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August 14, 1995
CONTACT: Emil Venere

Astronomers Focus Hubble on Enigmatic Odd-shaped Galaxies

The Wide Field Planetary Camera-2 was part of a package of corrective optics installed in December 1993, during a mission to repair an image-blurring mirror flaw. Griffiths was a member of the team of scientists responsible for building the new camera.

Although astronomers can see faint galaxies with the largest telescopes on Earth, even the most powerful ground-based telescopes cannot resolve details. Astronomers had to wait until the space telescope's blurry vision was corrected before they could get a good look at the galaxies.

"We've known about them for nearly two decades, but we didn't know what they were," Griffiths said. "We couldn't say anything about their actual structure."

Scientists have learned that the galaxies are about one-tenth the size of the typical elliptical and spiral galaxies, like our Milky Way, previously thought to dominate the universe. Whereas giant galaxies common in today's cosmos might contain about a hundred thousand million stars, the very faint, distant galaxies contain roughly a thousand million stars, Griffiths said.

Not only are they much smaller, they also have irregular shapes, unlike the nearly symmetric spiral and elliptical galaxies.

Scientists think many of them may still exist but cannot be seen because they have become much fainter over the years. If that's true, perhaps large telescopes on the ground might be used to find the galaxies, Griffiths said.

"If you want to look at nearby galaxies of this kind you have to do very large area surveys," he said. The space telescope can take in only small areas of the sky at a time, an area about one-tenth the diameter of the moon.

Astronomers are puzzled about whether the small galaxies were the building blocks of large galaxies like the Milky Way, or whether they have simply faded into obscurity.

While some of the odd-shaped galaxies might have merged to form large galaxies, it now appears more likely that many of them did not.

"They are apparently not merging into big spirals because we don't see them doing that, at least not several billion years ago," Griffiths said. "We see them everywhere and they're mainly isolated objects, not in pairs or groups. The deepest images from the Hubble may show the big spirals forming, even further into the past, but we need more of these deep images."

The Medium Deep Survey's main goal was to learn the nature of the faint blue galaxies. "We've come a long way to solving that particular problem," he said.

However, the survey data are yielding many additional cosmic fruits, besides the faint galaxy findings. For example, the survey, which began in 1990, has enabled scientists to learn details about the frequency of exceptionally bright galactic cores, called active galactic nuclei. It also has provided insights into "gravitational lenses," the bending and shaping of light by massive objects. The Hopkins team has discovered two such lenses; in each case, light from a faint blue galaxy or quasar located exactly behind the center of an elliptical galaxy is distorted as it passes through the center of the elliptical galaxy. As seen by viewers on Earth, the light is forced into the shape of a cross.

Astronomers on the survey team have published about 30 scientific papers so far.

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