Absence of dark matter startles astronomers


The spin of extremely old and distant galaxies shows they are dominated by regular matter rather than the dark stuff. Cathal O'Connell reports.


A spiral galaxy.
NASA

We know that galaxies today move according to the wishes of an invisible choreographer—the mysterious dark matter. But perhaps it wasn’t always so.

New work, published in Nature shows that once upon a time the galaxies danced freestyle, with almost no influence of any invisible component. The unexpected result is at odds with how cosmologists think galaxies formed.

Back in the 1970s, the American astronomer Vera Rubin measured the rotation speeds of our galactic neighbours and found stars at the edges were moving way too fast to be accounted for by visible matter alone. She concluded that most of the mass must exist in some form we can’t see.

“This ‘dark matter’ now underpins much of our understanding of the large-scale structure of the universe, and also defines how galaxies form and evolve,” writes the Durham University astronomer Mark Swinbank in a perspective piece, also in Nature.

Just like how the tip of an iceberg jutting above the sea represents only a small fraction of the immensity beneath, galactic discs are the visible component of a much larger structure – the huge cloud of dark matter that surrounds them.

The new work seems to contradict this picture – uncovering ancient galaxies that appear to be naked, unclothed in a dark matter shroud. A

team led by Reinhard Genzel at the Max Planck Institute for Extraterrestrial Physics used the 8.2-metre Very Large Telescope in Chile to peer at six galaxies that existed about 10 billion years ago, when the universe was just one fifth of its current age. The authors chose these galaxies in particular because they had the right rate of star formation, and the right mass, to eventually turn into large spiral galaxies like the Milky Way.

To observe the spin of the galaxies, the team focused on hydrogen gas, watching how its well known fingerprint was shifted by the relative speeds of the galaxies — a bit like how the sound of a car shifts to a higher pitch as it approaches you, compared with when it is going away.

The resulting data set and level of detail are “remarkable”, according to Swinbank, who was not involved in the study.

And the data told a conclusive story: the rotation rates of the galaxies decreased with distance from the centre. That’s exactly what you expect from a system, like our solar system, where most of the mass is located around a central core. The Earth whizzes around the sun at 30 kilometres per second, while Pluto, by comparison, creeps along at less than five.

This indicates a surprising lack of dark matter in and around the studied galaxies. However, it does not mean that there was less dark matter in the early universe.

We know the dark matter content very well from the cosmic microwave background radiation, the “echo” of the Big Bang, which tells us about 85% of the matter in the universe is dark. But the result does shed a surprising new light on how galaxies form and evolve.

As we currently understand it, galaxies form inside haloes of dark matter. The visible stuff collides with itself in a galactic scale demolition derby, and so tends to accumulate into a relatively dense core, which gradually forms a disc.

Meanwhile the dark matter, because it can interact only through gravity (and possibly the weak force), passes through everything like a ghost walking through a wall in an old horror flick, and forms a dispersed cloud.

To align their result with this model, Genzel and his team suggest that the rapid pile-up of visible matter must be ruling these early proceedings. It’s also possible that the dark matter is still being recruited from extra-galactic space—and will later form a more dominant halo.

While the work will need to confirmed with a much larger sampling of galaxies, Swinbank says, the new work is “an important step towards identifying the dominant physical processes responsible for galaxy formation.”

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Cathal O'Connell is a science writer based in Melbourne.