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We present results from a survey of weak MgII absorbers in the VLT/UVESspectra of 81 QSOs obtained from the ESO archive. In this survey, we identified112 weak MgII systems within the redshift interval 0.4 < z < 2.4 with 86%completeness down to a rest-frame equivalent width of W_r(2796) = 0.02A,covering a cumulative redshift path length of deltaZ=77.3. From this sample, weestimate that the number of weak absorbers per unit redshift dN/dz increasesfrom 1.06 +/- 0.04 at <z>=1.9 to 1.76 +/- 0.08 at <z>=1.2 and thereafterdecreases to 1.51 +/- 0.09 at <z>=0.9 and 1.06 +/- 0.10 at <z>=0.6. Thus wefind evidence for an evolution in the population of weak MgII absorbers, withtheir number density peaking at z=1.2. We also determine the equivalent widthdistribution of weak systems at <z>=0.9 and <z>=1.9. At 0.4 < z < 1.4, there isevidence for a turnover from a powerlaw of the form n(W_r) \propto W_r^{-1.04}at W_r(2796) < 0.1A. This turnover is more extreme at 1.4 < z < 2.4, where theequivalent width distribution is close to an extrapolation of the exponentialdistribution function found for strong MgII absorbers. Based on these results,we discuss the possibility that some fraction of weak MgII absorbers,particularly single cloud systems, are related to satellite clouds surroundingstrong MgII systems. These structures could also be analogs to Milky Way highvelocity clouds. In this context, the paucity of high redshift weak MgIIabsorbers is caused by a lack of isolated accreting clouds on to galaxiesduring that epoch.Comment: 14 pages, 11 figures, ApJ accepte

A Survey of Weak MgiiAbsorbers at 0.4 <z< 2.4