Locating the Cosmic Dark Bodies Among Us
Written by Linda Vu, Spitzer Science Center
May 30, 2007
|An artist concept of Spitzer and Earth observing OGLE-2005-SMC-001 (which isn't visible) in the direction of the Small Magellanic Cloud.
NASA/JPL-Caltech/R. Hurt (SSC)
Like cosmic "ghosts," dark planets, black holes, and failed stars lurk invisibly among us. These objects do not produce light, and are too faint to detect from Earth.
Although astronomers cannot see these "dark bodies," they can sense their presence from the way background light acts around them. While this process works for detecting invisible objects -- it does not tell scientists whether the object is near or far, in our own Milky Way galaxy or a neighboring galaxy.
For years, astronomers have struggled to find a way to determine this mystery distance. Now, they have evidence that NASA's Spitzer Space Telescope and a simple trigonometric trick called "parallax" can be used to solve this mystery.
"The parallax technique has been used to determine the distance of stars for centuries. But this Spitzer observation is the first time a space telescope has used the technique to determine the distance of a dark body," said Dr. Andrew Gould, of Ohio State University, Columbus, Ohio.
In July 2005, Gould led a Spitzer Target of Opportunity observation to determine that a dark body called OGLE-2005-SMC-001 lies within the halo, or outskirts, of our Milky Way galaxy. The event was first discovered by Dr. Andrzej Udalski, of the Optical Gravitational Lens Experiment (OGLE), and Warsaw University Observatory, Warszawa, Poland.
Udalski discovered the dark object through a technique called "gravitational microlensing." Microlensing works because everything in the universe with mass has gravity, and gravity naturally warps the space around it. When a really bright object lines up behind a dark celestial body -- light from the bright object travels through space warped by the dark body's gravity, and is magnified. Presto! Astronomers have sensed an otherwise invisible object. When the observer, dark body, and bright object are most closely aligned -- the microlensing event will reach its peak brightness.
Once an object is discovered, scientists can use parallax to determine its distance.
What is Parallax?
Ever notice that when you are driving down a highway, the lamppost just a few feet away from the road is moving faster than the mountain behind it? That's parallax!
You can also test parallax by holding your hand at arms length away from your face. Now, look at the hand with only your right eye. Then look with only the left eye. Notice how your hand moves? If you hold your thumb up close to your face, and do the same trick, it seems as if the thumb moves more. That's parallax!
With two perspectives, close objects seem to move greater distances than those that are further away. Basically, your two eyes look at things from two different perspectives. Then, your brain takes this information and instantaneously processes how far away the object is. This is how astronomers discovered that OGLE-2005-SMC-001 is in our Milky Way galaxy. Before the Spitzer observations, scientists debated about whether objects like this were located in our own galaxy, or in satellite galaxies called the Small Magellanic Cloud (SMC) and the Large Magellanic Cloud (LMC).
To determine the distance of this invisible object, scientists first looked at the microlensing event's peak brightness from Earth with a ground-based telescope. This is the moment when Earth, OGLE-2005-SMC-001, and the bright star system in the SMC, were most closely aligned. This provided one perspective. Think of this as looking at an object with only your left eye.
To get the other perspective, the scientists then looked at peak brightness with Spitzer, which is currently in an Earth-trailing orbit around the Sun. This is the moment when Spitzer, OGLE-2005-SMC-001, and the bright star system in the SMC, were most closely aligned. Think of this like looking at the same object with only your right eye. At the time of the observation, Spitzer was traveling approximately 1/4 astronomical units (AU), or about 40 million kilometers, behind Earth. The telescope continues to drift about 15 million kilometers further away from Earth every year.
Because scientists knew the exact distance between Earth and Spitzer, they could infer the dark body's speed by timing how long it took for Spitzer to see peak brightness, after astronomers observed peak brightness on Earth. In other words, how long it took the right eye to see an object after the left eye observed it. Using trigonometric equations and graphs to do the "brain's" job, the scientists determined that the dark body most likely sits in the outskirts, or halo, of our Milky Way galaxy.
"Theoretically, this should have been a very easy equation. However, there are many variables that can affect how the gravitational microlens's brightness varies with time," said Gould. "In this case, the dark object was a binary system. So this had to be taken into consideration when calculating the brightness of the microlensing event."
According to Gould, these variables made the modeling quite complex. To do an analysis of this event, Subo Dong, an Ohio State University graduate student, had to imagine the 12-dimensional parameter space that this event occurred in. In the end, Dong determined that there is a 95 percent chance OGLE-2005-SMC-001 lives in the halo of our Milky Way galaxy.
The distance of dark bodies is extremely important information for astronomers looking into the mystery of "dark matter." Galaxies are heavier than they look, and scientists use the term "dark matter" as an umbrella definition for all the invisible "heavy stuff." They believe that there are two components to dark matter. A large fraction of dark matter is made up of exotic materials, different from the ordinary particles that make up the familiar world around us. Meanwhile, some dark matter may consist of dark celestial bodies like OGLE-2005-SMC-001, which do not produce light or are too faint to detect from Earth.
"By locating the dark objects in our galaxy, we will have a better understanding of how much of our Milky Way is made up of dark celestial objects, and how much of it is made up of exotic dark matter," said Dong, whose paper on OGLE-2005-SMC-001 has been accepted for publication in Astrophysical Journal.
Gould attributes the success of this observation to "perfect timing."
"There was only a small window of time where we could take useful information of this event," said Gould. "We specifically needed to look at the event before it reached its peak brightness, and after it peaked."
However, in astronomy, observing a time-sensitive event is easier said than done. To ensure that a space telescope like Spitzer is used efficiently, observations are usually scheduled weeks in advance. Any disruption of the schedule requires major scrambling, and hours of work for members of the telescope's scheduling and operations teams. So before Gould sent in his request for Spitzer to "drop everything" and look at OGLE-2005-SMC-001, he had to be sure that this was a microlensing event.
"This was a very stressful time," Gould recalls. "If this wasn't a microlensing event, but just another variable star, we would have disrupted other people's observations, wasted the telescope's time, and money, all for nothing. The same would have been true if we didn't get Spitzer observations of the event before and after it peaked."
According to Gould, the fact that astronomers now know they can use Spitzer's infrared array camera (IRAC) instrument to locate invisible dark bodies is one of the greatest successes of this observation. This means that when Spitzer's liquid helium runs out, and it is no longer able to cool itself, the telescope will still be able to locate dark bodies.
"Forty years ago a visionary astronomer named Dr. Sjur Refsdal theorized that dark bodies could be located using parallax and a space telescope, it is truly remarkable that we have been able to prove him right with this Spitzer observation," Gould adds.
For more information on dark celestial objects in our Milky Way, please see the feature "Spitzer Seeks Out Milky Way Dark Matter."