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The Population of Weak MgII Absorbers. II The Properties of Single-Cloud Systems

We present an investigation of MgII absorbers characterized as single-cloud weak systems at z~1. We measured column densities and Doppler parameters for MgII and FeII in 15 systems found in HIRES/Keck spectra at 6.6 km/s. Using these quantities and CIV, Lyman alpha and Lyman limit absorption observed with FOS/HST (resolution ~230 km/s) we applied photoionization models to each system to constrain metallicities, densities, ionization conditions, and sizes. We find that: (1) Single-cloud weak systems are optically thin in neutral hydrogen and may have their origins in a population of objects distinct from the optically thick strong MgII absorbers, which are associated with bright galaxies. (2) Weak systems account for somewhere between 25% to 100% of the z < 1 Lyman alpha forest clouds in the range 15.8<log N(HI)<16.8 cm^-2. (3) At least seven of 15 systems have two or more ionization phases of gas (multiphase medium). (4) We identify a subset of weak MgII absorber that we term ``iron-rich''. These clouds are not alpha-group enhanced and are constrained to have sizes of ~10 pc. At that size, to produce the observed redshift path density, they would need to outnumber L* galaxies by approximately six orders of magnitude. We discuss these results and the implications that the iron-rich systems require enrichment from Type Ia supernovae. Further, we address how star clusters or supernova remnants in dwarf galaxies might give rise to absorbers with the inferred properties. This would imply far larger numbers of such objects than are presently known, even locally. We compare the weak systems to the weak kinematic subsystems in strong MgII absorbers and to Galactic high velocity clouds. (abridged)