Program: GS-2009B-Q-92

Large Quasar Groups (LQGs) are the largest structures seen in the early universe. We have discovered *four* new LQGs at z ~ 2, which are the highest redshift LQGs found in the contemporary, well-defined surveys. We anticipate that they will be the sites of early clusters and active star-formation. We propose to investigate these LQGs further by identifying the MgII 2796, 2803 doublet in the spectra of background quasars. MgII absorbers with rest-frame equivalent width W0(2796) > 0.3 Ang. are associated with galaxies to at least z ~ 2.5. The objectives are as follows. (1) A significant excess of MgII absorbers compared with published data for non-LQG fields will show that the enhancement in the density of quasars is indeed associated with a corresponding general enhancement of the mass (galaxies). (2) The MgII absorbers will allow us to estimate directly the fraction of the mass that is contained in LQGs at z ~ 2 for comparison with simulations of the largest structures. (3) The positions of the MgII absorbers should be productive sites for finding the highest-z clusters. Time has been awarded in 2009A for one of these LQGs (LGG03) and the observations proposed here are for two (LQG01, LQG02) of the other three. Given the recent indication that LQGs can correlate on still larger scales, the proximity of LQG01 and LQG02 (~ 330 Mpc) makes them intriguing targets.