The largest known structure in the high redshift universe is mapped by at least 18 quasars and spans \sim 5 ^ { \circ } \times 2.5 ^ { \circ } on the sky , with a quasar spatial overdensity of 6–10 times above the mean .
This large quasar group provides an extraordinary laboratory \sim 100 \times 200 \times 200 h ^ { -3 } comoving Mpc ^ { 3 } in size ( q _ { 0 } = 0.5 , \Lambda = 0 , H _ { 0 } = 100 h km s ^ { -1 } Mpc ^ { -1 } ) covering 1.20 < z < 1.39 in redshift .
One approach to establish 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 \sim 2.5 ^ { \circ } \times 2.5 ^ { \circ } subfield in the large quasar group , and found 38 absorbers to a rest equivalent width limit of W _ { 0 } = 0.3 Å over 0.69 < z < 2.02 .
Only 24 absorbers were expected , thus we find a 2 \sigma 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 \sim 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 | \Delta v| = 3000 to 4500 km s ^ { -1 } .
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 .