Lenticular galaxies are the cosmic quiet types: they have a disk but no bright spiral arms. For a long time, their origin remained a mystery. A team of scientists ran the Hydrangea computer simulation and traced the fate of each galaxy from its youth to the present day.
On the mass–size diagram, these galaxies line up in a V shape. Light and heavy S0 galaxies arrive at their shape by two different routes. The small ones are former spiral galaxies that 'fell asleep' (stopped forming stars) after falling into a large cluster. After falling asleep, they barely change in size: they remain disks. The large ones, on the other hand, are born in dramatic mergers: they collide, merge, and puff up in size. Their stars go out even before falling into the cluster.
The two paths explain not only the kink in the diagram but also the scatter in properties. The calm appearance of lenticular galaxies is deceptive: behind it lie either turbulent or, conversely, quiet histories.
The name 'lenticular' (S0) was coined by Edwin Hubble, who placed them between spiral and elliptical galaxies in his famous 'tuning fork' scheme.