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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 CO J=1-0 emission line in [H89]1821+643, one of the most optically luminous 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 corresponds to a molecular gas mass of ~ 8.0 x 10^{9} M_sun. The dynamical mass enclosed by the CO line emission region could amount up to ~ 1.7x10^{12} M_sun (80% of the total mass of the elliptical host). The bulk of the CO emission is located at ~9 kpc south-east from the nuclei position, and 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 for the CO source, which would imply a highly asymmetrical elliptical host. The close alignment between the CO emission major axis and the radio-plume suggests a possible role of the latter excitation of the ambient gas reservoir. The stacking technique was used to search for CO emission and 3mm continuum emission from galaxies in the surrounding cluster, however no detection was found either toward individual galaxies or the stacked ensemble of galaxies, with a 3 sigma limit of < 1.1x10^9 M_sun for the molecular gas.