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Waspie_Dwarf

May 31 2007, 01:46 AM

NASA Space Telescope Gives Scientists Depth Perception

For Release: For Release: May 30, 2007

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Astronomers now have a new "eye" for determining the distance to certain mysterious bodies in and around our Milky Way galaxy. By taking advantage of the unique position of NASA's Spitzer's Space Telescope millions of miles from Earth, and a depth-perceiving trick called parallax, they were able to pin down the most probable location of one such object. The findings will ultimately help astronomers better understand the different components of our galaxy.

"Forty years ago a visionary astronomer named Dr. Sjur Refsdal theorized that dark bodies could be located using parallax and a space telescope," said Andrew Gould of Ohio State University, Columbus, Ohio, who led the project. "It is truly remarkable that we have been able to prove him right with this Spitzer observation."

Spitzer is the only telescope that orbits the sun behind Earth, and is the farthest telescope from us with the ability to study distant stars. Currently, Spitzer is about 40 million miles (70 million kilometers) away from Earth. It will continue to drift farther and farther away at a rate of about 10 million miles (15 million kilometers) per year.

This great distance gives astronomers a great advantage. They can use Spitzer in the same way that a human brain uses two eyes to tell how far away objects are, a principle called parallax. With two eyes, we have two perspectives, which our brains combine to give us depth perception. In space, Spitzer acts as one eye, while a ground-based telescope acts as the other. With two very wide cosmic eyes, astronomers can determine the location of bodies within and just outside our galaxy.

Gould and his team are the first to use Spitzer to perform this astronomical feat. Their goal was to determine whether a previously identified dark matter candidate, called a massive compact halo object, or "Macho," is within our galaxy and contributing to its overall weight.

Our galaxy is heavier than it looks, with at least 80 percent of its mass consisting of mysterious, invisible dark matter. A large fraction of this dark matter is the exotic kind, different from the ordinary matter that makes up the familiar world around us. The rest might be so-called machos, which are ordinary-matter dark bodies that lurk in our galaxy's halo, the region that sits above and below its spiral disk. They are thought to be a combination of black holes, very faint stars and isolated planets.

Several suspected machos have been discovered in the past through a technique called microlensing, in which the dark bodies' gravity causes light from a passing background star to bend and brighten. But astronomers do not know whether these candidates are indeed machos in the galaxy halo, or other, non-macho objects just outside the Milky Way in small satellite galaxies. By pinpointing the location of the candidates, astronomers will learn whether they are in the halo and thus machos. This information, in turn, will help them figure out how much machos contribute to the total mass of our galaxy.

OGLE-2005-SMC-001 is one such macho candidate. It was first discovered by Andrzej Udalski, of the Optical Gravitational Lens Experiment (OGLE), and Warsaw University Observatory, Warszawa, Poland. Udalski and colleagues noticed that the dark object was causing a passing, background star to brighten. Gould and his team quickly sprang to action, following up with Spitzer observations of the short-lived event.

The data from both telescopes, or "eyes," were then combined and modeled through a series of complicated equations. The results indicate with 95 percent probability that OGLE-2005-SMC-001 is dark matter in our galaxy's halo and therefore a part of its overall mass.

In addition, the data show that OGLE-2005-SMC-001 consists of two bodies circling around each other. Gould and colleagues think the objects could be a pair of black holes, a very rare sighting in our universe. However, there is a small chance this feature is actually just a regular pair of orbiting stars in a neighboring, satellite galaxy.

"It will be very exciting to locate and measure the masses of more dark objects in the future by applying this technique. And we might finally be able to unravel the mystery of machos," said Subo Dong of Ohio State University, whose paper on OGLE-2005-SMC-001 has been accepted for publication in Astrophysical Journal. Dong presented the results today at a press conference, at the 210th meeting of the American Astronomical Society in Honolulu, Hawaii.

NASA's Jet Propulsion Laboratory, Pasadena, Calif., manages the Spitzer Space Telescope mission for NASA's Science Mission Directorate, Washington. Science operations are conducted at the Spitzer Science Center at the California Institute of Technology, also in Pasadena. Caltech manages JPL for NASA

Whitney Clavin 818-354-4673
Jet Propulsion Laboratory, Pasadena, Calif.

ssc2007-11

Source: NASA/CalTech - Spitzer- Newsroom

Waspie_Dwarf

May 31 2007, 01:54 AM

Depth Perception in Space

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Credit: NASA/JPL-Caltech/T. Pyle (SSC)

This artist's concept shows how astronomers use the unique orbit of NASA's Spitzer Space Telescope and a depth-perceiving trick called parallax to determine the distance of dark planets, black holes, and failed stars which lurk invisibly among us. These objects do not produce light, and are too faint to detect from Earth. However, astronomers can deduce their presence from the way they affect the light from background objects. When such a dark body passes in front of a bright star, its gravity warps the path of the star's light and causes it to brighten -- this process is called gravitational microlensing.

