Old idea spawns new way to study dark matter (Forwarded)

Research Communications
Ohio State University

Contact:
Andrew Gould, (614) 292-1892
and
Subo Dong, (614) 292-6893

Written by Pam Frost Gorder, (614) 292-9475

Embargoed for release: Until 10:00 a.m. HST (4:00 p.m. EDT) on Wednesday,
May 30, 2007, to coincide with poster presentation 121.11, "First
Space-based Microlens Parallax Measurement: Spitzer Observations of
OGLE-2005-SMC-001" at the American Astronomical Society meeting in
Honolulu. A press conference will be held at 11:30 a.m. HST (5:30 p.m.
EDT) in 321B, Hawaii Convention Center.

OLD IDEA SPAWNS NEW WAY TO STUDY DARK MATTER

COLUMBUS, Ohio -- An international team of astronomers led by Ohio State
University has examined dark matter in the outer reaches of our galaxy in
a new way.

For the first time, they were able to employ triangulation -- a method
rooted in ancient Greek geometry -- to estimate the location of dark
matter and calculate its mass.

The results, reported May 30 at the meeting of the American Astronomical
Society in Honolulu, suggest that this technique could help astronomers
detect dark matter of a particular mass range for which there were
previously no reliable tests.

It could also settle longstanding questions about the composition of dark
matter in the outer reaches of the Milky Way -- the so-called galactic
"halo."

Dark matter is, by its nature, invisible. But astronomers can watch the
sky for those rare moments when dark matter affects visible objects. One
such opportunity is a gravitational lensing event -- when one dark object
in space acts like a lens to magnify the light from a star shining behind
it.

"Astronomers have discovered more than a dozen lensing events that could
have been caused by dark matter objects lying in the halo," said team
leader Andrew Gould, professor of astronomy at Ohio State. "But because we
had no way to estimate their distance, we couldn't tell whether they were
really dark objects in our halo, or just garden-variety stars in another
galaxy."

This study marks the first time that anyone has triangulated a lensing
event by observing it from the ground and from space at the same time.

On Earth, surveyors triangulate an object by observing it from two
different vantage points. The two vantage points and the object itself
form the vertices of a triangle. Knowing the distance between the vantage
points and the angles of the triangle, surveyors can calculate how far
away the object is.

To study dark matter, Gould and his team used triangulation a little
differently. Over the summer of 2005, they watched a lensing event from
two locations: Earth, and NASA's Spitzer Space Telescope, which is
orbiting the sun some 25 million miles away. Earth, the Spitzer telescope,
and the dark matter formed a giant cosmic triangle.

Credit for discovering the OGLE-2005-SMC-001 microlensing event goes to
the Optical Gravitational Lensing Experiment (OGLE) led by Andrzej
Udalski. OGLE found the event in its very early phase using the Warsaw
Telescope at Las Campanas Observatory in Chile, which enabled intensive
follow-up observations from the ground and space

The astronomers didn't measure the angles of the triangle to calculate the
distance to the object as a surveyor would on Earth. That's because
lensing events are all about timing. The dark matter lens is moving, and
so astronomers learn about the lens by watching how quickly the light
brightens and fades over a brief few days as the lens passes by. Seen from
Earth and from the Spitzer telescope, the peak brightness would occur at
slightly different times.

So when Gould and his team triangulated OGLE-2005-SMC-001, they didn't
directly measure its distance, but rather the velocity with which it was
moving across the sky.

Since astronomers know roughly how fast an object in our galactic halo
should be moving, compared to an object in another galaxy -- in this case,
the Small Magellanic Cloud (SMC) -- they could infer from the velocity
whether the lens was a halo object or an SMC object.

They calculated a 95 percent likelihood that the lens was in the halo.
That would place the dark matter some 16,000 light years away from Earth.
A light year is the distance light travels in a year -- approximately six
trillion miles.

