By Liat Clark, Wired UK
Galaxies need 10 times less dark matter to sustain star formation than previously thought, but just the right amount can set off rapid star formation, a recent study suggests.
The discovery was made after European Space Agency photos showed evidence of dark matter 300 billion times the mass of our Sun supporting ancient galaxies. The galaxies -- over 10 billion light years from Earth -- are some of the most active in the universe, producing thousands of stars each year compared to the 10 a year the Milky Way produces on average.
"If you start with too little dark matter, then a developing galaxy would peter out," Asantha Cooray, the University of California astrophysicist who led the study, said in a press release. "If you have too much, then gas doesn't cool efficiently to form one large galaxy, and you end up with lots of smaller galaxies. But if you have the just the right amount of dark matter, then a galaxy bursting with stars will pop out."
The 300-billion-solar-mass size of ancient galaxies studied seems to encourage star formation more than any other previously recorded mass. It does not just sustain star formation, it facilitates and promotes it, changing previous theories of how galaxies are formed.
It is thought that dark matter, which is believed to make up around 20 percent of the universe's energy density, lays the groundwork for galaxy formation. Its gravitational pull attracts gas and dust, which gather and soon condense to form stars. The dark matter then collects around young galaxies in the form of giant spheres known as halos.
It does not reflect light and is therefore not visible to us. Cooray and his team detected it by measuring its gravitational pull on other, visible matter. Using the ESA's Herschel telescope they took infrared images at wavelengths 1,000 times longer than those visible to the naked eye. The photos can permeate dust-filled galaxies and Cooray used them to measure light emitted by the galaxies.
Their research concentrated on the Lockman Hole -- an area of sky about the size of Earth's moon within the Ursa Major constellation. It is an ideal test area because there is minimal dust blocking the view.
The photos' web-like patterns are actually an intricate map of galaxies illuminated in infrared. They revealed that distant galaxies produce stars at a rate three to five times higher than young galaxies more easily visible to us. The images are far more detailed than those taken using the Hubble telescope, and by illuminating galaxies in this way much can be learned.
Jamie Bock, who studies Herschel's Spectral and Photometric Imaging Receiver at NASA's Jet Propulsion Laboratory in California, explained the photos are a huge advancement for galaxy formation studies.
"It turns out that it's much more effective to look at these patterns rather than the individual galaxies," he said in the release. "This is like looking at a picture in a magazine from a reading distance. ... Herschel gives us the big picture of these distant galaxies, showing the influence of dark matter."
Galaxies may be sustained on even less dark matter, but they would be short-lived. Supernovas would be common in this instance, it is theorized, and without enough gravitational pull supplied by dark matter the remaining gases would dissipate.
Image: A computer simulation of dark matter distribution when the universe was about 3 billion years old. Blue shows the basic distribution of dark matter particles, red shows dark matter halos model, and yellow shows dark matter halos that are most likely to fuel starburst galaxies. (Alexandre Amblard/The Virgo Consortium/ESA)
Source: Wired.co.uk
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