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An international consortium of scientists participating in the so-called MACHO Collaboration has concluded that gravitational microlensing events observed in the Milky Way are indeed related to the influence of massive compact halo objects (MACHOs), and that their measurements of these objects constitute the first definite observation of dark matter. However, the MACHOs can account for only about 19 percent of the presumed dark matter halo shrouding our galaxy, leaving open the question of what makes up the rest of the undetected mass of our universe.
The presence of MACHOs, such as non-radiating neutron stars or white dwarfs and substellar objects such as planets, is invoked to partially explain the rapid rotation of the outer parts of the Milky Way. Several groups have been searching for MACHOs by scanning over 20 million stars in the central galactic bulge and in the overhead Large Magellanic Cloud (LMC). They look for instances of microlensing, a phenomenon in which the star's light is brightened by the gravitational focusing effect of a foreground MACHO. (On an extragalactic scale, large lensing effects have been observed in which a distant quasar's image is split in two by the gravity of foreground galaxies.)
The MACHO collaboration recently completed its analysis and concluded that they had detected four microlensing events towards the LMC and more than 40 events towards the galactic bulge - significantly more than had been predicted by the standard model of the central structure of the Milky Way. Because the size and shape of the dark halo that is needed to explain the motion of stars in our galaxy depends upon the structure and amount of luminous matter in the galaxy, these observations are forcing a revision of the standard models.
"Combining our results for the dark halo and the galactic center is forcing a revision of the standard model of our galaxy's structure, as well as its dark halo," said Kem Cook, a physicist at Lawrence Livermore National Laboratory who reported on the collaboration's findings at a Monday morning session of the April Meeting.
Many astronomers have clung to the theory that the invisible mass that should make up about 90 percent of the universe is in the form of normal matter, such as stars, planets, asteroids, and quiescent black holes, but so dim or dark that it has eluded detection. However, as searches using increasingly powerful and versatile technologies continue to rule out the simplest explanations, the likelihood increases that the solution lies in exotic hypothetical forms of matter such as weakly interacting massive particles (WIMPs) that pass unscathed and undetected through planets and people.
Cook reported that he and his colleagues were now convinced that the LMC events are indeed related to the influence of truly nonluminous objects. They therefore assert that these measurements constitute the first definitive observation of dark matter in our galaxy. The halo dark matter is most likely in the form of sub-stellar objects known as brown dwarfs, which are about 4 percent as massive as the sun. Furthermore, they calculate that the mass of the MACHOs in the halo added up to about 7.6 x 1010 solar masses and that the MACHO fraction of the dark halo was about 19 percent.
Unlike the LMC events, the lensing events seen in the direction of the galactic bulge are probably caused by ordinary stars and not dark matter objects. Still, the bulge events are of interest partly because they may offer a way of looking for extra-solar planets. One observed lensing event entailed a double-cusped brightening, suggesting to the MACHO scientists that some lensing objects are binary systems; some of these might be star-planet systems.
The MACHO Project is currently releasing information about microlensing events as they begin using its near real time analysis system now in place. The lensing technique should soon provide a powerful new tool for detecting extra-solar planetary systems. "The possibility that we can discover planets orbiting distant stars is an exciting dividend which arises from the rather esoteric search for baryonic (normal) dark matter in the halo of our galaxy," said Cook
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