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 ( \hbox { { Fe~ { } } \kern 1.0 pt { \sc ii } } ) \sim N ( \hbox { { Mg~ { } } \kern 1.0 pt { % \sc ii } } ) , in which case the density is constrained to n _ { \hbox { { H } } } > 0.05 cm ^ { -3 } .
Other clouds in which N ( \hbox { { Fe~ { } } \kern 1.0 pt { \sc ii } } ) < < N ( \hbox { { Mg~ { } } \kern 1.0 pt { \sc 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 ( \hbox { { Fe~ { } } \kern 1.0 pt { \sc ii } } ) < < N ( \hbox { { Mg~ { } } \kern 1.0 pt { \sc ii% } } ) , identified at both low ( z \sim 1 ) and high ( z \sim 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 \alpha -enhanced interstellar gas expelled from star-forming galaxies , in correlated supernova events .
The N ( \hbox { { Mg~ { } } \kern 1.0 pt { \sc ii } } ) and N ( \hbox { { Fe~ { } } \kern 1.0 pt { \sc ii } } ) / N ( \hbox { { Mg~ { } } \kern 1.0 pt { \sc 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 ( \hbox { { Fe~ { } } \kern 1.0 pt { \sc ii } } ) / N ( \hbox { { Mg~ { } } \kern 1.0 pt { \sc ii% } } ) from z = 2.4 to z = 0.4 , with an absence of weak Mg ii clouds with N ( \hbox { { Fe~ { } } \kern 1.0 pt { \sc ii } } ) \sim N ( \hbox { { Mg~ { } } \kern 1.0 pt { % \sc ii } } ) at high- z .
The N ( \hbox { { Fe~ { } } \kern 1.0 pt { \sc ii } } ) \sim N ( \hbox { { Mg~ { } } \kern 1.0 pt { % \sc 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 .