  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.
