2019A&A...628A.118B

We report on the first ALMA observation of the CO(3-2) and rest-frame ∼340GHz continuum emission in PDS 456, which is the most luminous, radio-quiet QSO in the local Universe (z~=0.18), with a bolometric luminosity L_Bol∼10⁴⁷erg/s. ALMA angular resolution allowed us to map scales as small as ∼700pc. The molecular gas reservoir traced by the core of the very bright CO(3-2) emission line is distributed in a compact rotating disk, with a size of ∼1.3kpc, seen close to face-on (i∼25deg). Fast CO(3-2) emission in the velocity range v∈[-1000,500]km/s is also present. Specifically, we detect several blue-shifted clumps out to ∼5kpc from the nucleus, in addition to a compact (R≤1.2kpc), broad emission component. These components reveal a galaxy-wide molecular outflow, with a total mass M_mol^out∼2.5x10⁸M_☉ (for an α_CO=0.8M_☉/(K.(km/s).pc²) and a mass outflow rate M_mol∼290M_☉/yr. The corresponding depletion time is τ_dep∼8Myr, shorter than the rate at which the molecular gas is converted into stars, indicating that the detected outflow is potentially able to quench star-formation in the host. The momentum flux of the molecular outflow normalised to the radiative momentum output (i.e. L_Bol/c) is ≤1, comparable to that of the X-ray ultra-fast outflow (UFO) detected in PDS 456. This is at odds with the expectations for an energy-conserving expansion suggested for most of the large-scale outflows detected in low-luminosity AGNs so far. We suggest three possible scenarios that may explain this observation: (i) in very luminous AGNs such as our target the molecular gas phase is tracing only a fraction of the total outflowing mass; (ii) a small coupling between the shocked gas by the UFO and the host-galaxy interstellar medium (ISM); and (iii) AGN radiation pressure may be playing an important role in driving the outflow.