杏吧原创

A nearby galaxy cluster called the Fornax Cluster is ripping apart its dwarf galaxies. They appear to be tearing up far more easily than we would expect, suggesting that they may not contain any dark matter. That may mean there is something fundamentally wrong with our understanding of the universe.

In the standard model of cosmology, called lambda-CDM, most galaxies should contain a healthy dollop of dark matter. The gravity of this invisible substance helps hold聽a galaxy together in the face of external perturbations. But when at the University of Bonn in Germany and her colleagues analysed 353 dwarf galaxies in the Fornax Cluster, far more of them than expected were distorted by the pull of other galaxies, especially near the centre of the cluster. This seems to indicate that they have little to no dark matter to hold them together.

鈥淲hat we鈥檙e seeing is if you move a dwarf galaxy into a galaxy cluster, the dwarf suddenly seems to have a lot less self-gravity,鈥� says at the University of St Andrews in the UK, part of the research team. 鈥淭hat wouldn鈥檛 be the case if the dwarf [galaxy] has dark matter 鈥� there鈥檚 no reason the dark matter should suddenly evaporate when you move a dwarf galaxy into a cluster.鈥�

However, with no dark matter, it is difficult to understand how these galaxies could exist at all. 鈥淚t鈥檚 not possible that these dwarf [galaxies] have the type of dark matter described by lambda-CDM,鈥� says Asencio. 鈥淚f you鈥檙e not explaining these galaxies with dark matter, you need to explain them another way.鈥�

The researchers performed simulations of how these galaxies would behave under an alternative model for gravity called modified Newtonian dynamics, or MOND. In this model, isolated galaxies are expected to behave differently from those in a cluster that are subject to strong external gravitational fields.

The team found that the MOND model correctly predicted the warping of the Fornax Cluster鈥檚 dwarf galaxies. 鈥淚t鈥檚 very difficult to explain it any other way,鈥� says Banik.

However, MOND is controversial among astrophysicists, largely because it doesn鈥檛 match established observations of the cosmic microwave background (CMB) 鈥� a sea of radiation left over from the big bang 鈥� and is difficult to reconcile with some long-accepted theories such as Albert Einstein鈥檚 general theory of relativity. The researchers are now working on testing MOND in other ways, which could help lend weight to their proposed explanation of the shredded galaxies in the Fornax Cluster.

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