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A&A 568, A46 (2014) Astronomy DOI: 10.1051/0004-6361/201424283 & c ESO 2014 Astrophysics Tracing a high redshift cosmic web with quasar systems, Maret Einasto1, Erik Tago1, Heidi Lietzen1,2,3, Changbom Park4, Pekka Heinämäki5, Enn Saar1,6, Hyunmi Song4, Lauri Juhan Liivamägi1,7, and Jaan Einasto1,6,8 1 Tartu Observatory, 61602 Tõravere, Estonia e-mail: [email protected] 2 Instituto de Astrof˜isica de Canarias, 38205 La Laguna, Tenerife, Spain 3 Universidad de La Laguna, Dept. Astrofísica, 38206 La Laguna, Tenerife, Spain 4 School of Physics, Korea Institute for Advanced Study, 85 Hoegiro, Dong-Dae-Mun-Gu, 130-722 Seoul, Korea 5 Tuorla Observatory, University of Turku, Väisäläntie 20, Piikkiö, Finland 6 Estonian Academy of Sciences, 10130 Tallinn, Estonia 7 Institute of Physics, Tartu University, Tähe 4, 51010 Tartu, Estonia 8 ICRANet, Piazza della Repubblica 10, 65122 Pescara, Italy Received 27 May 2014 / Accepted 20 June 2014 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 that range from 1.0 ≤ z ≤ 1.8 by using the quasar (QSO) data from the SDSS DR7 QSO catalogue. Methods. We apply a friend-of-friend algorithm to the quasar and random catalogues to determine systems at a series of linking length 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 the 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 a quasar sample at a 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. Quasar luminosities in systems are not correlated with the system richness. Conclusions. Quasar system catalogues in our web pages and at the Strasbourg Astronomical Data Center (CDS) serve as a database for searching superclusters of galaxies and for tracing the cosmic web at high redshifts. Key words. large-scale structure of Universe – quasars: general 1. Introduction us to describe the cosmic web in our neighbourhood in detail. One source of information about the cosmic structures at high The contemporary cosmological paradigm tells us that the struc- redshifts is the distribution of quasars. Quasars lie at centres of ture in the Universe, the cosmic web formed and evolved from massive galaxies (Hutchings et al. 2002; Kotilainen et al. 2009; tiny density perturbations in the very early Universe by hi- Padmanabhan et al. 2009; Letawe et al. 2010; Kotilainen et al. erarchical growth, is driven by gravity (see, e.g., Loeb 2008; 2013; Falomo et al. 2013; Floyd et al. 2013; Falomo et al. 2014; van de Weygaert & Schaap 2009, and references therein). Karhunen et al. 2014, and references therein), which typically lie Groups and clusters of galaxies and their filaments are formed by in small groups and poor clusters of galaxies (Wold et al. 2000; −1 density perturbations on a scale up to 32 h Mpc. Superclusters Soechting et al. 2002; Söchting et al. 2004; Coldwell & Lambas of galaxies, the largest density enhancements in the local cosmic 2006; Hutchings et al. 2009; Trainor & Steidel 2012; Ueda et al. −1 web, are formed by larger-scale perturbations up to 100 h Mpc. 2013). Nearby quasars are typically located in a relatively low- Still, larger scale density perturbations modulate the richness of density, large-scale environment near superclusters of galaxies galaxy systems (Einasto et al. 2011a; Suhhonenko et al. 2011). (Coldwell & Lambas 2006; Lietzen et al. 2009, 2011). To understand how the cosmic web is formed and evolved, it Galaxy luminosities, morphological types, colours, star for- is of paramount importance to describe and quantify it at low and mation rates, and other properties are closely related to their en- high redshifts. Large galaxy redshift surveys like SDSS enable vironment at small and large scales (Einasto et al. 1974; Dressler 1980; Einasto & Einasto 1987; Mo et al. 1992; Park et al. 2007; Appendix A is available in electronic form at Park & Choi 2009; Skibba 2009; Tempel et al. 2011; Lietzen http://www.aanda.org et al. 2012; Einasto et al. 2014). The studies of the quasar envi- The catalogues are only available at the CDS via anonymous ftp to ronment in the cosmic web helps us to understand the relation cdsarc.u-strasbg.fr (130.79.128.5)orvia between the galaxy and quasar