On the outskirts of the Virgo Cluster, where galaxies crowd together but haven't yet clumped into a dense mass, lies VCC 693. Outwardly, it's an unremarkable dwarf: a small cloud of stars and gas. But astronomers suspected that behind its calm appearance lurks a turbulent past. Using the VLA and FAST radio telescopes, they peered into the distribution of hydrogen — the main building material for stars.
The picture resembles a kitchen after cooking: stars are scattered chaotically, like spilled flour, while the gas has settled into a neat rotating disk, as if someone had gathered the dough into a ball. Such a separation is a sign of a recent merger of two dwarf galaxies. One of them, it seems, was almost gas-free — a 'dry' merger, where mostly stars collide rather than gas clouds.
Simulations confirmed that after such a merger, the stellar streams should look exactly as observed in VCC 693. At the same time, the star formation rate and the fraction of heavy elements in the center turned out to be slightly higher than in ordinary dwarfs of the same mass. But overall, the galaxy stands out in no way, and that's the main surprise.
It turns out that major mergers of dwarfs can leave behind completely ordinary systems. This means that many 'ordinary' dwarf galaxies around us may be products of hidden collisions. The cluster's outskirts are like a way station where galaxies collide and mix before they fall into the center and are transformed by the pressure of the environment.
The Virgo Cluster contains over 2000 galaxies, and most of them are dwarfs that often collide with each other on the outskirts.