation of one or a few extreme objects.…”

“…Using these quasar survey data, Clowes et al (2013), Nadathur (2013), Einasto et al (2014), and Park et al (2015) found very large quasar groups and discussed the cosmological implications of the existence and properties of these extreme objects. Nadathur (2013), Einasto et al (2014), and Park et al (2015) pointed out that the cosmological interpretation by Clowes et al (2013) of large quasar groups, which questions the validity of the cosmological assumption of homogeneity and isotropy, is misleading.…”

“…They stressed the importance of a statistically precise analysis in order to draw conclusions on cosmological implications from the observation of one or a few extreme objects. Park et al (2015) also emphasized that statistical comparison with cosmological simulations must be employed as well. The quasar survey data are also used for studies to constrain cosmological parameters (Han & Park 2015;Risaliti & Lusso 2015).…”

Topology of large-scale underdense regions

“…Still larger structures based on the SDSS have been reported; in particular, the Sloan Great Wall 420 Mpc long (Gott et al 2005), and the largest filamentary structure extending above 1 Gpc found in the DR7QSO catalogue (Clowes et al 2013). However, statistical significance of the latter one was questioned by Park et al (2015) on the grounds that group finding algorithm used by Clowes et al (2013) was not sufficiently restrictive and structures formed by chance were recognized as physical quasar group.…”

“…A question of identifying structures is of statistical nature and has a long history. It was recently discussed by Park et al (2015). Here we examine statistics of large voids found in the SDSS DR7 quasar catalogue.…”