Galaxy mergers can rapidly shut down star formation

TL;DR

Galaxy mergers can cause rapid shutdown of star formation, but this effect is observed mainly after the galaxies have coalesced, confirming long-standing theoretical predictions.

Contribution

This study provides the first observational evidence that galaxy mergers can rapidly quench star formation after coalescence, supporting theoretical models.

Findings

Post-mergers show 30-60 times higher rapid star formation shutdown than controls.

No excess of rapid quenching is observed in close galaxy pairs.

Mergers lead to star formation cessation primarily after coalescence.

Abstract

Galaxy mergers trigger both star formation and accretion onto the central supermassive black hole. As a result of subsequent energetic feedback processes, it has long been proposed that star formation may be promptly extinguished in galaxy merger remnants. However, this prediction of widespread, rapid quenching in late stage mergers has been recently called into question with modern simulations and has never been tested observationally. Here we perform the first empirical assessment of the long-predicted end phase in the merger sequence. Based on a sample of ~500 post-mergers identified from the Ultraviolet Near Infrared Optical Northern Survey (UNIONS), we show that the frequency of post-merger galaxies that have rapidly shutdown their star formation following a previous starburst is 30-60 times higher than expected from a control sample of non-merging galaxies. No such excess is found in a sample of close galaxy pairs, demonstrating that mergers can indeed lead to a rapid halt to star formation, but that this process only manifests after coalescence.