Dark matter clues in oldest stars

From: http://news.bbc.co.uk/2/hi/science/nature/6993870.stm

Sep 14, 2007

Simulation: With cold dark matter, structures become clumpy

A computer model of the early Universe indicates the first stars could have formed in spectacular, long filaments.

These structures, which may have been thousands of light-years across, would have been shaped by "dark matter".

Scientists know very little about this type of matter, even though it accounts for most of the mass in the cosmos.

The researchers told the British Association (BA) Festival of Science that their work could reveal the true nature of dark matter.

Liang Gao and Tom Theuns from Durham University, UK, also reported their findings in the journal Science.

Quick or slow

Astronomers believe that more than three-quarters of the matter in our Universe may be "dark". It does not reflect or emit detectable light, and so cannot be seen directly - but it does gravitationally pull on normal matter (the gas, stars, and planets we see in space).

It is this interaction that allows scientists to predict its existence - even if they cannot say what it is. Various types of exotic particle seem to be the favoured theory, with equally exotic names such as neutralinos, axions and gravitinos.

The new research, though, may give some clues as to dark matter's properties. Computer modelling suggests there is a link between the structures assumed by early stars and the temperature of the dark matter amongst them.

Tom Theuns, from Durham's Institute for Computational Cosmology, told the festival: "What we found for the first time is that the nature of the dark matter is crucial to the nature of the first stars.

"In cold dark matter the particles move very slowly; in warm dark matter they move very quickly," he explained.

"We found that if the dark matter consists of these fast moving particles, then the first stars form in very long, thin filaments.

"The filaments have a length about a quarter the size of the Milky Way and contain an amount of matter and gas about 10 million times the mass of the Sun, so that provides a lot of fuel for many stars."

Exotic collection

Some of the stars that formed within the filaments would have had a relatively low mass, which is of interest to astronomers as they have a long lifespan and could still survive today.

Dr Theuns added: "In stark contrast, what happens in (the simulation with) cold dark matter is very, very different.

"Here, the first stars formed in little lumps of dark matter, and just one star per dark matter lump. And these stars are probably very massive as well: 100 solar masses.

"Because these stars are so massive, they die very quickly; so you wouldn't find such stars in the Milky Way today," he said.

Scientists believe that the temperature of the dark matter indicates what kind of particles it is made of.

Observational pointers

The research team hopes answers could come from astronomers who are now scouring the skies to find signs of very old stars.

If dark matter is warm, then some of these very first stars may be in the Milky Way today.

However, detecting the massive stars formed in cold dark matter would require very powerful telescopes capable of "peering into the very distant Universe," Dr Theuns added.

"We don't know what the dark matter is, we don't know what the first stars are. If we bring these two problems together, when we know more about one, then we can say something about the other."