SCIENCE / ASTRONOMY : Hubble’s Galaxy Photos Show Universe in Flux
Scientists led by a Pasadena astronomer said Tuesday they have used an extraordinarily long exposure from the Hubble Space Telescope to create sharp images of a 4-billion-year-old galaxy cluster that confirms theories that the universe is evolving--”and at a pretty rapid rate.”
At the same time, in the background of those images, the astronomers serendipitously stumbled upon what appears to be a previously unknown, 10-billion-year-old galaxy cluster. When studied further, that cluster may answer fundamental questions about how and why galaxies formed.
Along with other recent finds, the images should help partially rehabilitate the Hubble’s reputation. The $1.5-billion space telescope had been written off as “virtually blind” after the National Aeronautics and Space Administration found a fundamental flaw in its main imaging mirror after its 1990 launch.
Salvaging images from the Hubble’s current wide-field and planetary camera--an updated model is scheduled to be installed late next year--has not been simple, but it is possible, said Alan Dressler of Pasadena, the Carnegie Institution astronomer who announced the findings Tuesday.
For example, Dressler’s team kept the space telescope fixed on one spot for six hours spread over 10 Earth orbits just to gather enough light to see the galaxy cluster clearly. Even then, the images had to be enhanced by computer to produce the photographs made public on Tuesday in Washington.
Daniel Weedman, an astronomy professor at Pennsylvania State University, praised the effort behind the discovery as “almost heroic.”
These new images--produced by Dressler, Augustus Oemler of Yale University, James E. Gunn of Princeton University and Harvey Butcher of the Netherlands Foundation for Research in Astronomy--show some galaxies colliding and violently ripping one another apart. This phenomenon, which astronomers refer to as a “cosmic Cuisinart,” could explain how some galaxies evolve.
“We have actually seen galaxies change over time. That’s very important to proving the Big Bang theory,” said Dressler, a pioneer in studying the morphology, or structure, of galaxies. “The universe was a much different place even 4 billion years ago than it is today. . . . The universe is evolving, and at a pretty rapid rate.”
Other astronomers were equally enthusiastic about the images of the distant galaxies, which are so far away that light from them has taken 4 billion years to reach Earth--and, thus, give scientists the opportunity to see how those galaxies appeared about the time our own Milky Way galaxy was created.
“It’s like we are truly in a time machine with the Hubble telescope,” Weedman said.
“For the very first time, we can see ‘before’ and ‘after’ pictures of the universe,” said Bruce Margon, chairman of the University of Washington’s astronomy department. “Things people have been speculating about for 20 or 30 years--here, with one picture, the Hubble proved to be true.”
Dressler said that nearly 30% of the 4-billion-year-old galaxies photographed by the Hubble are spiral-shaped. That is six times the percentage found in galaxies today, Dressler said.
This finding, based on direct photographic evidence made possible with the space telescope, confirmed a postulation first advanced by Butcher and Oemler in 1978. At that time, they noted that distant clusters seemed to contain a higher percentage of hot, “blue” galaxies, usually associated with the kind of active star formation found in spiral galaxies.
Since nearby, contemporary clusters are dominated by elliptical “red” galaxies in which star-formation had long since ceased, Butcher and Oemler gave astronomers the first hint that the universe may once have contained far more spiral galaxies than it does now.
Using the wide-field/planetary camera--one of the Hubble’s five imaging instruments--Dressler and his colleagues produced the photographs that they say prove that change.
This, he said, could indicate that spiral galaxies, with their eerie arms reaching out into space, evolve into elliptical galaxies. Ironically, that process is the exact opposite of an early postulation by Edwin P. Hubble, the venerated Nobel laureate from Caltech after whom the space telescope is named.
But the observation also could indicate something completely different, Dressler added. It could, for example, be evidence that spiral galaxies are chewed up or otherwise changed by collisions with other galaxies, a frequent occurrence in that relatively crowded section of the universe. Yet another possibility is that many spirals have merged in a tumultuous process that results in spherical, elliptical galaxies. This theory was advanced by Alar Toomre of the Massachusetts Institute of Technology and Francois Schweizer of the Carnegie Institution.
To resolve such uncertainties, Dressler said astronomers will need more time to study other galaxy clusters with the degree of sharpness promised by the computer-enhanced space telescope images. Galaxy clusters are attractive targets for in-depth study, he added, because they have samples of all types of stellar formations in one place at one time.
Divining how galaxies were created and why they take different shapes--from delicate spirals with their many newborn stars to regular ellipticals with older stars to the curious irregular galaxies full of wisps of hydrogen--is one of the most avidly pursued goals in astronomy today.
One intriguing prospect for further study, Dressler said, is the 10-billion-year-old galaxy cluster found while making images of the much closer 4 billion-year-old galaxy cluster. The older cluster, which is so far from Earth that it took 10 billion years for light to reach here, appears as faint and fuzzy dots in the background of those photographs.
Astronomers who had seen this ancient cluster found it particularly tantalizing because not all of the objects within it seem large enough to count as full galaxies. They said this could mean that these small objects are some wholly new kind of galaxy, or even that they are a much sought-after galactic building block known as a “protogalaxy.”
Dressler said the ancient cluster, which is seen as it appeared only 1 billion or 2 billion years after the Big Bang theoretically created the known universe, is particularly interesting because, it seems to be relatively unremarkable. As such, he said, it is much more like our own Milky Way, and thus more likely to help explain how the Milky Way came to exist.
Ironically, news about the discovery of this “ordinary” galaxy cluster came a month after another group of scientists using the Hubble said that they had recorded images of “luminous knots” at the core of another 10-billion-year-old galaxy cluster. This other ancient cluster, which emits strong electromagnetic signals, already had been discovered by radio astronomers.
George Miley of Leiden University in the Netherlands said the knots could be giant clusters of as many as 10 billion stars in the process of formation, or they could be clouds of dust or gas caught in a “searchlight” beam of energy being radiated from a massive black hole at the galaxy’s core.
Because of its optical problems, the space telescope is unable to send back detailed images of either ancient cluster, even with computerized image enhancement. Scientists hope the newer wide-field camera to be installed next year will improve their view of these targets.
Galactic Mug Shots
Distant, 4 - billion-year-old galaxies that appeared as little more than shapeless smudges to ground-based telescope operators have been rendered much sharper with computer-enhanced images from the Hubble Space Telescope.
Astronomers have organized these galactic mug shots in a “Hubble atlas” ranging from some nearly spherical elliptical galaxies, on the left, to wispy spiral galaxies and lumpy irregulars, on the right. All these galaxies were found in one cluster.
About 30% of these galaxies are spiral-shaped. That is about six times the percentage among more modern galaxies, indicating a significant and relatively rapid evolution in the structure of the universe.
