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

Abstract

We present Atacama Large Millimeter/submillimeter Array observations of a massive (${M}_{\star }\approx {10}^{11}\,{M}_{\odot }$) compact (${r}_{{\rm{e}},\mathrm{UV}}\approx 100$ 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.