Abstract

High-velocity outflows from quasars are revealed by the absorption signatures that they produce in the spectrum of the quasar. Clues on the nature and origin of these flows are important for our understanding of the dynamics of gas in the central regions of the active galactic nucleus but also of the metal enrichment of the intergalactic space. Line radiation pressure has often been suggested to be an important process in driving these outflows; however, no convincing evidence has been given so far. Here we report observation of a highly structured flow, toward Q1511 + 091, where the velocity separations between distinct components are similar to O VI, N V and C IV doublet splittings, with some of the profiles matching perfectly. This strongly favours the idea that the absorbing clumps originate at similar physical location and are driven by radiative acceleration caused by resonance lines. The complex absorption can be understood if the flow is highly collimated so that the different optically thick clouds are aligned and cover the same region of the background source. One component shows saturated H I Lyman series lines together with absorptions from excited levels from C II and Si II, but covers only 40 per cent of the source of continuum. The fact that clouds cover only part of the small continuum source implies that the flow is located very close to it.