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We present initial results from the first systematic survey for MgII quasarabsorption lines at z > 2.5. Using infrared spectra of 46 high-redshiftquasars, we discovered 111 MgII systems over a path covering 1.9 < z < 6.3.Five systems have z > 5, with a maximum of z = 5.33 - the most distant MgIIsystem now known. The comoving MgII line density for weaker systems (Wr < 1.0A)is statistically consistent with no evolution from z = 0.4 to z = 5.5, whilethat for stronger systems increases three-fold until z \sim 3 before decliningagain towards higher redshifts. The equivalent width distribution, which fitsan exponential, reflects this evolution by flattening as z approaches 3 beforesteepening again. The rise and fall of the strong absorbers suggests aconnection to the star formation rate density, as though they trace galacticoutflows or other byproducts of star formation. The weaker systems' lack ofevolution does not fit within this interpretation, but may be reproduced byextrapolating low redshift scaling relations between host galaxy luminosity andabsorbing halo radius to earlier epochs. For the weak systems,luminosity-scaled models match the evolution better than similar models basedon MgII occupation of evolving CDM halo masses, which greatly underpredictdN/dz at early times unless the absorption efficiency of small haloes issignificantly larger in the early universe. Taken together, these observationssuggest that the general structure of MgII-bearing haloes was put into placeearly in the process of galaxy assembly. Except for a transient appearance ofstronger systems near the peak epoch of cosmic star formation, the basicproperties of MgII absorbers have evolved fairly little even as the(presumably) associated galaxy population grew substantially in stellar massand half light radius.

A Survey of MgII Absorption at 2 < z < 6 with Magellan / FIRE: I: Sample and Evolution of the MgII Frequency