2002ApJ...578..708W

The largest known structure in the high-redshift universe is mapped by at least 18 quasars and spans ∼5°x2°.5 on the sky, with a quasar spatial overdensity of 6-10 times above the mean. This large quasar group provides an extraordinary laboratory ∼100x200x200 h^–3 comoving Mpc³ in size (q₀=0.5, Λ=0, H₀=100 h km.s^–1 Mpc^–1) covering 1.20<z<1.39 in redshift. One approach to establishing how large quasar groups relate to mass (galaxy) enhancements is to probe their gas content and distribution via background quasars. We performed a survey for Mg II absorption systems in a ∼2°.5x2°.5 subfield in the large quasar group and found 38 absorbers to a rest equivalent width limit of W₀=0.3 Å over 0.69<z<2.02. Only 24 absorbers were expected; thus, we find a 2 σ overdensity over all redshifts in our survey. We have found the large quasar group to be associated with 11 Mg II absorption systems at 1.2<z<1.4; 0.02%-2.05% of simulations with random Mg II redshifts match or exceed this number in that redshift interval, depending on the normalization method used. The minimal spanning tree test also supports the existence of a structure of Mg II absorbers coincident with the large quasar group and additionally indicates a foreground structure populated by Mg II absorbers and quasars at z∼0.8. Finally, we find a tendency for Mg II absorbers over all redshifts in our survey to correlate with field quasars (i.e., quasars both inside and outside of the large quasar group) at a projected scale length on the sky of 9 h^–1 Mpc and a velocity difference|Δv|=3000-4500 km.s^–1. While the correlation is on a scale consistent with observed galaxy-active galactic nucleus distributions, the nonzero velocity offset could be due to the ``periphery effect'', in which quasars tend to populate the outskirts of clusters of galaxies and metal absorption systems, or to peculiar velocity effects.