Galaxy Clusters in the Line of Sight to Background Quasars. I. Survey Design and Incidence of MgiiAbsorbers at Cluster Redshifts

Abstract

Quasar absorption line systems are redshift-independent sensitive mass tracers. Here we describe the first optical survey of absorption systems associated with galaxy clusters at z = 0.3 − 0.9. We have cross-correlated quasars from the third data release of the Sloan Digital Sky Survey with high-redshift cluster/group candidates from the Red-Sequence Cluster Survey. In a common field of ≈ 20 square degrees, we have found 442 quasar-cluster pairs for which the Mg II λλ2796, 2803Å doublet might be detected at a transverse (physical) distance < 2 h −1 71 Mpc from the cluster centers. In addition, we have found 33 other pairs in the literature and we have discovered 7 new quasars with foreground clusters. To investigate the incidence (dN/dz) and equivalent-width distribution n(W ) of Mg II systems at cluster redshifts, two statistical samples were drawn out of these pairs: one made of high-resolution spectroscopic quasar observations (46 pairs), and one made of quasars used in Mg II searches found in the literature (375 pairs). The total redshift path from an ad-hoc definition of 'cluster redshift path' is ∆z cluster = 6.3 and ∆z cluster = 57.0 for the two samples, respectively. We estimate the completeness level to be nearly 100 % for W detection thresholds of W 2796 0 > 0.05 and W 2796 0 > 1.0Å in the two samples, respectively. The results are: (1) the population of strong Mg II systems (W 2796 0 > 2.0Å) near cluster redshifts shows a significant (> 3σ) overabundance (up to a factor of 15) when compared with the 'field' population; (2) the overabundance is more evident at smaller distances (d < 1 h −1 71 Mpc) than larger distances (d < 2 h −1 71Mpc) from the cluster center; and, (3) the population of weak Mg II systems (W 2796 0 < 0.3Å) near cluster redshifts conform to the field statistics. Unlike in the field, this dichotomy makes n(W ) in clusters appear flat and well fitted by a power-law in the entire W -range. We assess carefully all possible selection and systematic effects, and conclude that the signal is indeed due to the presence of clusters. In particular, a sub-sample of the most massive clusters yields a stronger and still significant signal. Since either the absorber number density or fillingfactor/cross-section affects the absorber statistics, an interesting possibility is that we have detected the signature of truncated halos due to environmental effects. Thus, we argue that the excess of strong systems is due to a population of absorbers in an overdense galaxy environment, and the lack of weak systems to a different population, that got destroyed in the cluster environment.Finally, comparison with models of galaxy counts show that there is proportionally less cold gas in more massive clusters than in low-mass systems, and two orders of magnitude less Mg II cross-section due to weak systems than due to stronger systems. Zibetti et al. 2007) for which extensive field surveys exist. The Mg II doublet has been used extensively in spectroscopic quasar surveys because it is a strong and an easy-to-find t...

4098 galaxy clusters to z∼ 0.6 in the Sloan Digital Sky Survey equatorial Stripe 82

“…A powerful observational technique is the exploitation of continuum‐bright background sources to search for spectroscopic evidence of absorption of continuum light by intervening intergalactic/intracluster material. For example, Lopez et al (2008) present a study of 442 cluster–quasar pairs (sightlines) within the Red Sequence Cluster Survey (RCS; Gladders & Yee 2000, 2005) where the Mg ii λλ2796, 2803 doublet could be detected at 0.3 < z < 0.9. The study of the distribution of the equivalent width of absorption‐line systems within clusters could provide a means of studying environmental effects such as gas‐stripping in dense environment.…”

Large-scale structure in absorption: gas within and around galaxy voids

“…Borgani et al ; Padilla et al ) and observed (e.g. Lewis et al ; Lopez et al ; Padilla, Lambas & González ) to have non‐negligible effects on the gas and galaxy properties. Given that baryonic matter is expected to fall into the considerably deeper gravitational potentials of dark matter, the IGM gas and galaxies are expected to be predominantly found at such locations forming the so‐called ‘cosmic web’ (Bond, Kofman & Pogosyan ).…”

Galaxy group at z=0.3 associated with the damped Lyman α system towards quasar Q1127-145

“…In contrast, the d N /d z of weak Mg ii systems [ W r (2796) < 0.3Å] in clusters is consistent with those derived from environmentally unbiased samples. Lopez et al (2008) argue that the detected over‐abundance of strong systems is a result of the over‐density of galaxies in a cluster region. The lack of an over‐abundance of weak systems may imply that they were destroyed by the cluster environment.…”

“…Padilla et al (2009) modelled these results and found environmental evidence of truncated Mg ii halo sizes as a function of cluster radii. Their models require a median Mg ii halo size of r < 10 h −1 kpc, compared to 35–85 h −1 kpc for field galaxies, in order to reproduce the observed absorption‐line statistics of Lopez et al (2008). These X‐ray‐selected clusters represent more extreme environments than in galaxy groups where the majority of galaxies reside.…”