2002ApJ...565..743R

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.

Weak systems account for somewhere between 25% and 100% of the z<1 Lyα forest clouds in the range 10^15.8cm^–2≤N(HI)≤10^16.8 cm^–2.

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).

We identify a subset of weak Mg II absorbers, those with logN(FeII)/N(MgII)>-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(FeII)/N(MgII) 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.