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We present high resolution spectroscopic VLT observations of the outflow seenin QSO 2359-1241. These data contain absorption troughs from five resonance FeII lines with a resolution of ~7 km/s and signal-to-noise ratio per resolutionelement of order 100. We use this unprecedented high quality data set toinvestigate the physical distribution of the material in front of the source,and by that determine the column densities of the absorbed troughs. We findthat the apparent optical depth model gives a very poor fit to the data andgreatly underestimates the column density measurements. Power-law distributionsand partial covering models give much better fits with some advantage topower-law models, while both models yield similar column density estimates. Thebetter fit of the power-law model solves a long standing problem plaguing thepartial covering model when applied to large distance scale outflow: How toobtain a velocity dependent covering factor for an outflow situated atdistances thousands of time greater than the size of the AGN emission source.This problem does not affect power-law models. Therefore, based on the betterfit and plausibility of the physical model, we conclude that in QSO 2359-1241,the outflow covers the full extent of the emission source but in anon-homogeneous way.

Measuring Column Densities in Quasar Outflows: VLT Observations of QSO 2359-1241