Mg II Selected Absorbers: Ionization structures and a Survey of Weak Systems

Abstract

First results from a study of high ionization absorption properties in roughly 30 Mg II absorption selected galaxies are presented. We have tested for correlations of Mg II, C IV, Si IV, N V, and O VI equivalent widths with the galaxy properties and Mg II gas kinematics. The results are suggestive of multi-phase halos with little to no global ionization gradient with impact parameter. C IV may arise in both the Mg II - Lyman alpha clouds and a high ionization &#34;halo&#34; traced by O VI. We also report on an unbiased survey for weak Mg II systems using HIRES/Keck spectra. At <z> = 0.9, we find dN/dz = 1.6 +/- 0.1 for 0.02 < W(2796) < 0.3 [A] and that weak systems comprise roughly 65% of all Mg II absorbers. In all but one case, the weak Mg II systems exhibit Fe II absorption with <log N(FeII)/N(MgII)> = -0.3 +/- 0.4 measured for the sample. We suggest that weak Mg II absorbers comprise a substantial yet-to-be explored population. If weak systems select the LSB and/or dwarf galaxy population, then the weakest Mg II absorbers may provide one of the most sensitive tracers of chemical enrichment and evolution of the UV background from z = 2 to z = 0.