Overweight galaxies force-fed by dark matter tendrils

Overweight galaxies force-fed by dark matter tendrils

* 00:01 07 April 2009 by Rachel Courtland

A clutch of massive galaxies that seem to be almost fully-formed just 5 billion years after the big bang challenge models that suggest galaxies can only form slowly. Tendrils of dark matter that fed the young galazxies on gas could be to blame (NASA/CXC/ESO/P Rosati et al)

Some of the biggest galaxies in the early universe seem to have grown quicker than thought possible and may have bulked up on streams of gas flowing along filaments of dark matter, say researchers.

Monster galaxies have long been thought to take a long time to form, growing slowly by gobbling up smaller galaxies like a giant amoeba absorbing food.

But overeating in this way cannot explain why some of the brightest galaxies at the heart of five clusters dating from relatively soon after the big bang – more than 8 billion years ago – grew so large, so fast, according to new research performed using the Subaru telescope in Hawaii.

Models suggest that if they snacked only on other galaxies, those ancient leviathans should have been just a fifth as massive as the biggest galaxies in similar clusters today that have had longer to eat their smaller neighbours. But instead the ancient galaxies appear to be roughly 90 per cent as massive as their present-day counterparts.

"It could be the tip of the iceberg. It might mean the simulations [of the early universe] need to be significantly altered," says Chris Collins of Liverpool John Moores University in Birkenhead, UK.


Rewriting history
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Either simulations of large galaxies gobbling up smaller ones have misjudged some physical principles, like star formation and the behaviour of gas, or the rapid growth was fed by a completely different diet, says Collins.

There is a limit on how quickly galaxies can draw in gas needed to fuel star formation, since pulling it in too fast raises its temperature to create a shock-wave-like barrier that prevents more gas from entering.

However, simulations published earlier this year suggest early galaxies could feed more quickly if they were situated on filaments of dark matter that act like pipes, allowing gas to flow rapidly into a galaxy while staying cool.

These filaments may already have been spotted in mystery blobs of hydrogen recently found surrounding other galaxies, Collins says. Finding such features in the dust around these large galaxies could be additional evidence that dark matter may be responsible.

"Overall the results are every intriguing. They show the history of these monster galaxies is more complex than we expected," says Kenneth Rines of Western Washington University in Bellingham, Washington.

But Rines, who has studied a nearby galaxy that seems to be in the process of eating three neighbours, notes it may be too early to eliminate the merger scenario entirely. But he acknowledges that there is probably more than one explanation for the way such galaxies grow.