Line-driven radiative outflows in luminous quasars

An analysis of ≃19 500 narrow ( ≲ 200 km s−1) C iv λλ1548.2,1550.8 absorbers in ≃34 000 Sloan Digital Sky Survey quasar spectra is presented. The statistics of the number of absorbers as a function of outflow velocity shows that in approximately two-thirds of outflows, with multiple C iv absorbers present, absorbers are line-locked at the 500 km s−1 velocity separation of the C iv absorber doublet; appearing as ‘triplets’ in the quasar spectra. Line-locking is an observational signature of radiative line-driving in outflowing material, where the successive shielding of ‘clouds’ of material in the outflow locks the clouds together in outflow velocity. Line-locked absorbers are seen in both broad absorption line (BAL) quasars and non-BAL quasars with comparable frequencies and with velocities out to at least 20 000 km s−1. There are no detectable differences in the absorber properties and the dust content of single C iv doublets and line-locked C iv doublets. The gas associated with both single and line-locked C iv absorption systems includes material with a wide range of ionization potential (14–138 eV). Both single and line-locked C iv absorber systems show strong systematic trends in their ionization as a function of outflow velocity, with ionization decreasing rapidly with increasing outflow velocity. Initial simulations, employing cloudy, demonstrate that a rich spectrum of line-locked signals at various velocities may be expected due to significant opacities from resonance lines of Li-, He- and H-like ions of O, C and N, along with contributions from He ii and H i resonance lines. The simulations confirm that line-driving can be the dominant acceleration mechanism for clouds with N(H i) ≃ 1019 cm−2.