Mergers are one of the main evolutionary mechanisms for galaxies in the universe. Under the influence of gravity and over billions of years, galaxies collide with small and large companions, altering their shapes and creating new stars.

Now, a Dutch-American team has created a computer simulation to look at an unusual merger: the collision between a gas-rich dwarf galaxy and a “dark satellite.” They found that these events could explain the mysterious origins of irregular dwarf or compact blue dwarf galaxies.

Dark satellites are clumps of dark matter that orbit galaxies. Since dark matter does not emit or interact with light, the satellites are not visible to us. Dark matter does interact gravitationally, however, so the merger can have a significant impact on the final shape of the merged galaxy.

The team's simulation looked at small galaxies, normally called dwarf galaxies, that have 10 to 1,000 times less mass than a large galaxy like the Milky Way. The Milky Way itself has about 30 confirmed companions, from irregular Magellanic Clouds to many smaller objects not visible to the naked eye.

The researchers looked at what happened when a dark satellite interacts with one of these small galaxies. They discovered that a dark satellite stretches and compresses the gas found in a dwarf galaxy, inducing a significant episode of star formation. These “starbursts” happen upon the dark satellite's closest approach and from the intense torque generated by the collision.

The strong gravitational forces are not just responsible for creating new stars in the dwarf galaxies, but also for altering their shape significantly. In the simulation, they tested scenarios in which the galaxies were thin disks in order to see how they would evolve over time: the final object looked similar to many observed galaxies.

“Our initial systems and their remnants compare well with the observational properties of a large selection of irregular dwarf galaxies and blue compact dwarfs,” the author wrote in the paper published in Astronomy & Astrophysics. “Even systems that are strongly perturbed as a result of a merger with a dark satellite fall within the scatter seen in the observations. This implies that such events might well be happening but may not be fully evident. We have yet to identify the 'smoking gun' of the dark merger scenario.”