The way in which stars, galaxies and other cosmic structures originally formed depends strongly on the nature of dark matter, claim physicists in the UK and Belgium. By simulating the early effects of “warm” dark matter, which some researchers consider the most likely form of dark matter, the researchers found that the first stars would have been produced in massive “filaments” of dense gas -- thus going against physicists’ conventional view of star formation (Science 317 1527).
As yet undiscovered, dark matter is thought to exist because galaxies seem to be held together by the gravitational attraction of much more mass than we can see through telescopes. Dark-matter particles could be “hot”, meaning that they are light and fast, although simulations suggest that hot dark matter would not explain how cosmic structure formed after the Big Bang. Most physicists therefore think “cold” or slower-moving particles are more likely because they do a better job of accounting for the universe’s evolution. But cold dark matter appears to be at odds with the observed densities of certain sub-galactic structures.
In the last few years, however, some researchers have latched onto the possibility of warm dark matter, which would still get the universe’s large-scale evolution right but would agree better with observations of smaller-scale structures. Now Liang Gao from Durham University and Tom Theuns from the University of Antwerp have performed numerical simulations on early star formation that give further credence to warm dark matter.
According to cold dark-matter theory, the first stars formed from “minihalos” inside huge isolated clouds of gas and dark matter — unlike stars in our present era, which form in molecular gases inside galaxies. In Gao and Theuns’s simulation, a generic warm dark-matter particle would change this picture so that the minihalos are replaced with trailing “filaments” of accumulated gas. These filaments would have been massive — about 9,000 light years long or a quarter the size of the Milky Way.
The researchers say that the filaments could well have produced a large number of low-mass stars that could exist to this day, and over time may have collapsed to seed the supermassive black holes that we appear to detect at the centres of most large galaxies.