The Chemical and Ionization Conditions in Weak Mg II Absorbers

We present an analysis of the chemical and ionization conditions in a sample of 100 weak Mg II absorbers identified in the VLT/UVES archive of quasar spectra. In addition to Mg II, we present equivalent width and column density measurements of other low ionization species such as Mg I, Fe II, Al II, C II, Si II, and also Al III. We find that the column densities of C II and Si II are strongly correlated with the column density of Mg II, with minimal scatter in the relationships. The column densities of Fe II exhibit an appreciable scatter when compared with the column density of Mg II, with some fraction of clouds having N(Fe II) ~ N(Mg II), in which case the density is constrained to n_H > 0.05 cm^-3. Other clouds in which N(Fe II) ll N(Mg II) have much lower densities. From ionization models, we infer that the metallicity in a significant fraction of weak Mg II clouds is constrained to values of solar or higher, if they are sub-Lyman-limit systems. Based on the observed constraints, we hypothesize that weak Mg II absorbers are predominantly tracing two different astrophysical processes/structures. A significant population of weak Mg II clouds, those in which N(Fe II) ll N(Mg II), identified at both low (z ~ 1) and high (z ~ 2) redshift, are likely to be tracing gas in the extended halos of galaxies, analogous to the Galactic high-velocity clouds. These absorbers might correspond to α-enhanced interstellar gas expelled from star-forming galaxies, in correlated supernova events. The N(Mg II) and N(Fe II)/N(Mg II) in such clouds are also closely comparable to those measured for the high-velocity components in strong Mg II systems. An evolution is found in N(Fe II)/N(Mg II) from z = 2.4 to z = 0.4, with an absence of weak Mg II clouds with N(Fe II) ~ N(Mg II) at high-z. The N(Fe II) ~ N(Mg II) clouds, which are prevalent at lower redshifts (z < 1.5), must be tracing Type Ia enriched gas in small, high-metallicity pockets in dwarf galaxies, tidal debris, or other intergalactic structures.