One of the best candidates for finding dark matter – dwarf galaxies – appear to be sending weaker signals than previously thought.

Dark matter is the ‘missing’ material thought to make up about 85 per cent of the Universe’s matter. Its presence is predicted by astrophysical observations, but the question of whether this missing matter consists of particles – and if so, of which type – is still unanswered.

Now, by modelling the evolution of dwarf galaxies, researchers have shown that the dark matter signal coming from them is weaker than previously thought.

The team, including Imperial College London researchers, suggests physicists now need to revise one of the key search strategies for dark matter discovery. Their results are published in Physical Review D Rapid Communications.

From the Big Bang to the present

Physicists believe the nature of dark matter may be revealed by looking at the signal produced when two dark matter particles annihilate each other and produce gamma rays. These rays can be detected by instruments such as the Fermi Gamma-ray Space Telescope orbiting the Earth.

We were surprised to find that including all of the science about how the universe grows and evolves over aeons has a very strong impact on the dark matter conclusions we can draw from the dwarf galaxies today in our immediate neighbourhood. Dr Alex Geringer-Sameth

Some of the best targets for such a search strategy are small satellite galaxies that are trapped in the gravitational pull of our own Milky Way. Dwarf galaxies are full of dark matter but little else, meaning they are nearly pristine laboratories to look for tell-tale signatures of dark matter annihilation without the complications of other astronomical phenomena.

In order to perform the search, the team combined theoretical and observational methods to put together the first ‘end to end’ analysis that models the dwarf galaxies in their cosmic context.

Lead author Shin’ichiro Ando, of the Institute of Physics at the University of Amsterdam, said: “Our method starts at the big bang, follows the evolution of dark matter and the growth of galaxies across cosmic time, and uses this as the basis to understand how dark matter might be distributed within the individual dwarf galaxies we see in the sky today. This understanding is then used as the basis to search for the gamma-ray signal which reveals information about the microscopic properties of dark matter particles.”

Co-author Dr Alex Geringer-Sameth, from the Departments of Physics and Mathematics at Imperial, said: “We were surprised to find that including all of the science about how the universe grows and evolves over aeons – something prior work tended to neglect – has a very strong impact on the dark matter conclusions we can draw from the dwarf galaxies today in our immediate neighbourhood.”

Weak signal

The team concluded that the expected signal of the dark matter annihilation is much weaker than earlier estimates. Dark matter is thus harder to detect using dwarf galaxies than previously expected, and the gamma-ray data actually tell scientists less about dark matter than they previously thought it did.

One immediate consequence is that there is still a possibility that dark matter annihilation explains one of the great current astronomical mysteries — the anomalous and unexplained gamma-ray glow that we see emerging from the center of our own galaxy. Until now, it was difficult to see how dark matter annihilation could power the galactic center anomaly but avoid generating a similar signal from the dwarf galaxies.

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‘Structure formation models weaken limits on WIMP dark matter from dwarf spheroidal galaxies’ by Shin’ichiro Ando, Alex Geringer-Sameth, Nagisa Hiroshima, Sebastian Hoof, Roberto Trotta, and Matthew G. Walker is published in Physical Review D Rapid Communications.

Top image: An optical image of the Sculptor dwarf spheroidal galaxy (left) alongside an illustration of the gamma-ray signal that might arise from dark matter annihilating within the galaxy (right). Image left: Giuseppe Donatiello. Image right (background): NASA/DOE/Fermi LAT Collaboration

Based on a press release by the University of Amsterdam.