Death by Impact: Evidence for Merger-driven Quenching in a Collisional Ring Galaxy at Cosmic Noon

Abstract The role of interactions and mergers in the rapid quenching of massive galaxies in the early Universe remains uncertain, largely due to the difficulty of directly linking mergers to quenching. Collisional ring galaxies provide a unique opportunity, as their morphology allows precise dating of the interaction, which can then be compared to quenching timescales inferred from star formation histories. We study a gravitationally bound system at z = 1.61 in the Ultra Deep Survey field, composed of a host galaxy ( M ⋆ = 10 11.4 M ⊙ ) with a collisional ring and an X-ray active galactic nucleus (AGN), and the Bullet Galaxy ( M ⋆ = 10 11.2 M ⊙ ), located at a projected distance of ∼8 kpc. Combining JWST and Hubble Space Telescope imaging with Keck/MOSFIRE spectroscopy, we find compelling evidence for an ongoing starburst in the host concurrent with rapid quenching in the Bullet Galaxy. The ring, ∼20 kpc in diameter, is expanding at <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mn>12</mml:mn> <mml:msubsup> <mml:mrow> <mml:mn>7</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>29</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>72</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> km s −1 , implying the galaxies first collided 47–96 Myr ago. This timeline is consistent with the host’s current starburst and the Bullet Galaxy’s sudden quenching, strongly suggesting both phenomena were triggered by the interaction. Crucially, the Bullet Galaxy shows no evidence of a preceding starburst, ruling out rapid gas consumption as the primary quenching channel. Instead, we suggest that merger-driven processes—such as enhanced turbulence and disk instabilities—may have suppressed star formation. An additional possibility, which we term the dragon effect , is that AGN-driven outflows from the host disrupted the Bullet Galaxy’s low-density molecular gas, thereby preventing efficient star formation and accelerating quenching.