The kinematics and ionization structure of the extended emission-line region of QSO E1821+643

The most luminous quasars are created by major, gas-rich mergers and E1821+643, an optically luminous quasar situated at the centre of a cool-core cluster, appears to be in the late stages of the post-merger blowout phase. This quasar is also identified as a gravitational recoil candidate, in which the supermassive black hole (SMBH) has received a recoil kick due to anisotropic emission of gravitational waves during the coalescence of a progenitor SMBH binary. We analyse long-slit spectra of the extended, ionized gas surrounding E1821+643 to study its kinematics and ionization. We have identified three kinematically distinct components, which we associate, respectively, with a wide-angle polar wind from the nucleus, kinematically undisturbed gas, and a redshifted arc-like structure of gas, at a distance of 3-4 arcsec (13-18 kpc) from the nucleus. The latter component coincides with the northern and eastern extremities of an arc of [O iii] emission seen in HST images. This feature could trace a tidal tail originating from a merger with a gas-rich galaxy to the south-east of the nucleus, whose presence has been inferred by Aravena et al. from the detection of CO emission. Alternatively, the arc could be the remnant of a shell of gas swept up by a powerful quasar wind. The emission-line ratios of the extended gas are consistent with photoionization by the quasar, but a contribution from radiative shocks cannot be excluded.