Ten years ago, a group of astronomers set out to find
invisible, or dark, matter in the outer fringes of our
galaxy. Long postulated to make up a significant chunk of
our universe, dark matter may be partly made up of massive,
celestial objects hiding in the halos of galaxies. The
astronomers spent six years scanning a large patch of sky
and sensed something, but they weren't sure if they were
really seeing dark matter or a different class of nearby
objects getting in the way.
Now, NASA's Spitzer Space Telescope has set its infrared
eyes on this mystery matter and verified that at least one
of 17 invisible objects observed years ago lies within the
body of our Milky Way galaxy, thereby supporting the latter
hypothesis. More observations are needed to draw definitive
conclusions; nonetheless, the findings illustrate the power
of Spitzer to finally put together the pieces of this decade-
old puzzle.
"Historically, searches for unseen matter have been part of
the justification for Spitzer," said Dr. Michael Werner, the
Spitzer project scientist at NASA's Jet Propulsion
Laboratory, Pasadena, Calif., and an investigator for the
new research. "We are very excited about these initial
results."
Matter as we know it doesn't add up to all the matter in the
universe. At least 10 times more unseen, or dark, matter
exists than known matter. Most dark matter is exotic, made
up of something other than everyday atoms. But the rest of
it may take the form of celestial objects that are too faint
to see because theyre very cool. These objects, referred to
as "massive compact halo objects," or "machos," are thought
to be lurking in the far reaches, or halos, of galaxies.
They might include black holes and failed stars called brown
dwarfs.
Beginning in 1992, Dr. Charles Alcock, who was then at the
Lawrence Livermore National Laboratory, Calif., and is now
at of the University of Pennsylvania, Philadelphia, and his
colleagues went on a hunt for machos. Rather than scan for
the objects themselves, the team looked for the objects'
gravitational tug on starlight emanating from behind them.
In this technique, called gravitational microlensing, a lens
object (the invisible matter) causes the source object (a
star) to brighten for a brief period of time. Alcock and his
team surveyed 12 million stars for these events in the
nearby Large Magellanic Cloud, which lies on the far side of
our galaxy halo. They detected 17.
But, based on the predicted numbers of faint stars in our
galaxy, the astronomers had expected to see much fewer than
17. Either there is a significant amount of dark matter in
the galaxy halo, or there is invisible matter in our own
galaxy that cannot yet be understood. Either way, the
findings challenged scientific descriptions of matter.
That's where Spitzer comes in. Because it can see objects
that are too cool to be seen with other telescopes, it may
be able to detect the heat from many of these invisible
lenses. To test this ability, a group of astronomers,
including Alcock and Werner, and led by Dr. Hien Nguyen of
JPL, used Spitzer to observe the macho event referred to as
MACHO-LMC-5. This event is the only one of its type that
could be seen by NASA's Hubble Space Telescope. Data
obtained by Alcock and others using Hubble beginning in
2001, and most recently analyzed by Dr. Andrew Gould of Ohio
State University, Columbus, suggest that the lens object for
MACHO-LMC-5 is a low-mass star about 1,500 light-years away
within our galaxys disc.
The new Spitzer data for this event independently confirm
this finding. "By luck, Hubble was able to see the lens in
one of 15 events it looked at, whereas Spitzer should be
able to see many more, if these microlensing events are
indeed caused by nearby cool objects," Nguyen said.
Nitya Kallivayalil, a graduate student at the University of
Pennsylvania, with critical insight from Dr. Daniel Stern of
JPL, carefully measured the brightness of the lens using the
Spitzer data. Dr. Brian M. Patten of the Harvard-Smithsonian
Center for Astrophysics, Cambridge, Mass., used these
measurements to establish that the lens is a very low mass,
faint star. "The data are fantastic," said Kallivayalil.
"When Brian showed us that they confirmed the nature of the
star, we were ecstatic."
Added Patten, "With this new capability, we'll be able to
determine the properties of many more lenses, and determine
their contribution to dark matter in our galaxy."
The team of astronomers recently collected data for four
additional macho events and have plans to study nine more.
For more information about the Spitzer Space Telescope,
visit http://www.spitzer.caltech.edu.
The Spitzer Space Telescope is managed by JPL for NASA's
Office of Space Science, Washington. Science operations are
conducted at the Spitzer Science Center at the California
Institute of Technology in Pasadena. JPL is a division of
Caltech.
