The gentle monster PDS 456: the kpc scale molecular outflow and its implications for QSO feedback

Abstract

We report on the first ALMA observation of the CO(3$-$2) and rest-frame ~340 GHz continuum emission in PDS 456, which is the most luminous, radio-quiet QSO in the local Universe ($z$~0.18), with a bolometric luminosity $L_{\mathrm{Bol}}\sim 10^{47}$ erg s${}^{-1}$. ALMA angular resolution allowed us to map scales as small as ~700 pc. The molecular gas reservoir, traced by the core of the very bright CO(3$-$2) emission line, is distributed in a compact rotating disk, with size of ~1.3 kpc, seen close to face-on ($i$~25 deg). Fast CO(3$-$2) emission in the velocity range $v\in [-1000,+500]$ km s${}^{-1}$ is also present. Specifically, we detect several blue-shifted clumps out to ~5 kpc from the nucleus, in addition to a compact ($R\lesssim 1.2$ kpc), broad emission component. These components reveal a galaxy-wide molecular outflow, with a total mass $M_{\mathrm{mol}}^{\mathrm{out}}\sim 2.5\times 10^8$ $M_{\odot }$ and a mass outflow rate ${\dot{M}}_{\mathrm{mol}}\sim 290$ $M_{\odot }$ yr${}^{-1}$. The corresponding depletion time is ~8 Myr, 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. $L_{\mathrm{Bol}}/c$) is $\lesssim 1$, 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 AGN so far. We suggest three possible scenarios that may explain this observation: (i) in very luminous AGN 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 ISM (iii) AGN radiation pressure may play an important role in driving the outflow.

Accepted in A&A