High-resolution study of associated C IV absorption systems in NGC 5548

Abstract

We present the results of a careful analysis of associated absorption systems toward NGC 5548. Most of the well-resolved narrow components in the associated system, defined by the Lyα, C IV, and N V profiles, show velocity separation similar (to within 10 km s^-1) to the C IV doublet splitting. We estimate the chance probability of the occurrence of such pairs with velocity separation equal to C IV doublet splitting to be 6 × 10^-3. Thus, it is more likely that most of the narrow components are line locked with C IV doublet splitting. This will mean that radiative acceleration plays an important role in the kinematics of the absorbing clouds. We build grids of photoionization models and estimate the radiative acceleration due to all possible bound-bound transitions. We show that the clouds producing absorption have densities less than 10⁹ cm^-3 and are in the outer regions of the broad emission line region (BLR). We note that the clouds that are line locked cannot produce appreciable optical depths of O VII and O VIII and hence cannot be responsible for the observed ionized edges in the soft X-ray range. We discuss the implications of the presence of optically thin clouds in the outer regions of the BLR for the models of broad emission lines.