Then, by factoring in other information about the timing of the
brightness, they were able to determine that the lens in this case was
made of two separate dark objects: one roughly seven times more massive
than our sun, and the other three times more massive than our sun. The
objects circle each other, separated by a distance roughly the same as
that between Jupiter and the sun.

All this calculation adds up to one tantalizing possibility: that the dark
matter lenses are black holes.

"If this lens is in the halo, it is a 10 solar-mass black-hole binary,
which would be very exciting," said Ohio State doctoral student Subo Dong,
lead author of the study. "But we cannot completely rule out the
possibility of the lens being in the SMC. In fact, there is still a 5
percent chance that it is a pair of ordinary stars in the neighboring
galaxy."

At its outermost edges, the Milky Way retains a halo of material left over
from when the galaxy first formed billions of years ago. The halo contains
gas and very old stars, but its chief ingredient is dark matter. Most
astronomers agree that a small minority of halo dark matter -- no more
than 20 percent -- could be made of planets, dim stars, or black holes.
These are called Massive Compact Halo Objects, or MACHOs.

But there's a gap in astronomers' knowledge when it comes to detecting
MACHOs of a particular mass range -- 10-100 solar masses. For those
objects, there have been no reliable methods -- until now.

With a combined mass of about 10 solar masses, the two dark objects that
Gould's team detected fall within that range. So the method they used
could finally enable astronomers to take a survey of 10-100 solar mass
dark objects in the halo.

Dong sees a lot of potential for future discoveries with this
observational method.

"It will be very interesting to locate and measure the mass of more dark
objects in the future by applying this technique," he said. "And we might
finally be able to unravel the mystery of MACHOs."

When astronomer Sjur Refsdal of the Hamburg Observatory in Germany
proposed using triangulation to study dark matter in 1966, no one could
attempt it, because his technique involved combining at least two separate
and distant views of an object. Since the launch of the Spitzer telescope
in 2003, researchers have had an opportunity to get the right kind of
space-based measurement.

Gould is confident that over the next few years, he and his colleagues
will be able to capture a few more lens events with the Spitzer telescope,
and will gain a better perspective on the variety of dark objects that may
populate the halo. But he also looks forward to a new satellite
instrument, NASA's SIM PlanetQuest (formerly the Space Interferometry
Mission), now set to launch in 2015, which could provide more answers.
Gould is on the SIM science team, and leads the project that will use the
satellite to search for lensing events.

"Right now we know with a high probability that these objects we found are
in the halo, but with SIM we could just directly measure the distance, as
well as the mass of the objects," he said. "We wouldn't be dealing with
probabilities anymore."

For this project, Gould and Dong collaborated with other astronomers at
Ohio State, as well as Warsaw University Observatory; Spitzer Science
Center and Michelson Science Center, both at the California Institute of
Technology; Auckland Observatory; Georgia State University; Notre Dame
University; Harvard-Smithsonian Center for Astrophysics; University of
California, San Diego; Universidad de Concepción in Chile; the Institute
of Astronomy at the University of Cambridge; Princeton University
Observatory; and the Observatories of the Carnegie Institute of
Washington.

The Jet Propulsion Laboratory (JPL), which is managed by the California
Institute of Technology (Caltech), operates the Spitzer Space Telescope;
observations taken with this telescope were supported by NASA through a
contract issued by JPL and Caltech. Other funding for the study came from
the National Science Foundation, the Polish Ministry of Science and Higher
Education, and NASA. Some computing resources were provided by Cluster
Ohio, an initiative of the Ohio Supercomputer Center (OSC), the Ohio Board
of Regents, and the OSC Statewide Users Group.

An image is available to accompany the story:
http://researchnews.osu.edu/archive/halolenspix.htm

Editor's note: Dong will attend the AAS meeting and can be reached through
Pam Frost Gorder in the AAS Press Room (321A, Hawaii Convention Center) at
808-792-6612. Gould will be traveling elsewhere, but can also be reached
through Frost Gorder.


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