A pair of strange galaxies seem to defy our standard model of cosmology. We can鈥檛 pin down how far away they are, but both probable distances come with problems. If they are relatively distant, they appear to have clusters of stars that are brighter than any seen before. If they are closer, then it looks like they are moving far too fast.
In early 2018, a group led by Pieter van Dokkum at Yale University announced that they had found a strange galaxy that appeared to have very little dark matter, or none at all. It was called 1052-DF2, or just DF2. Then, about a year later, they announced another one called 1052-DF4.
Both galaxies have an extremely low concentration of stars and this wispiness makes it hard to say how far away they are. We work out the characteristics of galaxies 鈥� things like how much dark matter and how many stars they have 鈥� based partly on how bright they are and how far away.
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If the galaxies are about 65 million light years away, as van Dokkum鈥檚 team originally claimed, they appear to lack dark matter, and clumps of stars within the galaxies called globular clusters are far bigger and brighter than any we鈥檝e seen before. If they are closer, as some other teams since then have claimed, they are relatively normal galaxies 鈥� just unusually large and moving strangely fast.
Now, Moritz Haslbauer at the University of Bonn in Germany and his colleagues have performed an analysis of the听DF2 galaxy, focusing on that one because we have much more data on it than we do for DF4.
They used five different simulations of the universe as we know it and allowed the universe to grow to maturity, although the physics that went into each one was slightly different 鈥� for example, some include the effects of magnetic fields while others do not. The team then searched for galaxies that appeared similar to DF2 at various distances.
It found very few that were a good match. Those they did find were most commonly about 37 million light years away. Even in the simulation where DF2-like galaxies were most common, they calculated only a 0.01 per cent chance of finding one at that distance. And the chance of finding one at 65 million light years away was more than 200 times lower again.
This means, says Haslbauer, that DF2 and DF4 听are are inconsistent with our standard model of cosmology.听That would mean that something about our understanding of the universe is incorrect.
However, other astronomers expressed concern that the problem may not be with our model of cosmology, but with the simulations themselves. 鈥淭hese galaxies are definitely interesting, definitely weird, but whether they suggest that lambda-CDM is wrong or that the simulations are wrong could go either way,鈥� says Marla Geha at Yale University. 鈥淚 would put my money on the simulations being a little wrong rather than the overall paradigm.鈥�
Ignacio Trujillo at the Institute of Astrophysics of the Canary Islands says they don鈥檛 so much show that the galaxies conflict with lambda-CDM, but that the models are simply not precise enough. 鈥淭hese galaxies are definitely at the limits of what we can simulate,鈥� he says. 鈥淲hen you don鈥檛 have agreements between the simulations, it鈥檚 normal to be sceptical about what you are concluding from them.鈥�
What kind of strange
Recent measurements from the Hubble Space Telescope may soon resolve the dispute about how far away these wispy galaxies are. This should at least tell us what kind of strange they are: either empty of dark matter but full of unexpectedly bright globular clusters, or full of dark matter but moving extremely quickly.
If they turn out to be the former sort, that may be most exciting. 鈥淭hat would be really unique and hard to explain, because we don鈥檛 have any good ideas about how huge and bright globular clusters like that could be formed,鈥� says Trujillo.
鈥淣o matter what the distance is, these galaxies are somehow special,鈥� he says. Figuring out exactly how will also help us determine how much of a threat they are to our view of the universe.
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