An unusual observational case: an object with a mass of a million solar masses, which has no clear “parallel” among known objects, may be the result of a collision between different types of dark matter
Could dark matter—the gravitational “glue” that holds galaxies and galaxy clusters together—behave differently than we’re used to thinking? A new paper in Nature Astronomy presents an unusual observational case: an object with a mass on the order of a million solar masses that has no clear “parallel” among known objects. If the interpretation holds up, it could pose a challenge to both common models of cold and hot dark matter.
How do you even “see” dark matter??
Dark matter neither emits nor absorbs light in a way that allows us to see it directly. So astronomers use a trick of the universe: gravitational lensing. Mass bends space-time, and so it also bends the path of light that passes by it. When a massive object stands “in the line of sight” between us and a distant light source, it can stretch, bend, and multiply the source’s light—creating thin arcs or “Einstein rings.” Inside these curved objects, you can sometimes spot small “bumps” that hint at small gravitational bodies along the way—even if they themselves are dark.
What did the researchers find, and why is it strange??
The study focuses on the object, which was discovered to be “sitting” on an extremely thin gravitational arc. The team performed extensive tests of possible models for its mass profile and distance, trying to see which model best explains the distortion in the arc. The preferred outcome, according to the abstract, is a model with two components:
- A very compact component that behaves like a point mass at the upper limit of a tiny radius, up to about 10 parsecs – that is, tens of light-years.
- On top of the first component is an extended component with a nearly constant surface density up to a cutoff radius of 139 parsecs.
This combination does not resemble the structure of a normal star cluster, a typical dwarf galaxy, or any other known category. The researchers emphasize: If this is an object dominated by dark matter, this structure does not fit the simple predictions of cold, “smooth,” non-collision dark matter, nor necessarily with hot versions in which the particles had a higher initial velocity that obliterates small objects.
So what could explain this??
One possibility the paper suggests is Self-Interacting Dark Matter. That is, dark matter particles have not only gravity but also “friction”/weak collisions between them. In such models, the internal evolution of a halo could be very different, including scenarios where the central region “collapses” and eventually a central black hole is formed. The researchers suggest that this may be a trajectory that is able to approach the anomalous picture measured – but it is important to emphasize: this is still an interpretation, not a fact.
What is the next step??
As always in science, “one strange case” is not the end of the story, but the beginning. We need to look for more similar cases, improve observations, and make sure that the signal is not due to technical details of the model or additional effects along the line of sight. Still, this story reminds us why astronomers love gravitational lenses: they allow us to peer right into the places where our models are most vulnerable—the small scale where dark matter is supposed to produce countless tiny “lumps.” . (Nature)
More of the topic in Hayadan:
