Galaxy-scale ionised winds driven by ultra-fast outflows in two nearby quasars

Abstract

We used MUSE adaptive optics data in narrow field mode to study the properties of the ionised gas in MR 2251-178 and PG 1126-041, two nearby (z0.06) bright quasars (QSOs) hosting sub-pc scale ultra-fast outflows (UFOs) detected in the X-ray band. We decomposed the optical emission from diffuse gas into a low- and a high-velocity components. The former is characterised by a clean, regular velocity field and a low (∼80 km s-1) velocity dispersion. It traces regularly rotating gas in PG 1126-041, while in MR 2251-178 it is possibly associated with tidal debris from a recent merger or flyby. The other component is found to be extended up to a few kpc from the nuclei, and shows a high (∼800 km s-1) velocity dispersion and a blue-shifted mean velocity, as is expected from outflows driven by active galactic nuclei (AGN). We estimate mass outflow rates up to a few Mpdbl yr-1 and kinetic efficiencies LKIN/LBOL between 1-4 × 10-4, in line with those of galaxies hosting AGN of similar luminosities. The momentum rates of these ionised outflows are comparable to those measured for the UFOs at sub-pc scales, which is consistent with a momentum-driven wind propagation. Pure energy-driven winds are excluded unless about 100× additional momentum is locked in massive molecular winds. In comparing the outflow properties of our sources with those of a small sample of well-studied QSOs hosting UFOs from the literature, we find that winds seem to systematically lie either in a momentum-driven or an energy-driven regime, indicating that these two theoretical models bracket the physics of AGN-driven winds very well.