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 ($T{\gtrsim }10^{6}{\rm {\ K}}$) is expected to dominate the energy content, but is challenging to observe due to its long cooling time and low emissivity. The cool phase ($T{\lesssim }10^{4}{\rm {\ K}}$) 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 $T{\approx }10^{6-7}{\rm {\ K}}$ which fuels the formation of a long-lived ($\ge 5\ \rm {Myr}$) 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 $D\simeq 3\!-\!4\ \rm {kpc}$. For quasar luminosities of $L_{\rm {AGN}}{\simeq } 10^{45-46}\rm {\ erg\ s^{-1}}$, the resulting X-ray luminosity is equivalent to that expected from star formation rates $\rm {SFR}\simeq 10\!-\!200\ \rm {M_\odot \ yr^{-1}}$, 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 ($z\lesssim 0.11$) using high spatial-resolution X-ray observatories such as Chandra, or proposed missions such as AXIS and Lynx.