Dark matter filaments stoked star birth in early galaxies

18:58 21 January 2009 by Rachel Courtland Tendrils of dark matter channelled gas deep into the hearts of some of the universe's earliest galaxies, a new computer simulation suggests. The result could explain how some massive galaxies created vast numbers of stars without gobbling up their neighbours.Dramatic bursts of star formation are thought to occur when galaxies merge and their gas collides and heats up. Evidence of these smash-ups is fairly easy to spot, since they leave behind mangled pairs of galaxies that eventually merge, their gas settling into a bright, compact centre.But several years ago, astronomers began finding disc-like galaxies with crowded stellar nurseries that seemed to bear no hallmarks of a past collision. These galaxies, which thrived when the universe was just 3 billion years old, were at least as massive as the Milky Way, but created stars at some 50 times our galaxy's rate.Blow awayIt was not clear how these galaxies could harbour such intense bursts of star formation without collisions. Smaller galaxies are thought to form when gas falls in from all directions. But this process would not work with larger galaxies - those about the Milky Way's size or heavier. These galaxies grow so hot and dense they create a shock-wave-like barrier that heats incoming gas and prevents it from falling in.But Avishai Dekel of Hebrew University in Jerusalem thinks an influx of gas could be responsible for the star formation after all. This gas could flow along filaments of dark matter that make up a cosmic web still seen today in the distribution of galaxies across the sky.Dekel and colleagues used fluid dynamics simulations to model the cosmic web of gas and dark matter at a time when the universe was some 3 billion years old. They tracked how gas accumulated in galaxies lying at the nodes of the web, where dark matter filaments intersect.Shock resistantThe team found that gas in the tendrils was so dense that collisions between particles would dissipate energy quickly, making it less susceptible to shocks in the surrounding, hotter gas. The cool gas could then fall into the galaxy's disc fast enough to fuel dramatic starbursts."We found the gas can penetrate all the way through the hot material," Dekel told New Scientist. "This is solving the riddle of where this star formation is coming from."Reinhard Genzel of the Max Planck Institute for Extraterrestrial Physics in Garching, Germany, says the explanation could work, but adds that the simulation cannot estimate how rapidly the gas can be converted to stars, which would be a crucial test.More detailed simulations and studies of the galaxies themselves could confirm the model. "We need more time to test it out, but it smells like the right answer," Genzel told New Scientist.copyright URL http://www.newscientist.com/article/dn16462-dark-matter-filaments-stoked-star-birth-in-early-galaxies.htmlDARK MATTER

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