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 they're 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 galaxy's 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 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.