2006ApJ...640...81L

The exact nature of weak Mg II absorbers [those with W_r(2796)<0.3 Å] is a matter of debate, but most are likely related to areas of local star formation or supernova activity outside of giant galaxies. Using 18 QSO spectra obtained with the Ultraviolet and Visual Echelle Spectrograph (UVES) on the Very Large Telescope (VLT), we have conducted a survey for weak Mg II absorbers at 1.4<z<2.4. We searched a redshift path length of Δz=8.51, eliminating regions badly contaminated by atmospheric absorption so that the survey is close to 100% complete to W_r(2796)=0.02 Å. We found a total of nine weak absorbers, yielding a number density of absorbers of dN/dz=1.06±0.12 for 0.02≤W_r(2796)<0.3 Å. Narayanan et al. found dN/dz=1.00±0.20 at 0<z<0.3, and Churchill et al. found dN/dz=1.74±0.10 at 0.4<z<1.4. Therefore, the population of weak Mg II absorbers appears to peak at z∼1. We explore the expected evolution of the absorber population subject to a changing extragalactic background radiation (EBR) from z=0.9 to z=1.78 (the median redshift of our survey), and find that the result is higher than the observed value. We point out that the peak epoch for weak Mg II absorption at z∼1 may coincide with the peak epoch of global star formation in the dwarf galaxy environment.