On the column density of AGN outflows: The case of NGC 5548

We reanalyze the Hubble Space Telescope high-resolution spectroscopic data of the intrinsic absorber in NGC 5548 and find that the C IV absorption column density is at least 4 times larger than previously determined. This increase arises from accounting for the kinematical nature of the absorber and from our conclusion that the outflow does not cover the narrow emission line region in this object. The improved column density determination begins to bridge the gap between the high column densities measured in the X-ray and the low ones previously inferred from the UV lines. Combined with our findings for outflows in high-luminosity quasars, these results suggest that traditional techniques for measuring column densities-equivalent width, curve of growth, and Gaussian modeling-are of limited value when applied to UV absorption associated with active galactic nucleus outflows.