What Determines the Depth of Broad Absorption Lines? Keck HIRES Observations of BALQSO 1603+3002

We find that the depth and shape of the broad absorption lines (BALs) inBALQSO 1603+3002 are determined largely by the fraction of the emitting sourcewhich is covered by the BAL flow. In addition, the observed depth of the BALsis poorly correlated with their real optical depth. The implication of thisresult is that abundance studies based on direct extraction of column densitiesfrom the depth of the absorption troughs are unreliable. Our conclusion isbased on analysis of unblended absorption features of two lines from the sameion (in this case the Si IV doublet), which allows unambiguous separation ofcovering factor and optical depth effects. The complex morphology of thecovering factor as a function of velocity suggests that the BALs are producedby several physically separated outflows. The covering factor is ion dependentin both depth and velocity width. We also find evidence that in BALQSO1603+3002 the flow does not cover the broad emission line region.Comment: 13 pages, 2 figures, accepted for publication in Ap