A collision strips dark matter, starts star formation

The Universe's first galaxies are thought to have formed at sites where a lot of dark matter coalesced, providing the gravitational pull to draw in enough regular matter to create stars. And, to date, it's impossible to explain the behavior of almost all the galaxies we've observed without concluding that they have a significant dark matter component.

Almost, but not all. Recently, a handful of galaxies have been identified that are dim and diffuse, and appear to have relatively little dark matter. For a while, these galaxies couldn't be explained, raising questions about whether the observations had provided an accurate picture of their composition. However, researchers recently identified one way the galaxies could form: A small galaxy could be swallowed by a larger one that keeps the dark matter and spits out the stars.

Now, a second option has been proposed, based on the behavior of dark matter in a galaxy cluster. This model may explain a series of objects found near the dark matter-poor galaxies. And it may suggest that galaxy-like objects could be formed without an underlying dark matter component.

Bullet time

The galaxy cluster that's the inspiration for this model is called the Bullet Cluster. First described in 2006, this huge grouping of galaxies is the product of a collision between two previously distinct clusters. Because dark matter doesn't interact physically, the dark matter portion of each of the two clusters passed gracefully through the collision site and continued on its way. The regular matter, in contrast, experienced an actual collision, with shockwaves developing within the large amounts of gas that accompanied the galaxy clusters.

Observations of gravitational lensing indicated that most of the mass was with the dark matter, which had moved past the collision site. But most of the visible matter is still near where the collision initially took place. This method of separating regular and dark matter has held up well to further observations and modeling.

The new work relies on extending the mechanism involved in creating the Bullet Cluster down to the scale of individual galaxies. The physics works the same way: A collision slams normal matter into a messy collision driven by its interactions, while dark matter passes smoothly through the mess. It's not clear how much of the regular matter structures can survive this sort of mess. But, because there can be a lot of gas present after the dark matter has moved on, it's possible the regular matter can form structures that lack a dark matter component.

The new research applies this logic to the two best-established dark matter-free galaxies, called DF2 and DF4, which are dwarf galaxies that exist near a normal, large galaxy called NGC 1052.

This goes to 11

It's easy to model collisions between dwarf galaxies that create a situation akin to the Bullet Cluster, with dark and regular matter separated. Collectively, these are referred to as "bullet dwarf" collisions. (Dwarf bullet would seem to be more descriptive, but that wasn't chosen for some reason.)

But in this case, the researchers were able to put a lot of constraints on the model based on the physical situation around NGC 1052. One of those constraints was provided by NGC 1052, the large galaxy in the area. There's no real reason to expect these sorts of galaxy collisions to occur near a large galaxy like that. Its presence in the area suggests that the proximity was central to the collision: One of the smaller galaxies involved in the collision was in orbit around NGC 1052.

Obviously, having both in orbit would make a collision more probable. But it would also mean that the dwarf galaxies wouldn't have a combined speed that would create a sufficiently violent collision. So at least one of the galaxies would have to come in from outside the system and pick up speed while being drawn in toward NGC 1052.

The other major constraint they have is the existence of the two dark matter-poor galaxies, DF2 and DF4, as well as a sense of their relative motion. The relative motion allowed the researchers to trace the galaxies' movements backward through time and conclude that any collision probably took place about 8 billion years ago, which is in good agreement with the age of some of the stars in DF2.

Models of the collision suggest that, in addition to DF2 and DF4, this collision should produce two dark matter-rich dwarf galaxies, and those should appear to be roughly along the line defined by DF2 and DF4. So the researchers looked in a catalog of objects for other dwarf galaxies in the region that might have emerged from the collision. Instead of four total objects, they found 11.