Spitzer Seeks Out Milky Way Dark Matter
Written by Linda Vu, Spitzer Science Center
February 21, 2007
|An artist concept of a planet around an M-dwarf star.
Our Milky Way galaxy is heavier than it looks, and it's not too much ice cream, or cookies, that is responsible for the extra weight -- it's "dark matter."
Dark matter is one of the greatest mysteries in modern astronomy. Scientists use the term as an umbrella definition for all the invisible "heavy stuff" in the universe. Astronomers currently believe that there are two components to dark matter. One part of dark matter is made up of exotic materials, different from the ordinary particles that make up the familiar world around us. The other part consists of dark celestial bodies -- like planets, black holes, or failed stars -- which do not produce light or are too faint to detect from Earth.
Astronomers suspect that about most of our galaxy's and universe' weight comes from dark matter. For almost a century, they scoured our Milky Way galaxy for both exotic dark matter and dark celestial bodies in hopes of accounting for the missing weight.
Now, research is showing that NASA's Spitzer Space Telescope may be able to play an important role in identifying the "invisible" celestial bodies that are weighing our galaxy down.
Astronomers refer to dark celestial objects like undetected planets, black holes, and failed stars, as massive compact halo objects, or MACHOs, because they hide in the far reaches, or halo, of galaxies.
Nearly a decade ago, a team of astronomers scoured the halo of our Milky Way galaxy for dark celestial bodies. They called their project MACHO, after the dark bodies they were searching for -- and with ground-based telescopes managed to indirectly sense the gravitational presence of 17 of them. However, technology at the time was not sensitive enough to tell the scientists what the objects were.
In a follow up survey, graduate student Nitya Kallivayalil of the Harvard-Smithsonian Center for Astrophysics, Boston, Mass., used Spitzer's supersensitive infrared eyes to zoom in on two of the previously detected MACHOs and identify them as puny stars called M-dwarfs. These stars are about 4,000 times dimmer than our Sun and cannot be seen from Earth with the naked eye. This was the first time any of the previously detected sources had ever been identified.
"This is a really exciting discovery," said Kallivayalil, whose paper on the discovery was published in the December 2006 issue of Astrophysical Journal Letters.
Although MACHOs are invisible to most telescopes, the MACHO team originally located them through a technique called "gravitational microlensing." Everything in the universe with mass has gravity. The more massive an object is, the more gravity it wields. Gravity is helpful for identifying invisible MACHOs, because it warps the space surrounding that object.
When the MACHO moves in front of a distant star, light from the star travels through the warped space and becomes magnified. By looking for magnified starlight, astronomers can sense a MACHO.
Once the location of a MACHO is known, astronomers can use Spitzer's infrared eyes detect heat coming off of the object and identify it. Kallivayalil used Spitzer to identify MACHOs known to astronomers as LMC- 5 and LMC-20 as small, dim M-dwarf stars.
"It is important to know what may constitute 'MACHOs', and what their contribution is to the dark matter in the Milky Way," says Kallivayalil, who is currently in the process of analyzing Spitzer's observations of the other 15 previously detected MACHO sources.
Weighing the Milky Way
There are two ways commonly used by astronomers to calculate the weight of a galaxy. One way is to look at the galaxy's brightness and convert it into mass, or weight. The other way is to track the movements of the galaxy's stars.
Everything in the universe moves. In our own Milky Way galaxy, our Earth moves around the Sun, and the Sun moves around the galaxy's center. By measuring how fast stars at the edge of the galaxy move, astronomers can calculate the Milky Way's weight. The faster the outer stars move, the heavier the galaxy is.
Problems began to show up when twentieth century astronomers compared the results of these two calculations for multiple galaxies -- they noticed that the numbers consistently did not match. The brightness calculation showed that the galaxies were lighter than the movement of its stars indicated.
Since both these techniques have been proven successful for determining an object's weight, astronomers concluded that there must be more to a galaxy than just the bright objects. They referred to the invisible stuff as "dark matter."