The physics and the structure of the quasar-driven outflow in Mrk 231

Abstract

Massive AGN-driven outflows are invoked by AGN-galaxy co-evolutionary models to suppress both star formation and black hole accretion. Massive molecular outflows have been discovered in some AGN hosts. However, the physical properties and structures of these AGN-driven molecular outflows are still poorly constrained. Here we present new IRAM PdBI observations of Mrk 231, the closest quasar known, targeting both the CO(1−0) and CO(2−1) transitions. We detect broad wings in both transitions, which trace a massive molecular outflow moving with velocities of up to 800 km s-1. The wings are spatially resolved at high significance levels (5−11σ), indicating that the molecular outflow extends to the kpc scale. The CO(2−1)/CO(1−0) ratio of the red broad wings is consistent with the ratio observed in the narrow core, while the blue broad wing is less excited than the core. The latter result suggests that quasar-driven outflow models invoking shocks (which would predict higher gas excitation) are inappropriate for describing the bulk of the outflow in Mrk 231. However, we note that within the central 700 pc the CO(2−1)/CO(1−0) ratio of the red wing is slightly, but significantly, higher than in the line core, suggesting that shocks may play a role in the central region. We also find that the average size of the outflow anticorrelates with the critical density of the transition used as a wind tracer. This indicates that, although diffuse and dense clumps coexist in the outflowing gas, dense outflowing clouds have shorter lifetimes and that they evaporate into the diffuse component along the outflow or, more simply, that diffuse clouds are more efficiently accelerated to larger distances by radiation pressure.