Astronomers Unveil First Detection
of Dark Matter Object in the Milky Way
December 5, 2001
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."
Composite figure showing the geometry of a microlensing event. The
insets show a picture of the Large Magellanic Cloud and the Great
Melbourne Telescope in Canberra, Australia where the MACHO Project
collected microlensing survey data for eight years. A MACHO in the
halo of the Milky Way is shown bending light from a star in the Large
Magellanic Cloud on its path to the telescope in Australia.
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.
Three color Hubble Space Telescope-image of LMC-5. We show a
three-color composite image of the WFPC2 V, R, and I band images of
LMC-5. The microlensing source star is the blue star near the center of the
figure which is partially blended with a much redder object (indicated
by arrow) displaced by 0.134''. The direction of motion of the lens on the
sky derived from the Hubble data (Theta_{HST,sky} = -92 degrees) and the
microlensing parallax fit (Theta_{PAR,sky} = -100 degrees) are both shown.
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.
We show a composite European Southern Observatory Very Large
Telescope FORS2 spectrum of the LMC-5 source-lens system from
4 x 1500 second exposures on February 2, 2001. The potassium,
sodium and titanium oxide features from the lens are marked. The calcium lines
are a blend from both the lens and the LMC source star (spectral type F). The
presence of KI, NaI, the absence of CsI + RbI and the TiO band at 7100
Angstroms with corresponding absence of the VO band at 7450
Angstroms lead us to conclude that the lens is of spectral type M4-5V.
The spectrum has been put on a relative flux scale and smoothed to a
resolution of about 3 Angstroms
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.
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