In April, astronomers with the European Southern Observatory led by Christian Moni-Bidin of the University of Concepcion in Chile raised eyebrows by suggesting that our cosmic neighborhood is empty of the extra mass needed to hold the galaxy together. But researchers Jo Bovy and Scott Tremaine from the Institute for Advanced Study in Princeton, NJ, have submitted a paper claiming that the results reported by Moni Biden et al are “incorrect”, and based on an “invalid assumption” of the motions of stars within — and above — the plane of the galaxy.
The most popular current theories say that dark matter is a hitherto undetected particle called a WIMP (weakly interacting massive particle. But several underground detectors searching for the elusive WIMPs have come up empty, or with conflicting results. If the galaxy is so full of dark matter, why hasn't it been detected yet?
The ESO team, led by Christian Moni Bidin of the Universidad de Concepción in Chile, mapped over 400 stars near our Sun, spanning a region approximately 13,000 light-years in radius to estimate the mass of matter – visible and dark – in the sun's local neighborhood. . Their report identified a quantity of material that matched what could be directly observed: stars, gas, and dust… but no dark matter. “Our calculations show that it should have shown up very clearly in our measurements,” Bidin had stated, “but it was just not there!”
Dark matter is the mysterious, invisible stuff that makes up 83 per cent of the matter in the universe. It is responsible for keeping galaxies from flying apart and has aided our understanding of how structures in the universe formed.
The primary flaw of the ESO team is that they assumed that the stars' speeds would be the same no matter how far they were from the galactic centre. Observations of dust clumps have shown that this assumption is true for young stars orbiting in the galactic disc, which mostly move in a near-perfect circle. But stars that orbit high above or far below the central disc can't have circular orbits, according to Jo Bovy says. The only stars that reach such great heights have been pushed from the disc by matter in the galaxy's spiral arms, which sent them on highly elliptical orbits.
"By assuming that stars rotate at the same velocity no matter at what distance they are from the centre of the Galaxy, they underestimated the total amount of matter in the solar neighbourhood and they concluded that there was no room for dark matter," said Bovy.
Bovy and Tremaine studies the ESO data and found that the amount of dark matter in the sun's neighbourhood agrees with previous predictions – if anything, there might be a little more nearby dark matter than we thought before. "We are now sure that the local dark matter density is what experimental particle physicists have long assumed when running their experiments," the team concluded.
The image at the top of the page shows the outer "halo" region of the Milky Way, as with other galaxies, contains the vast majority of the mass of the Galaxy. Yet the exact mass and shape of the halo are not known. Unlike the Galaxy's spiral arms, which contain bright stars, the halo is mostly dark. The few stars in the halo are the oldest stars in the Galaxy, and include the globular clusters, but they can account for only a small fraction of the halo's mass. Understanding the exact nature of the Milky Way halo may help to explain dark matter, which as the Princeton team asserts, seems to pervade the universe.