The Detection of Two Distinct High Ionization States in a QSO Lyman Limit Absorption System: Evidence for Hierarchical Galaxy Formation at z ~ 3?

Abstract

We have detected two high ionization phases of gas in the z ~ 2.77 partial Lyman limit system (LLS) towards QSO 1157-3143. We detect the first phase by CIV and SiIV absorption, and the second phase -- which is either warmer or undergoing larger random bulk motions than the first -- via OVI absorption. Both phases of gas are present in similar column density ratios in each of the 5 velocity components, making it appear that this LLS is constructed of 5 very similar building blocks. We find that this system displays some of the properties expected of a hierarchical merging event, although published models may have trouble explaining the SiIV absorption we observe. When different ions show similar velocity structure, we commonly assume that they arise in the same gas, and we compare their column densities to derive the ionization and abundances. For this one absorption system the different ions have similar velocity but they do not arise in the same gas.