The population of weak Mg II absorbers. I. A survey of 26 QSO HIRES/Keck spectra

We present a search for "weak" Mg II absorbers [those with W-r(2796) < 0.3 Angstrom] in the HIRES/Keck spectra of 26 QSOs. We found 30, of which 23 are newly discovered. The spectra are 80% complete to W-r(2796) = 0.02 Angstrom and have a cumulative redshift path of similar to 17.2 for the redshift range 0.4 less than or equal to z less than or equal to 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-r(2796) < 0.3 Angstrom sample. The equivalent width distribution follows a power law, N(W) proportional to W-delta, With delta similar to 1.0; there is no turnover down to W,(2796)= 0.02 Angstrom 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 similar to 5% of the Ly alpha forest clouds with W-r(Ly alpha) greater than or equal to 0.1 Angstrom will have detectable Mg II absorption to W-r(min)(2796) = 0.02 Angstrom and that this is primarily a high-metallicity selection effect ([Z/Z(.)] greater than or equal to -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-r(2796) greater than or equal to 0.02 Angstrom absorbers with that of both high and low surface brightness galaxies and find a fiducial absorber size of 35 h(-1)-63 h(-1) kpc, depending upon the assumed galaxy population and their absorption properties. The individual absorbing "clouds" have W-r(2796) less than or equal to 0.15 Angstrom, and their narrow (often unresolved) line widths imply temperatures of similar to 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.