19 August 1998

MACHO Researchers Weigh-Up Dark Matter

Observations at Mount Stromlo Observatory near Canberra, Australia, have shed some light on 'dark matter' - the unseen material astronomers have spent decades searching for. An international conference at the Academy of Science in Canberra was told yesterday that Australian and US researchers have found objects, lying in a 'halo' around our galaxy, that could account for about 50 per cent of our galaxy's unseen mass.

The hypothetical star-sized lumps were christened MACHOs - MAssive Compact Halo Objects.

The MACHO team hunted down its prey through 'gravitational lensing'. When a MACHO passes in front of a background star the MACHO's gravitational field acts like a lens, magnifying the star's light. The MACHO team has recorded about 15 such events.

The dark matter appears to be in lumps of about half the mass of the Sun. Exactly what the lumps are is a matter of some conjecture. "The favoured candidate is dim shrunken stars called 'white dwarfs'," Dr Alcock, the team leader from the US Lawrence Livermore National Laboratory, said. "But there are problems with those. When white dwarfs form there's a lot of associated chemical 'pollution' in space, and we're not seeing that."

Primordial black holes are another possibility, says Professor Freeman, another team member. "To be the right mass they must have formed early in the life of the Universe, about ten microseconds after the Big Bang," he said.

"We've known of this stuff for decades because of its gravitational effects," said Dr Alcock. "But we didn't know if it existed as big dark star-sized lumps or exotic elementary particles."

The observations were made with the Great Melbourne Telescope at Mount Stromlo Observatory. The telescope system can record the brightnesses of tens of millions of stars in a night. The background stars which the MACHOs magnified were in a small neighbouring galaxy called the Large Magellanic Cloud.

Professor Freeman cautioned that MACHOs help to solve the dark matter problem for our galaxy but not for the whole universe. "There is a great deal of dark matter out there and there are many other things it could be," he said.

The MACHO project has run for five years and will wind down after another two. Similar work has just been started by a French team using a telescope in Chile.