The population of weak Mg II absorbers. II. The properties of single-cloud systems

We present an investigation of Mg II absorbers characterized as single-cloud "weak systems" [denned by W_r(λ2796) < 0.3 Å] at z ∼ 1. We measured column densities and Doppler parameters for Mg II and Fe II in 15 systems found in High Resolution Echelle Spectrometer/Keck spectra at 6.6 km s^-1. Using these quantities and C IV, Lyα, and Lyman limit absorption observed with the Faint Object Spectrograph on the Hubble Space Telescope (resolution ∼ 230 km s^-1), we applied photoionization models to each system to constrain metallicities, densities, ionization conditions, and sizes. We find the following: 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 Mg II absorbers, which are associated with bright galaxies. 2. Weak systems account for somewhere between 25% and 100% of the z < 1 Lyα forest clouds in the range 10^15.8 cm^-2 ≤. N(H I) ≤ 10^16.8 cm^-2. 3. At least seven of the 15 systems have two or more ionization phases of gas (multiphase medium). The first is the low-ionization, kinematically simple Mg II phase and the second is a high-ionization C IV phase, which is usually either kinematically broadened or composed of multiple clouds spread over several tens of kilometers per second. This higher ionization phase gives rise to the majority of the Lyα absorption strength (equivalent width), although it often accounts for a minor fraction of a system's N(H I). 4. We identify a subset of weak Mg II absorbers, those with log [N(Fe II)/N(Mg II)] > -0.3, which we term "iron-rich." Although there are only three of these objects in our sample, their properties are the best constrained because of their relatively strong Fe II detections and the sensitivity of the N(Fe II)/N(Mg II) ratio. These clouds are not α-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 6 orders of magnitude. The clouds with undetected iron do not have well-constrained sizes; we cannot infer whether they are enhanced in their α-process elements. We discuss these results and the implications that the weak Mg II systems with detected iron absorption require enrichment from Type Ia supernovae. Furthermore, 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 Mg II absorbers and to Galactic high-velocity clouds. Although weak systems could be high-velocity clouds in small galaxy groups, their neutral hydrogen column densities are insufficient for them to be direct analogues of the Galactic high-velocity clouds.