Press release: “Giant cosmic tsunami wakes up comatose galaxies”, RAS 

Galaxies are often found in clusters, with many 'red and dead' neighbours that stopped forming stars in the distant past. Now an international team of astronomers, led by Andra Stroe of Leiden Observatory and David Sobral of Leiden and the University of Lisbon, have discovered that these comatose galaxies can sometimes come back to life. If clusters of galaxies merge, a huge shock wave can drive the birth of a new generation of stars – the sleeping galaxies get a new lease of life. The scientists publish their work in the journal Monthly Notices of the Royal Astronomical Society.

https://www.ras.org.uk/news-and-press/2619-giant-cosmic-tsunami-wakes-up-comatose-galaxies