High-resolution study of associated CIV absorption systems in NGC 5548

We present the results of a careful analysis of associated absorption syste ms toward NGC 5548. Most of the well-resolved narrow components in the asso ciated system, defined by the Ly alpha, C IV, and N V profiles, show veloci ty 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 v elocity separation equal to C IV doublet splitting to be 6 x 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 ail possible bound-bound transitions. We show that the clouds producing absorpt ion have densities less than 10(9) cm(-3) and are in the outer regions of t he 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 he nce 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.