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

“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

Founded in 1952, Lawrence Livermore National Laboratory is a national security
laboratory, with a mission to ensure national security and apply science and
technology to the important issues of our time. Lawrence Livermore National
Laboratory is managed by the University of California for the U.S. Department of
Energy’s National Nuclear Security Administration.