A Survey of MgII Absorption at 2 < z < 6 with Magellan / FIRE: I: Sample
  and Evolution of the MgII Frequency
  We present initial results from the first systematic survey for MgII quasar
absorption lines at z > 2.5. Using infrared spectra of 46 high-redshift
quasars, 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 MgII
system 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, while
that for stronger systems increases three-fold until z \sim 3 before declining
again towards higher redshifts. The equivalent width distribution, which fits
an exponential, reflects this evolution by flattening as z approaches 3 before
steepening again. The rise and fall of the strong absorbers suggests a
connection to the star formation rate density, as though they trace galactic
outflows or other byproducts of star formation. The weaker systems' lack of
evolution does not fit within this interpretation, but may be reproduced by
extrapolating low redshift scaling relations between host galaxy luminosity and
absorbing halo radius to earlier epochs. For the weak systems,
luminosity-scaled models match the evolution better than similar models based
on MgII occupation of evolving CDM halo masses, which greatly underpredict
dN/dz at early times unless the absorption efficiency of small haloes is
significantly larger in the early universe. Taken together, these observations
suggest that the general structure of MgII-bearing haloes was put into place
early in the process of galaxy assembly. Except for a transient appearance of
stronger systems near the peak epoch of cosmic star formation, the basic
properties of MgII absorbers have evolved fairly little even as the
(presumably) associated galaxy population grew substantially in stellar mass
and half light radius.
