Galactic filaments – the vast threads that link clusters of galaxies – may feed them with the matter required to make new stars, researchers now believe.
Galactic clusters are the largest known structures in the universe and they are strung together in a cosmic web of vast filaments and nodes that looks uncannily like the neuronal structure of a human brain.
Scientists believed that the filaments were made of a mixture of dark matter – the hypothetical material that researchers believe makes up most of the matter in the universe — and its normal counterpart. But they could find neither in the filaments.
‘We want to somehow produce a visual map of the distribution of this dark matter in the entire universe.’
Dr Julian Merten, University of Oxford, UK
That was until Dr Dominique Eckert, principal investigator for a project on the European Space Agency’s XMM-Newton telescope, announced last year that his team had found hot gas running through filaments during observations of the distant galaxy cluster Abell 2744.
He had been looking for dark matter, but what he found was normal matter in gigantic quantities.
‘It was serendipity,’ said Dr Eckert, who is based at the University of Geneva, Switzerland.
His discovery will help with a fundamental stock-taking problem for cosmology – that only half of the ordinary matter that was around a billion years after the Big Bang has been located today. But it may also explain how galaxies can make stars.
Galaxies manufacture stars and this requires huge amounts of gas. So far, however, researchers have only found a trickle of new gas travelling into the galaxies where it is needed.
‘There’s a huge mismatch,’ said Professor Erwin de Blok, of the Netherlands Institute for Radio Astronomy, who is trying to understand how galaxies evolve.
This apparent giant fuel deficit means that — though it may not be high on your worry list — galaxies should start to fizzle out in 2 billion years.
As part of the EU-funded GALGASSKA project, Prof. de Blok is now scouring the new plentiful gas supplies of the cosmic web for signs that they could be acting like giant petrol pump nozzles, fuelling hydrogen gas into the galaxies.
‘The cosmic web is a big gas reservoir that contains as much ... gas as we need,’ he said.
Unfortunately most of it is too hot to do the job. At tens of millions of degrees, it is so energetic that galaxies cannot capture it.
But the latest computer simulations predict that, just at their fringes, gas filaments such as Dr Eckert’s may drift close to the galaxies and cool at the last minute. Could this be the gas that Prof. de Blok is looking for?
To answer this question Prof. de Blok needs to scan as much of the sky as possible to a very great sensitivity.
‘The gas is at a very low density and it’s a huge challenge to observe it directly,’ he said.
And so he is making use of the data from telescopes using the technique of radio interferometry, in which the signals of many radio antennae are combined in a computer and treated as if they are parts of a much bigger telescope.
‘We are trying to detect a signal that is ten thousand times fainter than the gas we normally observe through radio telescopes,’ he said. ‘You are really pushing the system to the limits.’
While Prof. de Blok is searching for cool ordinary matter, others are trying to understand dark matter. Cosmologists believe dark matter must exist to prevent the ordinary matter in the universe flying apart, and it is thought to form the skeleton of the cosmic web.
It neither absorbs nor emits light, so it can’t be seen, but it does exert a gravitational pull, and thus, like normal matter, it bends light.
This gravitational lensing distorts the images of distant galaxies collected by optical telescopes, and astronomers can measure these distortions to work out just how much matter the light has passed on its journey to the telescope.
If X-rays can reveal how much normal matter there is in, say, a galaxy cluster, and an optical picture of the same cluster can reveal from its distortions how much matter is present overall, then subtracting one from the other can reveal how much dark matter there is.
Dr Julian Merten, a Marie Skłodowska-Curie fellow at the University of Oxford, UK, is doing just that.
‘We want to somehow produce a visual map of the distribution of this dark matter in the entire universe,’ he said. ‘Only in recent years do we have the data. You need very crisp optical images.’
One source for his EU-funded WEBMAP project is the famous Hubble space telescope but, despite its spectacular images over the years, it views, with each pointing, a mere thumbprint of the sky just an eightieth the size of the moon.
But a glut of telescopes that can see much more widely is coming.
There is the Dark Energy Survey, from one of the best digital cameras in the world, in Chile; the Kilo Degree Survey, using the Very Large Telescope at the European Southern Observatory, also in Chile; and the Hyper Suprime-Cam on the Subaru telescope in Hawaii, US.
Dr Merten also eagerly awaits the European Space Agency’s dark universe mission, Euclid, which will map half the sky with an image quality only achievable from space.
‘The data it will produce will be a kind of dream – crisp and not altered by Earth’s atmosphere,’ he said.
Ultimately he wants to be able to distinguish between competing theories about the nature of dark matter. Each theory predicts different shapes and distributions so, with good data, it may be possible to eliminate some of them.
‘This, ultimately, will bring us a little closer to one of the great cosmological questions: what dark matter is,’ he said.
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