Dark Matter may be hitting the right note in small galaxies

New study may explain distribution of dark matter in galaxies

Astronomers observed that the dark matter does not seem to clump very much in small galaxies, but their density peaks sharply in bigger systems such as clusters of galaxies. It has been a puzzle why different systems behave differently. (picture: Kavli IPMU - Kavli IPMU modified this figure based on the image credited by NASA, STScI)

When two dark matter particles approach each other, they tend to simply pass each other. (picture: Kavli IPMU)

If two dark matter particles aproach each other at a special speed, they “resonate” and stick together for a short moment, and move out to different directions afterwards, causing them to scatter. This way, dark matter can spread out so that we can understand the smooth profile in small galaxies (picture: Kavli IPMU).

Dark matter may scatter against itself only when it hits the right energy. This is what researchers from the Kavli Institute for the Physics and Mathematics of the Universe (Japan), DESY and the Austrian Academy of Science say in a study published in the recent issue of Physical Review Letters. Their idea can explain why galaxies, from the smallest to the biggest, have the shapes they do, providing a plausible solution for a long-lasting problem.

Dark matter is a mysterious and so far undetected form of matter that comprises more than 80 per cent of matter in today´s Universe. Its nature is unknown, but scientists think that it is responsible for forming stars and galaxies by its gravitational pull, a phenomenon that ultimately also led to our existence.

“Dark matter is actually the mother who gave birth to all of us. But we haven’t met her; somehow, we got separated at birth. Who is she? That is the question we want to know,” says author Hitoshi Murayama, professor at University of California at Berkeley and Principal Investigator at the Kavli Institute for the Physics and Mathematics of the Universe.

Astronomers have already found that dark matter does not seem to clump together as much as computer simulations suggest. If gravity is the only force through which dark matter can interact, only pulling and never pushing, then dark matter should become very dense towards the centre of galaxies. However, especially in small faint galaxies called dwarf spheroidals, dark matter does not seem to become as dense as expected toward their centres.

This puzzle could be explained if dark matter particles scatter with each other like billiard balls, allowing them to spread out more evenly after a collision. But one problem with this idea is that dark matter does seem to clump in bigger systems such as clusters of galaxies. What makes dark matter behave differently in dwarf spheroidals and clusters of galaxies? The authors of the study have developed an explanation that could solve this riddle, supporting the so-called self-interacting dark matter hypothesis, according to which dark matter consists of particles which can collide.

“If dark matter scatters with itself only at a low but very special speed or energy, this can happen often in dwarf spheroidals where the dark matter moves slowly. In clusters of galaxies, where it moves fast, the effect is rare. It needs to hit a resonance,”says Chinese physicist Xiaoyong Chu, a postdoctoral researcher at the Austrian Academy of Sciences.

Resonance is a phenomenon that appears every day. A playground swing has to be pushed at a special frequency so that it goes higher and higher. Or if you bring a swinging tuning fork near a guitar, the guitar string starts to vibrate only if it is tuned correctly. These are all examples for resonances, Murayama explains. The team suspects this is precisely what dark matter is doing: the dark matter particles are much more likely to hit each other when they are moving at a particular energy that corresponds to the resonance of the swing or the tuning fork.

“As far as we know, this is the simplest explanation to the puzzle. We are excited because we may soon know what dark matter is,” says Murayama. However, the team wondered whether such a simple idea would explain the existing observational data correctly. “First, we were a bit skeptical that this idea will explain the observational data; but once we tried it, it worked like a charm!,” says Colombian physicist Camilo Garcia Cely, a postdoctoral researcher at DESY.

For their idea to work, the mass of the resonance has to be close to twice the mass of one dark matter particle. The team believes it is no accident that dark matter can hit the exact right note. “There are many other systems in nature that show similar accidents: for example, for the carbon production in stars, alpha particles hit a resonance of beryllium, which in turn hits a resonance of carbon, producing the building blocks that gave rise to life on Earth,” says Garcia Cely.

“This behaviour of dark matter may also be a sign that our world has more dimensions than we see. If a particle moves in extra dimensions, it has a certain kinetic energy. For us who don’t see the extra dimensions, we observe the energy actually as mass, thanks to Einstein’s famous equation E=mc2 – energy is equivalent to mass. Perhaps some particle moves twice as fast in extra dimension, making its mass precisely twice as much as the mass of dark matter,” says Chu.

The team’s next step will be to find observational data that backs their theory. “If our theory is valid, future and more detailed observations of different galaxies will reveal that scattering of dark matter indeed depends on its speed,” says Murayama. He is also the leader of an international group that works on an instrument called the Prime Focus Spectrograph, currently under construction. The US$80 million instrument will be mounted on the Subaru telescope atop Mauna Kea on Big Island, Hawaii, and will be capable of measuring the speeds of thousands of stars in dwarf spheroidals, thus being able to provide the data needed to check this new hypothesis.


Original publication:

Velocity dependence from resonant self-interacting dark matter; Xiaoyong Chu, Camilo Garcia-Cely and Hitoshi Murayama; Physical Review Letters, 2019 DOI: 10.1103/PhysRevLett.122.071103