Posted: September 16, 2008
Unlike the familiar ‘normal’ matter that makes up stars, gas and dust, dark matter is invisible, yet it is thought to make up around 22 percent of the mass of the Universe, and its presence is inferred through its gravitational influence on surrounding stars and galaxies. In comparison, normal matter comprises just four percent of the mass of the Universe, and the mysterious dark energy makes up the remaining 74 percent.
Despite the pervasive nature of dark matter, however, no one is sure what it actually consists of, but through supercomputer simulations, scientists can model the initial fluctuations in dark matter a few hundred thousand years after the big bang, and track their evolution as they collapse under gravity to form the galaxies and galaxy clusters that we see today. Until now, most simulations have modelled the evolution of the dark matter component alone, since it comprises such a large element of the mass of the Universe, and it only interacts via gravity.
A simulated dark matter disc (red contours) from the new supercomputer simulations overlaid on a 2MASS image of the Milky Way. Although the dark disc is a tiny component of the Galaxy by mass, it is interesting because it is a source of very low velocity WIMPs that could be detectable by the XENON Dark Matter Search Experiment. Image: J. Read & O. Agertz.
“In our paper, we made the first attempt to model also the stars and gas in the Universe – the atoms from which we are made up – and their effect on the local distribution of dark matter in our Galaxy,” lead scientist Justin Read tells Astronomy Now. Prior to this work, it was thought that dark matter forms in roughly spherical lumps called ‘halos’, one of which envelopes the Milky Way. “In our Galaxy, for example, although most of its total mass is dark matter, interior to the orbit of our Sun (about eight kiloparsecs (kpc) from the Galactic centre) actually most of the mass is in stars and gas, not dark matter. The dark matter dominates the mass only further out, some 10 kpc from the Galactic centre. This means that at our local position within the Galaxy, the stars and the gas will have a strong influence on the local distribution of dark matter and this is what we set out to measure.”
In the simulations, the team observe that dark matter halos form hierarchically through merging events of smaller halos. Without considering the influence of stars and gas, the halos end up being roughly spherical. But as the gas cools down it settles into a disc, which gradually forms stars that produce the galaxies we see today.
“The key point about our new work is that we realised that this disc of gas and stars will affect the subsequent accretion of the dark matter halos, each of which contains its own little galaxy,” says Read. “The halos are now preferentially dragged towards the disc plane. As they are torn apart by tidal forces, their material then settles into a disc-like structure: this is the dark matter disc. We see precisely this effect in our simulations, which we use to predict how massive this dark matter disc should be.”
But Read says that the global structure of the dark matter halo is still roughly spherical, agreeing well with simulations that model dark matter alone, since only those halos that actually get close enough to hit the gas disc and are oriented close to the disc plane are strongly affected. “But locally, i.e. around the Earth and the Sun, there is this focusing effect and so extra to the standard dark matter halo there is a dark matter disc,” he says. “If we live in a Universe dominated by dark matter, then the dark matter disc is a natural consequence of such a theory and there must be one in our own Galaxy.”
The simulations also show that the dark matter disc has only about half of the density of the dark matter halo, but even so, the existence of such a disc has dramatic implications for the detection of dark matter here on Earth.
The Earth and Sun move at roughly 220 kilometres per second along a nearly circular orbit about the centre of our Galaxy. Since the dark matter halo only has a negligible rotation, from an Earth-based perspective it feels as if we have a ‘wind’ of dark matter flowing towards us at great speed. By contrast, the ‘wind’ from the dark disc is much slower than from the halo because the disc co-rotates with the Earth. This abundance of low-speed dark matter particles could be the key to detecting dark matter particles because, scientists say, they are more likely to excite a response in dark matter detectors than fast-moving particles.
“Current detectors cannot distinguish these slow moving particles from other background ‘noise’,” says Professor Laura Baudis, a collaborator at the University of Zurich and one of the lead investigators for the XENON direct detection experiment, which is located at the Gran Sasso Underground Laboratory in Italy. “But the XENON100 detector that we are turning on right now is much more sensitive. For many popular dark matter particle candidates, it will be able to see something if it’s there.”
The research is described in a recent issue of the journal Monthly Notices of the Royal Astronomical Society.