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Dark matter's such a pushover: Baby stars can shove weird stuff around dwarf galaxies

Mystery material 'a cold, collisionless fluid' apparently

Dark matter may be even more elusive than previously thought, as researchers believe the mysterious material hidden at the heart of galaxies can be moved around with the power of heat.

The process described as "dark matter heating" occurs in dwarf galaxies that are often found hovering around larger galaxies like the Milky Way. A group of international astrophysicists from the University of Surrey, Carnegie Mellon University, and ETH Zurich studied a total of 16 dwarf galaxies split into two shapes: irregular, amorphous blob-like ones and spheroid ones. All were at various stages of evolution, and the researchers estimated the amount of dark matter contained in each one.

They found that older galaxies with slower rates of star formation had denser cores of dark matter than younger galaxies. Galaxies older than six billion years had more than a whopping hundred million solar masses worth of dark matter at their core, whereas as ones younger than three billion years had "shallower" dark matter cores. The results were published yesterday in the Monthly Notices of the Royal Astronomical Society.

The researchers reckon that the copious amounts of energy emitted by growing stars heats up the centre of the galaxy, and nudges the dark matter away. As the stars form, strong gusts of electromagnetic radiation sweep gas and dust away, leaving the cores with less mass. The dwindling mass means there is less gravitational attraction, and the dark matter gains enough energy to move away from the galaxy's centre.

"We found a truly remarkable relationship between the amount of dark matter at the centres of these tiny dwarfs, and the amount of star formation they have experienced over their lives," said Justin Read, lead author of the study and a physics professor at the University of Surrey. "The dark matter at the centres of the star-forming dwarfs appears to have been 'heated up' and pushed out."

Dark matter makes up about 27 per cent of the universe, and although it's more abundant than visible baryonic matter, it has yet to be directly detected. It doesn't interact with light but is affected by gravity. The dynamic nature of the material has led the researchers to believe that dark matter is, in fact, "a cold, collisionless, fluid that can be kinetically 'heated up' and moved around". ®

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