1999ApJS..120...51C

We present a search for ``weak'' Mg II absorbers [those with W<sub>r</sub>(2796)<0.3 Å] in the HIRES/Keck spectra of 26 QSOs. We found 30, of which 23 are newly discovered. The spectra are 80% complete to W<sub>r</sub>(2796)=0.02 Å and have a cumulative redshift path of ∼17.2 for the redshift range 0.4≤z≤1.4. The number of absorbers per unit redshift, dN/dz, is seen to increase as the equivalent width threshold is decreased; we obtained dN/dz=1.74±0.10 for our 0.02 W<sub>r</sub>(2796)<0.3 Å sample. The equivalent width distribution follows a power law, N(W)∝W<sup>–δ</sup>, with δ∼1.0; there is no turnover down to W<sub>r</sub>(2796)=0.02 Å at <z≥0.9. Weak absorbers comprise at least 65% of the total Mg II absorption population, which outnumbers Lyman limit systems (LLSs) by a factor of 3.8±1.1; the majority of weak Mg II absorbers must arise in sub-LLS environments. Tentatively, we predict that ∼5% of the Lyα forest clouds with W<sub>r</sub>(Lyα)≥0.1 Å will have detectable Mg II absorption to W<sup>min</sup><sub>r</sub>(2796)=0.02 Å and that this is primarily a high-metallicity selection effect ([Z/Z<sub>☉</sub>]≥-1). This implies that Mg II absorbing structures figure prominently as tracers of sub-LLS environments where gas has been processed by stars. We compare the number density of W<sub>r</sub>(2796)≥0.02 Å absorbers with that of both high and low surface brightness galaxies and find a fiducial absorber size of 35 h<sup>–1</sup>-63 h<sup>–1</sup> kpc, depending upon the assumed galaxy population and their absorption properties. The individual absorbing ``clouds'' have W_r(2796)≤0.15 Å, and their narrow (often unresolved) line widths imply temperatures of ∼25,000 K. We measured W_r(1548) from C IV in Faint Object Spectrograph/Hubble Space Telescope archival spectra and, based upon comparisons with Fe II, found a range of ionization conditions (low, high, and multiphase) in absorbers selected by weak Mg II.