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Quasar Pairs with Arcminute Angular Separations A&A 372, 1–7 (2001) Astronomy DOI: 10.1051/0004-6361:20010283 & c ESO 2001 Astrophysics Quasar pairs with arcminute angular separations V. I. Zhdanov1,2 and J. Surdej1,? 1 Institut d’Astrophysique, Universit´edeLi`ege, Avenue de Cointe 5, 4000 Li`ege, Belgium 2 Astronomical Observatory of Kyiv University, Observatorna St. 3, UA- 04053 Kyiv, Ukraine Received 19 October 2000 / Accepted 16 February 2001 Abstract. We use the V´eron-Cetty & V´eron (2000) catalog (VV) of 13 213 quasars to investigate their possible physical grouping over angular scales 1000 ≤ ∆θ ≤ 100000. We first estimate the number of quasar pairs that would be expected in VV assuming a random distribution for the quasar positions and taking into account observational selection effects affecting heterogeneous catalogs. We find in VV a statistically significant (>3σ)excessofpairs of quasars with similar redshifts (∆z ≤ 0.01) and angular separations in the 5000−10000 range, corresponding to projected linear separations (0.2−0.5) Mpc/h75(ΩM =1, ΩΛ =0)or(0.4−0.7) Mpc/h75(ΩM =0.3, ΩΛ =0.7). There is also some excess in the 10000−60000 range corresponding to (1−5) Mpc in projected linear separations. If most of these quasar pairs do indeed belong to large physical entities, these separations must represent the inner scales of huge mass concentrations (cf. galaxy clusters or superclusters) at high redshifts; but it is not excluded that some of the pairs may actually consist of multiple quasar images produced by gravitational lensing. Of course, a fraction of these pairs could also arise due to random projections of quasars on the sky. The list of 11 pairs of quasars with redshift differences ∆z ≤ 0.02 and angular separations 5000 ≤ ∆θ ≤ 10000 is presented in order to stimulate further observational studies and to better understand the astrophysical and cosmological significance of these interesting objects. Key words. quasars – clusters – gravitational lenses – observations 1. Introduction over several Mpc linear scales, based upon an original sta- tistical analysis of the V´eron-Cetty & V´eron (1984) cat- Close pairs of quasars with small angular separations (typ- alog, which included at that time 2251 quasars. This has ≤ 00 ≤ ically ∆θ 10 ) and small redshift differences (∆z been later confirmed by correlation function analysis with 0.01) have attracted much attention in recent years be- smaller but homogeneous datasets (Andreani & Cristiani cause of their possible relevance to a number of challenging 1992; Mo & Fang 1993; Croom & Shanks 1996). astrophysical and cosmological problems (e.g. Kochanek et al. 1999). Identification of multiply imaged quasars It is obvious that alike for the case of close pairs of quasars, the identification of very large separation ones in optical and radio surveys has frequently been used (with ∆θ ∈ [1000−100000]) would have very challenging as- to set interesting constraints on cosmological parameters, trophysical and cosmological implications. Indeed, such including the Hubble constant (cf. the recent review by physical pairs of quasars could be associated with high Claeskens & Surdej 2001). However, little is known about redshift galaxy clusters and provide interesting clues on the statistics of physical pairs of quasars or of multiply im- 00 the origin and early evolution of clusters, dark matter, aged lensed quasars with angular separations ∆θ>10 , etc. Similarly, setting more stringent observational limits which would probe lensing masses over scales of galaxy clusters, or even superclusters. Maoz et al. (1997) have car- on the frequency of gravitational lensing at very large an- gular scales would enable us to rule out the existence of ried out a survey with HST for very large image separation galaxy clusters with very large M/L ratio, to set upper lensed quasars, but they failed to detect any secondary limits on the central density of clusters and superclusters, image near each of their 76 primarily selected quasars 00 00 to reject certain proposed cosmogonic models and to set with separations in the 7 –50 range. On the other hand, an independent upper limit on the cosmological constant Shaver (1984) has pointed out an excess of quasar pairs (Kochanek 1995; Wambsganss et al. 1995). Send offprint requests to: J. Surdej, In the present paper, we show that in the recent ver- e-mail: [email protected] sion of the V´eron-Cetty & V´eron catalog (2000, hereafter ? Also, Research Director at the National Research Science VV) the number Np of quasar pairs with similar red- Foundation, Belgium. shift (∆z ≤ 0.01) deviates significantly, within certain Article published by EDP Sciences and available at http://www.aanda.org or http://dx.doi.org/10.1051/0004-6361:20010283 2 V. I. Zhdanov and J. Surdej: Quasar pairs with arcminute angular separations specific angular separation intervals, from the number number Nexp from