The Population of Weak Mg ii Absorbers. I. A Survey of 26 QSO HIRES/Keck Spectra

We present a search for "weak" MgII absorbers those with Wᵣ (2796) 0. 1 A will have detectable MgII absorption to Wᵣ, min (2796) = 0. 02 A and that this is primarily a high-metallicity selection effect (Z/Zₛun > -1). This implies that MgII absorbing structures figure prominently as tracers of sub-LLS environments where gas has been processed by stars. We compare the number density of Wᵣ (2796) > 0. 02 A absorbers with that of both high and low surface brightness galaxies and find a fiducial absorber size of 35h^-1 to 63h^-1 kpc, depending upon the assumed galaxy population and their absorption properties. The individual absorbing "clouds" have Wᵣ (2796) <= 0. 15 A and their narrow (often unresolved) line widths imply temperatures of ~25, 000 K. We measured Wᵣ (1548) from CIV in FOS/HST archival spectra and, based upon comparisons with FeII, found a range of ionization conditions (low, high, and multi-phase) in absorbers selected by weak MgII.