Andrew Yee[_1_]
June 6th 07, 08:02 PM
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 Concepcion 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.
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 Concepcion 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.