Precious Metals in SDSS Quasar Spectra II: Tracking the Evolution of
  Strong, 0.4 < z < 2.3 MgII Absorbers with Thousands of Systems
  We have performed an analysis of over 34,000 MgII doublets at 0.36 < z < 2.29
in Sloan Digital Sky Survey (SDSS) Data-Release 7 quasar spectra; the catalog,
advanced data products, and tools for analysis are publicly available. The
catalog was divided into 14 small redshift bins with roughly 2500 doublets in
each, and from Monte-Carlo simulations, we estimate 50% completeness at rest
equivalent width W_r ~ 0.8 Angstrom. The equivalent-width frequency
distribution is described well by an exponential model at all redshifts, and
the distribution becomes flatter with increasing redshift, i.e., there are more
strong systems relative to weak ones. Direct comparison with previous SDSS MgII
surveys reveal that we recover at least 70% of the doublets in these other
catalogs, in addition to detecting thousands of new systems. We discuss how
these surveys come by their different results, which qualitatively agree but,
due to the very small uncertainties, differ by a statistically significant
amount. The estimated physical cross-section of MgII-absorbing galaxy halos
increased three-fold, approximately, from z = 0.4 --> 2.3, while the W_r >= 1
Angstrom absorber line density grew, dN_MgII/dX, by roughly 45%. Finally, we
explore the different evolution of various absorber populations - damped
Lyman-alpha absorbers, Lyman-limit systems, strong CIV absorbers, and strong
and weaker MgII systems - across cosmic time (0 < z < 6).
