  The largest known structure in the high redshift universe is mapped by at
least 18 quasars and spans ~5 deg x 2.5 deg on the sky, with a quasar spatial
overdensity of 6-10 times above the mean. This large quasar group provides an
extraordinary laboratory ~100 x 200 x 200 h^-3 comoving Mpc^3 in size (q0=0.5,
Lambda=0, H0=100h km/s/Mpc) covering 1.20<z<1.39 in redshift. One approach to
establish how LQGs relate to mass (galaxy) enhancements is to probe their gas
content and distribution via background quasars. We have found the large quasar
group to be associated with 11 MgII absorption systems at 1.2<z<1.4;
0.02%--2.05% of simulations with random MgII 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
MgII absorbers coincident with the LQG, and additionally indicates a foreground
structure populated by MgII absorbers and quasars at z~0.8. Finally, we find a
tendency for MgII absorbers in general to correlate with field quasars (i.e.
quasars both inside and outside of the LQG) at a projected scale length on the
sky of 9/h Mpc and a velocity difference |Delta v|=3000 to 4500 km/s. While the
correlation is on a scale consistent with observed galaxy-AGN 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.
