October 28, 2005
Piecing Together the History of the Universe
JPL -- Nowadays, much of the universe's matter seems to be organized, as stars, solar systems, galaxies, and galaxy clusters are familiar sights of the local universe. However this was not always the case.
According to the Big Bang theory, the universe was created approximately 13.7 billion years ago when a singular point exploded, unleashing energy and the fabric of space and time. Galaxies did not exist in this primordial universe and the extreme temperatures kept matter from condensing.
By using the Spitzer Space Telescope to conduct a wide-area sky survey, the Spitzer Wide-area Infrared Extragalactic Survey (SWIRE) Legacy team hopes to explain how mature nearby galaxies evolved from the "element clumps" of the early universe.
According to SWIRE Scientist Dr. Jason Surace, Spitzer is essential for studying the evolution of galaxies because its unprecedented infrared sensitivity allows astronomers to lift the dusty veil shrouding infant stars and gain valuable insights into the process of star formation itself.
"Spitzer is helpful because it brings us actual measurements of the number of stars forming in a galaxy and can tell us how rapidly interstellar dust heats up," says Surace.
The rapidly heating dust is important because it offers astronomers the first window into the star formation process. In the very beginning stages of star formation, long before a star "turns on" visibly, heat is emitted as gas and dust contract to form a star. This heat radiates in the far-infrared and can easily be detected by Spitzer's Multiband Imaging Photometer (MIPS).
Using the telescope's Infrared Array Camera (IRAC) the team can observe the history of star formation in galaxies, by studying the older stars that make up the structure's "skeleton."
"By comparing the rate of star formation with redshift data from optical telescopes, we learn a lot about galaxies," says Surace.
Witnessing the Evolution of Galaxies
Due to the nature of light travel, distance equals time in space. In other words, the farther away the object is from Earth the longer it takes for its light to travel to Earth, thus when scientists observe objects that are 10 billion light-years away, they are essentially observing a snapshot of the object from 10 billion years ago.
SWIRE principal investigator Dr. Carol Lonsdale stresses that the wide-area scans are essential to SWIRE's survey for precisely this reason. The wider the survey, the more distance and time is covered.
"The wide-area survey allows us to see different types of galaxies over large distances and large-scale structures like galaxy clusters," said Lonsdale.
"The ability to observe galaxies over large distances of space and time will give us a better idea of how galaxies formed and subsequently evolved, revealing the life cycle of the largest entities known to science" added SWIRE scientist Dr. Thomas Jarrett.
According to Jarrett, conducting a survey to determine how galaxies have evolved is like simultaneously witnessing ten different crime scenes and then trying to solve one of the mysteries by linking all these crimes together. In the case of SWIRE, each scientist observes a variety of galaxies at different distances, or times in the universe's history. After careful analysis, these astronomers link together their information to solve the mystery of how galaxies evolved throughout the universe's lifetime.
For example, as a member of the SWIRE Legacy team, Jarrett studies the local universe, or galaxies that are "only" millions of light-years away. Meanwhile, Lonsdale and Surace's expertise lay in researching galaxies billions of light-years away in the distant universe. After conducting their individual observations, Jarrett, Lonsdale, Surace, and other members of the team then attempt to connect their evidence to solve the mystery of how modern galaxies came to be. Their ultimate goal is to use observations of distant galaxies to explain behaviors of local galaxies.
"The benefit of working with the local universe is that you can actually see the structural details of nearby galaxies, for example, spiral arms, nuclei, and giant star formation regions," says Jarrett.
Because of the current technological limitations, galaxies in the distant universe can only be detected as fuzzy lights. Despite this, Spitzer's unprecedented sensitivity allows the telescope to take pictures of nearby galaxies in unrivaled detail. While the fuzzy distant galaxies may not provide scientists with detailed images of galactic behavior in the early universe, these attributes can be inferred from observations in the local universe. A prime example of this lays in SWIRE's recent observations of the "Tadpole Galaxy," located at a mere 400 million light-years from Earth, an example of a spectacular gravitational interaction between two galaxies.
Although astronomers recognize that interacting galaxies, or colliding galaxies, are less frequent occurrences in the local universe, they believe that this was a fairly common phenomenon in the early universe. Locally, galaxies are less likely to collide because over time the expanding universe has increased the distance between galaxies. Because Spitzer's low image resolution of galaxies billions of light-years away makes it difficult for scientists to discern the details of early galaxy interactions, this detailed image of the Tadpole Galaxy collision gives scientists insight into what the early galactic collisions may have looked like, and how this phenomenon helped galaxies evolve into normal galaxies being observed in the local universe.
According to Surace, a second reason for doing a wide-area survey is to give astronomers a greater chance of discovering and mapping rare phenomenon.
Like all the Spitzer Legacy Science projects, although SWIRE has a specific goal, it was chosen because the information acquired by its method of research will benefit astronomers across the field. In addition to studying the evolution of galaxies over the universe's history, the team will also be mapping the universe in unprecedented infrared detail for future astronomers. And while all the questions revolving around the physics of galactic evolution may not be answered by the end of this particular project, the information that the team has gathered will lay the foundation for future research.
"Our explicit goal is to see how galaxies evolved over billions of years," says Surace. "But because of Spitzer's sensitivity and the large area of our survey, the information that we've collected will also help scientists studying asteroids, [planet-forming, or] protoplanetary disks, and star formation."
By the end of this project, SWIRE will produce a catalogue of two to three million galaxies for Spitzer's public archive.
SWIRE: The Son of WIRE
For many SWIRE astronomers, Spitzer's Legacy Science Program represents a second chance to study the evolution of galaxies and the structure of the modern universe. Before they were members of SWIRE, most of these scientists were part of NASA's Wide-field Infrared Explorer (WIRE) team. That is why members of the SWIRE Legacy team often refer to their project as "the Son of WIRE" or, "S-WIRE" for short.
In March 1999, just a few days into WIRE's four-month mission, the telescope was declared dead due to a technical malfunction that caused the spacecraft to eject its protective cover while its heat-sensitive telescope was still pointing at the hot Earth. This glitch caused the block of hydrogen ice keeping the craft's telescope at a cool -436 degrees Fahrenheit to melt and evaporate, rendering the instrument ineffective.
"When WIRE's mission prematurely ended, we realized that we could do our technical observations with Spitzer," said Surace. "[Thus] SWIRE stemmed from WIRE's failed mission and SWIRE inherited its science team from WIRE, along with many additions, particularly from Europe."
According to Jarrett, a group of scientists led by Lonsdale interested in galaxies proposed the SWIRE Legacy project. Thus, the project's explicit goal is to survey the sky and study galactic evolution. However, as members of the astronomical community realized that detailed information on cosmic objects like asteroids, comets, and protoplanetary disks would also be captured in the survey, scientists with varied interests asked to participate and the team expanded to include them.
"The mission's failure was a tremendous disappointment," said Surace. "Many of the people associated with WIRE had been working long hours for more than half a decade only to have all that hard work trashed."
Using unprecedented technology, WIRE would have surveyed a large section of the sky at infrared wavelengths of 12 and 25 microns for extremely bright star clusters called starburst galaxies, which produce new stars at 10 times the rate of typical galaxies. It would have also looked at infant galaxies, or protogalaxies, giving scientists an insight into the process of galaxy, star and solar system formation. Now with NASA's Spitzer Infrared Telescope and the SWIRE survey, the major scientific goals of WIRE will finally be realized.
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