Mixing between AGN winds and ISM clouds produces luminous X-ray emission

Abstract

Active galactic nuclei (AGNs) drive powerful, multiphase outflows that are thought to play a key role in galaxy evolution. The hot, shocked phase of these outflows (T≳106 K) is expected to dominate the energy content, but is challenging to observe due to its long cooling time and low emissivity. The cool phase (T≲104 K) is easier to detect observationally, but it traces a less energetic outflow component. In prior simulations of the interaction between an energy-driven AGN outflow and a clumpy ISM, we found that mixing between hot wind and cool ISM clouds produces a new, highly radiative, phase at T≈106-7 K which fuels the formation of a long-lived (≥ 5 Myr) cool outflow. We investigate the X-ray emission generated by thermal Bremsstrahlung and high-ionization metal line emission in this mixing phase, finding that it could contribute significantly to the X-ray output of the outflow. This mixing-induced X-ray emission is strongest in the part of the outflow propagating equatorially through the disc, and is extended on scales of D ≃ 3-4 kpc. For quasar luminosities of LAGN ≃ 1045-46 erg s-1, the resulting X-ray luminosity is equivalent to that expected from star formation rates SFR ≃ 10-200 M⊙ yr-1, showing that it could be an important source of soft X-rays in AGN host galaxies. Our results suggest that this extended emission could be resolvable in local quasars (z ≲ 0.11) using high spatial-resolution X-ray observatories such as Chandra, or proposed missions such as AXIS and Lynx.