Hubble trouble: Astronomers baffled by ‘dark core’ of matter

By
Rob Waugh

Last updated at 6:38 PM on 2nd March 2012

Astronomers have been left scratching their heads after the Hubble Space Telescope found a huge, dark lump in space which should not exist.

The dark matter in the distant galaxy cluster Abell 520, 2.4 billion light years from earth, appears to have collected into a ‘dark core’ separate from the galaxies around it, after a collision between them.

But dark matter – a mysterious ‘glue’ thought to hold galaxies together – shouldn’t do that.

Dark matter

Dark matter, galaxies, and hot gas in the core of the merging galaxy cluster Abell 520, 2.4 billion light-years away, and formed from a violent collision of massive galaxy clusters. The blue areas show the location of most of the mass in the cluster, which is dominated by dark matter. Dark matter is an invisible substance that makes up most of the universe’s mass

The Hubble Space Telescope drifts through space: Astronomers confirmed the measurements of Abell 520 using the Hubble space telescope - and are now more puzzled than ever

The Hubble Space Telescope drifts through space: Astronomers confirmed the measurements of Abell 520 using the Hubble space telescope – and are now more puzzled than ever

The problem is that dark matter – a
mysterious ‘glue’ thought to hold galaxies together – should remain
stuck to galaxies.

The initial detection, made in 2007, was so unusual that astronomers shrugged it off as unreal.

However, new results from NASA’s Hubble Space Telescope confirm that the dark matter and galaxies parted ways in Abell 520.

The blend of blue and green in the center of the image reveals that a clump of dark matter resides near most of the hot gas, where very few galaxies are found.

Now astronomers have to work out why.

First detected about 80 years ago, dark
matter is thought to be the gravitational ‘glue’ that holds galaxies
together.

Astronomers
know little about dark matter, yet it accounts for most of the
universe’s mass.

‘This result is a puzzle,’ said astronomer James Jee of the University of California, Davis, leader of the Hubble study. ‘Dark matter is not behaving as predicted, and it’s not obviously clear what is going on.’

‘Observations like those of Abell 520 are humbling in the sense that in spite of all the leaps and bounds in our understanding, every now and then, we are stopped cold,’ explained Babul, the team’s senior theorist.

Is Abell 520 an oddball, or is the prevailing picture of dark matter flawed? Jee thinks it’s too soon to tell.

The team has proposed a half-dozen explanations for the findings, but each is unsettling for astronomers.

‘It’s pick your poison,’ said team member Andisheh Mahdavi of San Francisco State University in California, who led the original Abell 520 observations in 2007.

One possible explanation for the discrepancy is that Abell 520 was a complicated interaction between three galaxy clusters, instead of just two colliding systems.

A third possibility is that the core contained many galaxies, but they were too dim to be seen, even by Hubble. Those galaxies would have to have formed dramatically fewer stars than other normal galaxies.

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