2011MNRAS.410.1957H

We report on the discovery of a high-velocity narrow absorption line outflow in the redshift 2.3 quasar J212329.46-005052.9. Five distinct outflow systems are detected with velocity shifts from -9710 to -14 050 km/s and C IV λλ1548, 1551 linewidths of FWHM ≈ 62–164 km/s. This outflow is remarkable for having high speeds and a degree of ionization similar to broad absorption line (BAL) flows, but linewidths roughly 100 times narrower than BALs and no apparent X-ray absorption. This is also, to our knowledge, the highest-velocity narrow absorption line system confirmed to be in a quasar outflow by all three indicators of line variability, smooth superthermal line profiles and doublet ratios that require partial covering of the quasar continuum source. All five systems have stronger absorption in O VI λλ1032, 1038 than C IV with no lower ionization metal lines detected. Their line variabilities also appear coordinated, with each system showing larger changes in C IV than O VI and line strength variations accompanied by nearly commensurate changes in the absorber covering fractions. The metallicity is approximately twice solar.

These data require five distinct outflow structures with similar kinematics, physical conditions and characteristic sizes of order 0.01–0.02 pc (based on partial covering). The coordinated line variations, occurring on time-scales ≤ 0.63 yr (quasar frame), are best explained by global changes in the outflow ionization caused by changes in the quasar's ionizing flux. An upper limit on the acceleration, ≲ 3 km/s/yr, is consistent with blobs of gas that are gravitationally unbound and coasting freely ≳ 5 pc from the central black hole. Additional constraints from the variability time indicate that the full range of plausible distances is 5 ≲ Ru ≲ 1100 pc. However, if these small absorbing structures were created in the inner flow, they should be near the ∼ 5 pc minimum radius because they can travel just a few pc before dissipating (without external confinement). An apparent double line-lock in C IV suggests that the flow was radiatively accelerated and its present trajectory is within ∼ 16° of the radial (line-of-sight) direction. The absence of strong X-ray absorption shows that radiative shielding in the far-UV and X-rays is not needed to maintain moderate BAL-like ionizations and therefore, apparently, it is not needed to facilitate the radiative acceleration to high speeds. We argue that the ionization is moderated, instead, by high gas densities in small outflow substructures. Finally, we estimate that the kinetic energy yield from this outflow is at least 2 orders of magnitude too low to be important for feedback to the host galaxy's evolution.