evolution and to test the evolu- http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/568/A46 tionary schemes of different type of objects. Article published by EDP Sciences A46, page 1 of 10 A&A 568, A46 (2014) Already decades ago, several studies described large systems in quasar distribution (Webster 1982; Clowes & Campusano 1.4e-06 1991; Komberg et al. 1996; Williger et al. 2002; Clowes et al. 2012, 2013, and references therein), which are known as large 1.2e-06 quasar groups (LQGs). Miller et al. (2004) found significant 1.0e-06 −1 ) 3 200 h Mpc size over- and underdensities in the density field h -3 determined by quasars from the 2dF QSO Redshift survey. 8.0e-07 Komberg et al. (1996) showed that LQGs may trace distant galaxy superclusters. The large-scale distribution of quasar sys- n (Mpc 6.0e-07 tems gives us information about the cosmic web at high redshifts which are not yet covered by large and wide galaxy surveys. 4.0e-07 The goal of this study is to analyse the distribution of quasars 2.0e-07 at redshifts in the range of 1.0 ≤ z ≤ 1.8. We choose this red- shift range as we are interested in the study of the cosmic web at 2200 2400 2600 2800 3000 3200 3400 3600 high redshifts. This redshift range was also used by Clowes et al. D (Mpc/h) (2012, 2013); with this choice, we can compare LQGs found Fig. 1. Quasar number density versus distance. Dotted lines show the in these studies and our systems. We shall use the Schneider 98% central confidence limits. et al. (2010) catalogue of quasars based on the Sloan Digital Sky Survey Data Release 7 (SDSS DR7). We analyse clustering and η, and redshift limits as quasar samples. The number of properties of quasars with the friend-of-friend (FoF) algorithm, random points in random samples was equal to the number of compare them with random distributions, and determine systems quasars. of quasars at a series of linking lengths. We present catalogues of quasar systems and analyse their properties and large-scale distribution. 3. Method In Sect. 2 we describe the data we used and the method in We employed the FoF clustering analysis method introduced Sect. 3. In Sect. 4, we give the results. We discuss and summarise in cosmology by Zeldovich et al. (1982)andHuchra & Geller the results in Sect. 5. In Appendix A, we present data about the (1982) to determine systems in the quasar catalogue and random richest quasar systems. 1 catalogues. The FoF method is commonly used to detect groups We present the quasar system catalogues . An interactive pdf and clusters in the galaxy distribution (Tucker et al. 2000; Eke file showing the distribution of quasars is also available on our et al. 2004; Tago et al. 2006, 2008; Park et al. 2012; Tempel et al. web pages. 2012, 2014, and references therein) and superclusters of optical We assume the standard cosmological parameters: the −1 −1 and X-ray clusters of galaxies (Einasto et al. 1994, 2001; Chon Hubble parameter H0 = 100 h km s Mpc , the matter den- Ω = . Ω = . et al. 2013). Komberg et al. (1996) applied the FoF method to sity m 0 27, and the dark energy density Λ 0 73. search for rich quasar systems. The FoF method collects objects into systems if they have 2. Data at least one common neighbour closer than a linking length. At small linking lengths where only the closest neighbouring ob- We adopt a catalogue of quasars with a spectrum taken as a jects form systems, most objects in the sample remain single. As normal science spectrum (SCIENCEPRIMARY=1) and a photo- the linking length increases, more and more objects join systems, metric measurement that is designated PRIMARY in the BEST and the number of systems, their richness and size increase. At photometric database. From this catalogue, we select a subsam- a certain linking length, the largest system spans over the whole ple of quasars in the redshift interval 1.0 ≤ z ≤ 1.8andap- sample volume and a percolation occurs. We apply the FoF al- ply an i-magnitude limit i = 19.1. The full quasar catalogue gorithm to quasar and random samples with a series of linking by Schneider et al. (2010) is not homogeneous; Vanden Berk lengths to analyse the properties of systems and their percola- et al.