A survey of weak Mgii absorbers at redshift 〈z 〉 = 1.78 1

The exact nature of weak Mg ii absorbers (those with Wr(2796) < 0.3 Å) is a matter of debate, but most are likely related to areas of local star formation or supernovae activity outside of giant galaxies. Using 18 QSO spectra obtained with the Ultra-Violet 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 ∆z = 8.51, eliminating regions badly contaminated by atmospheric absorption so that the survey is close to 100% complete to Wr(2796) = 0.02 Å. We found a total of 9 weak absorbers, yielding a number density of absorbers of dN/dz = 1.06±0.12 for 0.02 ≤ Wr(2796) < 0.3 Å. Narayanan et al. (2005) found dN/dz = 1.00± 0.20 at 0 < z < 0.3 and Churchill et al. (1999) 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. Subject headings: intergalactic medium — quasars: absorption lines Based on public data obtained from the ESO archive of observations from the UVES spectrograph at the VLT, Paranal, Chile. Department of Astronomy and Astrophysics, The Pennsylvania State University, University Park, PA 16802, rlynch, [email protected] Institute of Astronomy, Madingley Road, Cambridge Astrophysikalisches Institut Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany, [email protected]