The Mystery of Ultra-diffuse Galaxies
How the discovery of Ultra-diffuse Galaxies led to more questions about dark matter and galaxy formation.
Recently, astronomers have observed extremely low luminosity galaxies, which are faint even when you compensate for how faraway they are. Astronomers call them ultra-diffuse galaxies for that very reason. Hai-bo Yu from the University of California, Riverside and his collaborators from Tufts University and the University of Groningen studied these galaxies with theoretical models to characterize their properties. They discovered several contradictions between the behavior they expected and the behavior they observed in the Illustris RNG simulation. Astrophysicists use the simulation with current physics models to experiment with properties of the universe since they cannot do so in reality.
Can you elaborate on the concept of self-interacting dark matter?
Physicists had a prevailing dark matter theory called a CDM paradigm, or Cold Dark Matter paradigm. So, you assume that dark matter is made of some hypothetical new particle that had effectively no collisions aside from gravity. You run computer simulations and look into the structure formation story. It turns out, if you look at the large-scale structures like clustering of dark matter [in] galaxies and pair it with simulations, they agree with each other. That’s how the CDM paradigm was established.
Why are self-interactions plausible?
If you have more detailed observations and compare it to simulations of tiny, tiny, dwarf galaxies (much smaller than the Milky Way), and you look at the motion of stars, you try to infer how much mass is inside a central region of those galaxies, in areas that show discrepancy between CDM simulation (the prevailing meta theory) and observations. So, there’s a long-standing debate of two camps. Is it a problem — a real problem? Is it an observational issue or is it new physics? You need to go beyond the prevailing CDM kind of scenario. My proposal, which includes the data with all the analysis we have done, is that maybe dark matter has self-interactions. Self-interactions means that, aside from gravity, those dark matter particles can collide to each other just like atoms can collide to each other
Do you have any hints about where ultra-diffuse galaxies fit in the galaxy formation process?
We know very little. Maybe ultra-diffuse galaxies are young because it takes time to consume the gas to convert into stars. Maybe those galaxies are relatively young so that [they do] not have enough time to convert the gas into stars. We realized that those halos [, which is dark matter on the outskirts of a galaxy,] are relatively young compared to normal halos. We try to put things in order. Maybe the high gas density isn’t related to the age of the galaxy because the halos we’re finding in the simulations are relatively young.
What’s next for you and your team concerning ultra-diffuse galaxies?
Currently, we are doing theory. We are looking to the details of the data in the Illustris simulation. Secondly, we are modeling the self-interaction effect, whether it can help reconcile some discrepancies. So, there is a theory side and observational side. We hope — we encourage observers to take more data to understand the dynamics better and to discover more ultra-diffuse galaxies so we can have a new level of comparison. On top of that, we can see whether the deviation from CDM is right or not. As high energy/particle physicists, our job is looking for new physics. I just turned to galaxies, looking for whether there’s new physics that [is needed] to explain the phenomena.
