The first galaxies in the Universe are thought to have formed at sites where large amounts of dark matter merged, providing the gravitational pull to attract enough regular matter to create stars. And, to date, it is impossible to explain the behavior of almost all the galaxies we have observed without concluding that they have a significant component of dark matter.
Almost, but not all. Recently, a handful of diffuse, dark galaxies have been identified that appear to have relatively little dark matter. For a time, these galaxies could not be explained, raising questions about whether observations had provided an accurate picture of their composition. However, researchers recently identified a way galaxies could form: a small galaxy could be swallowed up by a larger one that traps dark matter and spits out stars.
Now a second option has been proposed, based on the behavior of dark matter in a cluster of galaxies. This model may explain a series of objects found near dark matter-poor galaxies. And it may suggest that galaxy-like objects could be formed without an underlying dark matter component.
The cluster of galaxies that inspired this model is called the Bullet Cluster. First described in 2006, this huge cluster of galaxies is the product of a collision between two previously separate clusters. Because dark matter does not physically interact, the dark matter portion of each of the two clusters gracefully passed through the collision site and continued on its way. Ordinary matter, on the other hand, suffered a real collision, with shock waves developing in the large amounts of gas that accompanied the galaxy clusters.
Gravitational lensing observations indicated that most of the mass was with dark matter, which had moved past the collision site. But most of the visible matter is still near where the collision originally took place. This method of separating ordinary matter from dark matter has held up well to further observations and modeling.
The new work relies on extending the mechanism involved in the creation of the Bullet Cluster down to the scale of individual galaxies. Physics works the same way: a collision slams normal matter into a disorderly collision driven by its interactions, while dark matter smoothly passes through the disorder. It’s not clear how many structures of regular matter can survive this kind of mess. But, since there can be a lot of gas after dark matter has moved, it is possible that ordinary matter can form structures that lack a dark matter component.
The new research applies this logic to the two most established galaxies without dark matter, called DF2 and DF4, which are dwarf galaxies that exist near a large normal galaxy called NGC 1052.
It goes to 11
It is easy to model collisions between dwarf galaxies that create a situation similar to the cluster of bullets, with separate dark and regular matter. Collectively, these are referred to as “ball dwarf” collisions. (The dwarf ball would seem to be more descriptive, but that was not chosen for some reason.)
But in this case, the researchers were able to impose many constraints on the model based on the physical situation around NGC 1052. One of these constraints was provided by NGC 1052, the large galaxy in the region. There is no real reason to expect these kinds of galaxy collisions to occur near a large galaxy like this. Its presence in the area suggests that proximity was at the heart of the collision: one of the smaller galaxies involved in the collision was orbiting NGC 1052.
Obviously having both in orbit would make a collision more likely. But it would also mean that the dwarf galaxies would not have a combined velocity that would create a violent enough collision. So at least one of the galaxies should come from outside the system and pick up speed while being pulled towards NGC 1052.
The other major constraint they have is the existence of the two dark matter-poor galaxies, DF2 and DF4, and an idea of their relative motion. The relative motion allowed the researchers to trace the motions of the galaxies through time and conclude that any collisions likely took place around 8 billion years ago, which is in good agreement with the age of some of the stars. of DF2.
Collision models suggest that in addition to DF2 and DF4, this collision should produce two dark matter-rich dwarf galaxies, and these should appear roughly along the line defined by DF2 and DF4. The researchers therefore searched a catalog of objects for other dwarf galaxies in the region that might have emerged from the collision. Instead of four objects in total, they found 11.