Imagine looking up on a clear winter night and seeing the Milky Way’s white band stretching across the dark night sky.
What you are seeing is the light from several hundred billion stars in our spiral galaxy. Our own solar system is just a tiny speck located some distance from the centre of the galactic disk.
If you could turn your gaze in exactly the opposite direction, you would be looking out of the flat spiral. Out there, in the seemingly empty darkness, scattered like fan-shaped veils, lie the remnants of ancient galaxies.
“Throughout the history of the Universe, galaxies have merged. When this happens, stars are torn away, forming stellar streams. Together with other faint structures and diffuse starlight, these make up what we call the galaxy’s stellar halo,” explains researcher Sijing Shen.
Shen is a professor of cosmology at the University of Oslo. Cosmologists study questions such as how the Universe came into being and what it contains. And there is plenty for them to work on. Most of the cosmos consists of something whose nature we do not know – what we simply call dark matter.
“In today’s cosmological model, dark matter accounts for around 27 percent of the Universe’s total energy content, while all ordinary matter – hydrogen, helium, oxygen, stars, planets, and everything we know from the Standard Model of particle physics – accounts for only around five percent,” says Shen.
Shen is now coordinating Norway’s contribution to the European Space Agency’s latest mission: the ARRAKIHS telescope.
“ARRAKIHS is a telescope that will observe extremely faint structures around galaxies in the nearby Universe. We plan to study around 70–80 galaxies similar to the Milky Way, although the final number of targets has not yet been determined,” says Shen.
Dark Matter Halos
The large-scale structure of the Universe is like a vast cosmic web with filaments, like threads. Where the threads meet, there are dark matter halos.
In cosmologists’ standard model, these halos form first. Gas – ordinary matter – then flows along the filaments and into the halos. The gas cools and condenses at the centres of the halos, and eventually galaxies containing stars and planets form.
“Nearby galaxies can also move along the filaments and merge with the main galaxy and its halo,” Shen explains.
Galaxies can be subjected to tidal forces, the same physical mechanism that causes the tides on Earth. When a small galaxy approaches a much larger galaxy, the gravity of the larger galaxy pulls more strongly on the side of the smaller galaxy that is closest to it than on the side that is farther away.
“The stars can then be torn away from their galaxy. This creates stellar streams and diffuse stellar halos. Globular clusters – dense, spherical collections of up to millions of stars held together by gravity – can also be torn apart by tidal forces and form stellar streams,” Shen explains.
Cosmic Fossils
“Different galaxies of different masses will have their own dark matter halos,” Shen explains.
The reason researchers now want to take a closer look at the traces of ancient, cannibalized galaxies is that they were created by processes involving both dark matter and “ordinary” matter.
The faint stellar structures that the ARRAKIHS telescope will observe serve as fossil traces of cosmological events.
“Ordinary observations of galaxies primarily show the bright central regions. To see the extremely faint structures, we need much deeper observations, reaching very low levels of surface brightness. When we do that, we begin to see dwarf galaxies, remnants of cannibalized galaxies, and diffuse stellar halos. These structures act as fossils. They can tell us how galaxies formed and evolved,” Shen explains.
Her research group is responsible for developing models that will predict which structures ARRAKIHS can expect to observe.
“We aim to determine how different galaxy formation processes and different dark matter models can affect these faint structures.”
To do this, the researchers use computer simulations in which they create a virtual Universe. The simulations begin with the first density fluctuations that arose after the Big Bang and follow the evolution of the Universe all the way to the present day.
“In this way, we can investigate how galaxies form and evolve under different assumptions. For example, we can compare the standard cold dark matter model with alternative models, such as warm dark matter or models in which dark matter has self-interactions.”
Not All Observations Match the Theories
The standard model of dark matter assumes that it is cold and does not interact with other matter. It does not collide. But what if that is not the case after all?
“Dark matter is often described as collisionless. It is a model that describes the large-scale structure of the Universe very well, but it faces challenges when we study the smallest and faintest galaxies. Observations of dwarf galaxies and ultra-faint dwarf galaxies do not always agree with the predictions of the theories,” says Shen.
Computer models, for example, predict far more small satellite galaxies around large galaxies than researchers actually observe.
“Another problem concerns the distribution of dark matter within galaxies. Cold dark matter simulations predict that the density of dark matter should increase steeply toward the centres of galaxies. Observations, however, suggest that the distribution is often much flatter,” says Shen.
At the same time, she emphasizes that ARRAKIHS will mainly investigate stellar streams and diffuse stellar halos, rather than just dwarf galaxies.
So what does the researcher hope for most? Does she hope that the observations from ARRAKIHS will confirm their models, or that they will reveal something unexpected? Shen smiles when asked.
“Both would be interesting. If the observations agree well with the simulations, it means that we have understood the underlying processes quite well. But if the observations deviate from the models, that could be even more exciting. It could indicate that there are processes or physics that we do not yet understand.”
