LIVERMORE, Calif.—Astronomers from the Lawrence Livermore National Laboratory, in collaboration with an international team of researchers, have directly detected and measured the properties of a gravitational microlensing event in the Milky Way.
By fusing microlensing light data, high-resolution images and spectroscopy, researchers can finally view a complete picture of a MACHO (Massive Compact Halo Object) by measuring its mass, distance and velocity. This demonstrates that precision brightness measurements and extensive follow-up will allow astronomers to characterize a significant fraction of the Milky Way’s dark matter. The work is presented in the Dec. 6 issue of Nature .
The team of scientists used the NASA/ESA Hubble Space Telescope and the Europeans Southern Observatory’s Very Large Telescope to take images and make spectra of a MACHO microlens — which turned out to be a red star in the Milky Way.
The observation makes it possible to determine the mass of the MACHO and its distance from the Earth. In this case, the MACHO is a small star with a mass between 5 percent and 10 percent of the mass of the sun at a distance of 600 light-years. This makes the MACHO a dwarf star and a faint member of the disk population of stars in the Milky Way.
"For the first time, we’ve been able to determine the detailed characteristics of a lens," said Cailin Nelson, a UC Berkeley graduate
student working at Livermore with the MACHO team. "This shows that we will be able to determine the make-up of MACHOs and their role in the universe. We expected about one of our microlenses to belong to the normal, stellar component of the Milky Way, and it just happened that this was the one."
"In order to observe and then follow-up more unusual microlensing events such as this one, we need to find many more events," said Kem Cook, the Livermore team leader. "We are just beginning a new five-year microlensing survey using the Cerro Tololo Interamerican Observatory’s four-m telescope, which should yield the number of events we need to identify the nature of the main microlensing population."
For the past 10 years, active search projects have looked for possible candidate objects for the dark matter. One of the many possibilities is that the dark matter consists of atomic sized, weakly interacting, massive particles. Another possibility is that the dark matter consists of MACHOs, such as dead or dying stars (neutron stars and cool dwarf stars), objects similar to stars, but too small to ‘light up’ ( planets and brown dwarfs) ,or black holes of various sizes.
Previous research shows that if some of the dark matter were in the form of MACHOs, then its presence could be detected by the gravitational influence MACHOs would have on light from distant stars. If a MACHO object passes in front of a star in a nearby galaxy, such as the Large Magellanic Cloud, then the gravitational field of the MACHO will bend the light and focus it into telescopes.
The MACHO acts like a gravitational lens and causes the brightness of the background star to increase for the short time it takes for the MACHO to pass by. Depending on the mass of the MACHO and its distance from the Earth, this period of brightening can last days, weeks or months. Gravitational lensing can also be observed on much larger scales around large mass concentrations, such as clusters of galaxies. Since MACHOs are much smaller they are referred to as "microlenses."
The form and duration of the brightening caused by the MACHO (the microlensing light curve) can be predicted by theory and searched for as a clear signal of the presence of MACHO dark matter. But in a normal event, the brightening alone is not enough information to yield the distance to the MACHO, its mass and velocity as independent quantities. It is only for unusual events, such as this one, that more can be learned.
In 1991, a team of astronomers from LLNL, the Center for Particle Astrophysics at UC Berkeley and the Australian National University joined forces to form the MACHO Project. This team used a dedicated telescope at the Mount Stromlo Observatory in Australia to monitor the brightness of more than 10 million stars in the Large Magellanic Cloud over a period of eight years. The team discovered their first gravitational lensing event in 1993 and have now published approximately 20 examples of microlenses toward the Magellanic Clouds. These results demonstrate that there is a population of MACHO objects surrounding the Milky Way galaxy that could comprise as much as 50 percent of the total dark matter content.
The MACHO collaboration is made up of: K.H. Cook, A.J. Drake, S.C. Keller, S.L. Marshall, C.A. Nelson and P.Popowski of the Lawrence Livermore National Laboratory; C. Alcock and M.J. Lehner from the University of Pennsylvania; R.A. Allsman of the Australian National Supercomputing Facility; D.R. Alves of STScI; T.S. Axelrod, K.C. Freeman and B.A. Peterson of the Mount Stromlo Observatory; A.C. Becker of Bell Labs; D.P. Bennett of the University of Notre Dame; M. Geha of University of California at Santa Cruz; K. Griest and T. Vandehei of the University of California at San Diego; D. Minniti of Universidad Catolica; M.R. Pratt, C.W. Stubbs and A.B. Tomaney of the University of Washington; P.J. Quinn of the European Southern Observatory; W. Sutherland of the University of Oxford; and D. Welch of McMaster University.