GOODS: Going Deep into Our Cosmic Origins with NASA's Great Observatories
Written by Linda Vu, Spitzer Science Center
August 11, 2006
|The GOODS survey combines data from different NASA telescopes to see galaxies in the distant Universe.
Credit: NASA, ESA/JPL-Caltech/B. Mobasher (STScI/ESA)
What do you call someone who can see billions of years into our cosmic past? An astronomer, of course!
By combining the super sensitive eyes of NASA's Spitzer Space Telescope, Hubble Space Telescope, and Chandra X-ray Observatory, astronomers on the Great Observatories Origins Deep Survey (GOODS) team, see 13 billion years back in time and provide valuable insights into the origins of galaxies.
How is this possible? In astronomical terms, scientists do this by "going deep."
In astronomy, "going deep" means pointing a telescope at one small patch of the sky for many hours (sometimes days or months) to capture faint light from some of the Universe's most distant objects. Distance is important because the farther away a galaxy is, the farther back in time astronomers peer to catch a glimpse of it.
According to the GOODS Legacy project lead, Dr. Mark Dickinson, the project's ultimate goal is to understand how galaxies that were thriving 1 billion years after the Big Bang developed into modern galaxies like our Milky Way and its neighbors.
"GOODS is currently the only survey that can offer a detailed picture of the Universe at 1 billion years old," adds co-investigator Dr. Ranga Ram Chary.
How The Great Observatories Look Back
To look billions of years into the past, astronomers do not need any special time machines, like those science fiction fans read about in books or see in movies. Instead they rely on something much more natural: physics.
Due to the physics of light travel, the farther an object is away from Earth, the longer it takes for its light to travel to Earth. When the light finally reaches our telescopes, we are essentially seeing it the way it looked when it originally left the object millions or billions of years ago.
All light is made up of vibrating waves of electric and magnetic fields. The human eye is tuned to see these ripples at wavelengths called "visible light." However, there are many types of light that humans cannot see. The entire range of light is called the "electromagnetic spectrum" and includes: gamma rays, X-rays, ultraviolet, visible, infrared, microwaves, and radio waves. The only differences among these types of light are their wavelengths and frequencies. To view the types of light that are invisible to the human eye, astronomers use a variety of telescopes.
For light waves, space travel is not always smooth. Along the way, these ripples may encounter "roadblocks" in the form of cosmic dust. The expanding Universe also stretches light waves, thus altering their frequencies and wavelengths. To compensate for these variables, GOODS astronomers deeply observe the same patch of sky with a variety of ground-based telescopes and NASA's space-based "Great Observatories," which includes Spitzer, Hubble, and Chandra.
"GOODS was really the first project that proposed to integrate technologies and wavelengths at its inception to get a better view of the distant early Universe," said Dickinson.
"The panchromatic coverage is really the key to GOODS' success. The project is the first truly panchromatic deep survey," adds Dr. Mauro Giavalisco, lead investigator of the GOODS Hubble Treasury project.
Like the Spitzer Legacy projects, the Hubble Treasury projects are large observing projects with a public archive that is immediately updated once observational data is analyzed.
Hubble Space Telescope: Massive Stars
The Hubble Space Telescope's contribution to this peek back in time lies in its ability to detect the Universe's most massive stars and its superb resolution of very distant galaxies.
Because massive stars live fast and die young, they are good indicators as to whether a galaxy is actively forming stars. Their short lifespan of a few tens of millions of years also helps astronomers determine the star formation rate of a galaxy. Although these massive stars emit mostly ultraviolet light, Hubble can detect them because the expanding Universe stretches the short ultraviolet light waves into longer visible-light waves.
According to Giavalisco, Hubble's superb angular resolution also allows scientists to study the structure of very distant galaxies, some of them located as far as 13 billion light years away.
"A lot of what we know about galaxies both near and far is through their morphology, or structure," says Giavalisco.
Spitzer Space Telescope: Dust and Sun-like Stars
While Hubble can offer a great deal of information about the distant Universe on its own, it cannot provide the whole picture. After all, very massive stars represent only a minority of stars in a galaxy. The majority of a galaxy's stellar population actually consists of smaller Sun-like stars that emit mostly visible light. As the Universe expands, this visible light is stretched into infrared light, which Spitzer detects.
By combining Spitzer and Hubble observations astronomers can conduct an accurate stellar population census in the Universe's first galaxies. They can also tell where a galaxy's stars are forming and where star formation has stopped. If Hubble spots massive, short-lived stars in a region of a distant galaxy, then astronomers can infer that the galaxy is actively forming stars. Meanwhile, if Spitzer sees many older Sun-like stars, in an area where Hubble sees no massive young stars, astronomers might infer that star formation has probably stopped in that region of the galaxy.
Spitzer's dust piercing infrared eyes also help to capture starlight from dusty distant galaxies. In space, dust particles absorb visible and ultraviolet starlight and re-emit it at longer infrared wavelengths detectable by Spitzer.
"Spitzer really out performed our expectations," said Dickinson. "When we designed GOODS in 2000 we hoped to see the galaxies about 2 to 3 billion years after the Big Bang. Once Spitzer began observations we were able to see galaxies 1 billion years after the big bang."
Chandra X-ray Observatory: Black Holes
Astronomers have spotted at least one supermassive black hole in the "hearts" of many of our galactic neighbors. These mysterious and ravenous creatures can consume masses of millions and billions of suns; not even light can escape their voracious feedings. Astronomers don't know when these structures started appearing in the center of galaxies or how they got there, but the GOODS team is hoping to answer these questions with Chandra.
"Most people like black holes because they are exotic, but these structures actually play a very important role in how galaxies grow and develop over time," said Dickinson.
Because light cannot escape the gravitational pull of a black hole, they are difficult to detect with visible-light telescopes like Hubble. However, as matter falls into the hole, the material is heated to extreme temperatures and produces X-rays, which can be spotted with Chandra.
"In the GOODS data, we've detected some very massive galaxies back when the Universe was only 1 billion years old. Despite their large size, there were no supermassive black holes at their centers. This puts a limit to when they started appearing in galaxies," said Chary.
The GOODS Legacy
Team members agree that the legacy of GOODS lies in its contributions to the astronomical archives stored at the Spitzer Science Center, Space Science Telescope Institute, Harvard Smithsonian Center for Astrophysics, and European Southern Observatory.
GOODS observations are released into these public archives as soon as the data is extracted and analyzed by team members. Astronomers worldwide can access these troves of astronomical information from anywhere on the planet in real time via the Internet.
"I think one of GOODS' greatest achievements thus far occurred a few years ago when a team of non-GOODS astronomers used data from our archive to prove the existence of dark energy in the Universe," said Giavalisco.
Scientists believe that dark energy exists throughout the cosmos and is the force behind the Universe' expansion. Just one year after the GOODS survey began, Dr. Adam Riess of the Space Telescope Science Institute in Baltimore, Md. used maps in the GOODS archives to pick out a certain type of supernova, the explosive death of a very massive star, that occurred back when the Universe was half its current age. By studying light from these explosions, Riess' team determined the expansion rate of the Universe more than 6 billion years ago. Scientific models for the existence of dark energy show that the Universe' rate of expansion has varied at different stages of its life. Riess' finding and other studies showing the varied rate of expansion, serve as evidence for these astronomical models.
"The GOODS archive is setting the stage for future missions to see back to the beginning of time," says Chary.