A Survey of Weak Mgii Absorbers at 0.4 < z < 2.4 1

We present results from a survey of weak Mg ii absorbers in the VLT/UVES spectra of 81 QSOs obtained from the ESO archive. In this survey, we identified 112 weak Mg ii systems within the redshift interval 0.4 < z < 2.4 with 86 % completeness down to a rest-frame equivalent width of Wr(2796) = 0.02 Å, covering a cumulative redshift path length of ∆Z ∼ 77.3. From this sample, we estimate that the number of weak absorbers per unit redshift (dN/dz) increases from 1.06 ± 0.04 at < z >= 1.9 to 1.76 ± 0.08 at < z >= 1.2 and thereafter decreases to 1.51 ± 0.09 at < z >= 0.9 and 1.06 ± 0.10 at < z >= 0.6. Thus we find evidence for an evolution in the population of weak Mg ii absorbers, with their number density peaking at z = 1.2. We also determine the equivalent width distribution of weak systems at < z >= 0.9 and < z >= 1.9. At 0.4 < z < 1.4, there is evidence for a turnover from a powerlaw of the form n(Wr) ∝ W −1.04 r at Wr(2796) < 0.1 Å. This turnover is more extreme at 1.4 < z < 2.4, where the equivalent width distribution is close to an extrapolation of the exponential distribution function found for strong Mg ii absorbers. Based on these results, we discuss the possibility that some fraction of weak Mg ii absorbers, particularly single cloud systems, are related to satellite clouds surrounding strong Mg ii systems. These structures could also be analogs to Milky Way high velocity clouds. In this context, the paucity of high redshift weak Mg ii absorbers is caused by a lack of isolated accreting clouds on to galaxies during that epoch.