Evidence of the fast acceleration of AGN-driven winds at kiloparsec scales

Abstract

Supermassive black holes at the centres of galaxies gain mass through accretion disks. Different models predict that quasi-spherical winds, expelled by black holes during accretion, have a key role in galaxy evolution through regulating star formation and the distribution of metals over kiloparsec scales and sweeping ambient gas to the outskirts of galaxies. Nonetheless, the mechanism that drives these outflows and the amount of energy exchanged between the wind and the galaxy’s interstellar medium remain unclear. Here we analyse the kinematic properties of these winds in a sample of nearby active galaxies using the MOKA^3D model, which reproduces the clumpy nature of the interstellar medium. We provide evidence that outflows exhibit a regular radial velocity trend—initially constant or slightly decreasing, followed by rapid acceleration starting at approximately 1 kpc from the nucleus—despite the seemingly complex kinematics. The observed behaviour is consistent with current theoretical understanding of active galactic nucleus outflows, where a momentum-driven phase transitions to an energy-conserving phase beyond 1 kpc. The constant velocity of the momentum-driven wind is then rapidly accelerated following inefficient Compton cooling of post-shock material. The measured radial terminal velocities of the outflows are larger than the escape velocities from the host galaxies, confirming the role of outflows in shaping galaxy evolution as a manifestation of active galactic nucleus feedback.