Unveiling the Mask on the ULIRG-to-QSO Transition Object [H89]1821+643 at z = 0.3: A Gas-poor/Gas-rich Galaxy Merger and the Implications for Co-based Dynamical Mass Estimates

We report the detection of the ¹²CO J = 1-0 emission line in [H89]1821+643, one of the most optically luminous quasi-stellar objects (QSOs) in the local universe, and a template ULIRG-to-QSO transition object, located in a rich, cool-core cluster at z = 0.297. The CO emission is likely to be extended, highly asymmetric with respect to the center of the host elliptical where the QSO resides, and correspond with a molecular gas mass of ~8.0 × 10⁹ M _sun. The dynamical mass enclosed by the CO emission-line region could amount to ~1.7 × 10¹² M _sun (80% of the total mass of the elliptical host). The bulk of the CO emission is located at ~9 kpc southeast from the nuclei position, close to a faint optical structure, suggesting that the CO emission could either represent a gas-rich companion galaxy merging with the elliptical host or a tail-like structure reminiscent of a previous interaction. We argue that the first scenario is more likely given the large masses implied by the CO source, which would imply a highly asymmetric elliptical host. The close alignment between the CO emission's major axis and the radio plume suggests a possible role in the excitation of the ambient gas reservoir by the latter. The stacking technique was used to search for CO emission and 3-mm continuum emission from galaxies in the surrounding cluster. However, no detection was found toward individual galaxies or the stacked ensemble of galaxies, with a 3σ limit of <1.1 × 10⁹ M _sun for the molecular gas.