large angular separations to smaller Nexp that would be expected assuming a random distribu- ones. Similarly, a realistic estimate of the dispersion tion of quasars over the sky. Therefore, in these intervals, σ(Nexp) is necessary to check for possible deviations from there is an excess NPP = Np − Nexp of pairs that may ei- Poisson statistics, which could arise from hidden selection ther be caused by statistical fluctuations or consist of very effects. Estimates of Nexp and σ(Nexp) are made possi- interesting genuine physical pairs of quasars, or, possibly, ble thanks to the very large number of entries available doubly imaged quasars. in VV. We do this by considering the pairs of quasars Because the VV catalog is heterogeneous, it is impor- with small but definite redshift differences from 10 selected tant to obtain a reliable estimate of Nexp in spite of the comparison bins (i.e. ∆z ∈ [0.03, 0.04], ..., [0.12, 0.13]) and various unphysical selection effects (cf. different survey ar- the pairs of quasars with ∆z ≤ 0.01 and angular sepa- eas, limiting magnitudes, ...) that may accordingly affect rations ∆θ>500000 (our investigation for ∆θ>100000 the two-point correlation function. Shaver (1984) has pro- yields similar results). For both these sets of quasar pairs, posed to take into account such selection effects by us- no physical grouping is expected between the quasars (cf. ing information from the distribution of quasar pairs with Andreani & Cristiani 1992; Mo & Fang 1993; Croom & different, but nevertheless close redshifts, that may only Shanks 1996) and we may then use this information to es- 00 arise due to random coincidences. In our case, thanks to timate Nexp and σ(Nexp)for∆z ≤ 0.01 and ∆θ ≤ 1000 . the very large number of entries available in VV, it is The results are presented in terms of the pair numbers possible to supplement Shaver’s method with estimates of N(θ, ξ) having an angular separation θ ≤ ∆θ<θ+ δ, δ Nexp and their dispersions. We have applied this improved representing the width of the ring with angular radius θ, method to VV and report here on some interesting results and a redshift difference ξ ≤ ∆z<ξ+ ζ.Intheremain- in the 5000−60000 angular separation interval. der, we use the normalized pair numbers defined per unit interval of ζ and per unit area of the ring N(θ ≤ ∆θ<θ+ δ, ξ ≤ ∆z<ξ+ ζ) 2. Number estimates of quasar pairs n(θ, ξ)= · (1) 2πζδ(θ + δ/2) 2.1. Basic assumptions and conventions Using the data from 10 equal comparison bins (ξ =0.03+ In the remainder, we shall distinguish the random pairs kζ; k =0, 1, ..., 9; ζ =0.01), we estimate the average pair (RP) of quasars, arising from random positions over the number na = <n(θ, ξ) >ξ and rms deviation from this sky, from the physical pairs (PP) of quasars. The lat- average for every angular separation interval. ters have some physical relation which renders their dis- Our choice to discretize the redshifts in bins having a tribution over the sky different from a random one. PP width ∆z =0.01 prevents us to deal with the problem of quasars may be members of some clusters or they may redshift measurement errors, roundoffs in VV or peculiar have evolved from some common mass concentrations. We velocities which are always smaller than 0.01. also include into PP possible doubly imaged quasars pro- We define the normalized number of quasar pairs from duced by gravitational lensing. Our aim is to compare VV with approximately similar redshifts as np = n(θ, 0), the expected number of RP with similar redshifts with adopting ζ =0.01. We assume that PP may contribute 00 the observed number of pairs in VV at various angular to np up to 5000 , at maximum, but that they do not separations. contribute to nr = n(θ, ξ ≥ 0.03), neither to np for 00 00 In order to study the physical pairing of quasars, ∆θ ∈ [5000 , 20 000 ]. Accordingly, we denote nexp the Shaver (1984) has considered, for given angular separation expected value of n(θ, 0) that would be in VV, provided intervals, the ratio of the number of quasar pairs having PP were absent. Following Shaver (1984), we also assume nearly the same redshifts (e.g. ∆z ≤ 0.01, which may in- that for small values of ξ, the distribution of nr must bear clude physical pairs) to that with different, but neverthe- the same imprint of non physical effects as np. It should less close redshifts (e.g. 0.03 ≤ ∆z ≤ 0.2) that may only be noted that the values of nexp, nr and their dispersions arise due to random coincidences. need not necessarily be the same (though they appear Because several tens of multiply imaged quasars have to be rather close) because of the very irregular depen- been reported with angular separations ∆θ<1000 dence of the number counts of quasars with redshift, but (Claeskens & Surdej 2001) and since our search in the their dependence upon angular separations must be simi- VV catalog has not revealed any single pair with 1000 ≤ lar.
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