Violent Quenching: Molecular Gas Blown to 1000 km s-1 during a Major Merger

We present Atacama Large Millimeter/submillimeter Array observations of a massive ({M}_\star ≈ {10}¹¹ {M}_⊙ ) compact ({r}_{{e},{UV}}≈ 100 pc) merger remnant at z = 0.66 that is driving a 1000 km s^-1 outflow of cool gas, with no observational trace of an active galactic nucleus (AGN). We resolve molecular gas on scales of approximately 1-2 kpc, and our main finding is the discovery of a wing of blueshifted CO J(2 → 1) emission out to -1000 km s^-1 relative to the stars. We argue that this is the molecular component of a multiphase outflow, expelled from the central starburst within the past 5 Myr through stellar feedback, although we cannot rule out previous AGN activity as a launching mechanism. If the latter is true, then this is an example of a relic multiphase AGN outflow. We estimate a molecular mass outflow rate of approximately 300 M _⊙ yr^-1, or about one third of the 10 Myr-averaged star formation rate. This system epitomizes the multiphase “blowout” episode following a dissipational major merger—a process that has violently quenched central star formation and supermassive black hole growth.