By comparing the "peak brightness" of the microlensing event from two perspectives -- Earth and Spitzer -- scientists can determine how far away the dark object is. Peak brightness is the moment when the observer, the dark object, and background star are most closely aligned.

Humans naturally use parallax to determine distance -- this is commonly referred to as depth perception. In the case of humans, each eye sees the position of an object differently. The brain takes each eye's perspective, and instantaneously calculates how far away the object is. In space, astronomers can use the same trick to determine the distance of an invisible dark object.

In this illustration, the dark object is the moving black ball between Earth, Spitzer, and our neighboring galaxy the Small Magellanic Cloud (SMC; bottom right). To determine the object's distance, astronomers observe the microlensing event at its peak brightness from Earth when the dark object crosses our line-of-sight (dashed line) to a given star in the SMC. This represents one perspective, like looking at an object with only your left eye.

To get the other "right eye" perspective, astronomers also observe the peak brightness with Spitzer when the object later moves through its line-of-sight. Because astronomers know the exact distance between Earth and Spitzer, they can determine the dark body's speed by timing how long it took for Spitzer to see peak brightness after astronomers observed the event on Earth. Using trigonometric equations and graphs to do the "brain's" job, scientists can infer the dark body's distance.

The scales in this diagram are greatly exaggerated for clarity. The distance between Spitzer and the Earth is miniscule in comparison to the distance to the dark object and SMC. Since microlensing events require extremely precise alignments, even such a tiny separation is enough to measure these objects out to tremendous distances.

Source: NASA/CalTech - Spitzer- Newsroom

Waspie_Dwarf

May 31 2007, 01:56 AM

Distance to Dark Bodies

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Credit: NASA/JPL-Caltech/T. Pyle (SSC)

Using the unique orbit of NASA's Spitzer Space Telescope and a depth-perceiving trick called parallax, astronomers have determined the distance to an invisible Milky Way object called OGLE-2005-SMC-001. This artist's concept illustrates how this trick works: different views from both Spitzer and telescopes on Earth are combined to give depth perception.

Our Milky Way galaxy is heavier than it looks, and scientists use the term "dark matter" to describe all the "heavy stuff" in the universe that seems to be present but invisible to our telescopes. While much of this dark matter is likely made up of exotic materials, different from the ordinary particles that make up the world around us, some may consist of dark celestial bodies -- like planets, black holes, or failed stars -- that do not produce light or are too faint to detect from Earth. OGLE-2005-SMC-001 is one of these dark celestial bodies.

Although astronomers cannot see a dark body, they can sense its presence from the way light acts around it. When a dark body like OGLE-2005-SMC-001 passes in front of a bright star, its gravity causes the background starlight to bend and brighten, a process called gravitational microlensing. When the observing telescope, dark body, and star system are closely aligned, the microlensing event reaches maximum, or peak, brightness.

A team of astronomers first sensed OGLE-2005-SMC-001's presence when it passed in front of a star in a neighboring satellite galaxy called the Small Magellanic Cloud. In this artist's rendering, the satellite galaxy is depicted as the fuzzy structure sitting to the left of Earth. Once they detected this microlensing event, the scientists used Spitzer and the principle of parallax to figure out its distance. Humans naturally use parallax to determine distance. Each eye sees the position of an object differently. The brain takes each eye's perspective and instantaneously calculates how far away the object is.

To determine OGLE-2005-SMC-001's distance, astronomers measured the microlensing event over several months with both Spitzer in space and the Earth-based telescopes. Careful analysis of the data revealed the time of the peak brightness differed slightly between the two locations.

Because astronomers knew the exact distance between Earth and Spitzer and the time lag between the peak-observed brightness, they could determine OGLE-2005-SMC-001's speed. Using trigonometric equations and graphs to do the "brain's" job, scientists then inferred the dark body's location to be in the outer portion, or halo, of our galaxy.

The picture of the Small Magellanic Cloud in this concept is a two-color image from two Digitized Sky Survey 2 observations The Digitized Sky Survey is based at the Space Telescope Science Institute in Baltimore, Md.

Source: NASA/CalTech - Spitzer- Newsroom

Waspie_Dwarf

May 31 2007, 02:01 AM

Locating the Cosmic Dark Bodies Among Us

Written by Linda Vu, Spitzer Science Center
May 30, 2007

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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.

Perfect Timing

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."

Source: NASA/CalTech - Spitzer- Features

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