Q&A: Galaxies

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... Q&A: Galaxies (cont'd) ...
17. What are the Hubble Deep Fields?

The Hubble Deep Field project was inspired by some of the first deep images to return from the Wide Field and Planetary Camera 2 (WFPC2), after it was installed during the 1993 Hubble Space Telescope servicing mission. These images showed that the early universe contained galaxies in a bewildering variety of shapes and sizes. Some had the familiar elliptical and spiral shapes seen among galaxies today, but there were many peculiar shapes as well. Such images of the early universe are likely to be one of the enduring legacies of the Hubble Space Telescope. Few astronomers had expected to see this activity presented in such amazing detail.

Impressed by the results of earlier observations such as the Hubble Medium Deep Survey, astronomers at the Space Telescope Science Institute (STScI), and STScI's Director, Robert Williams, realized that they could provide a service to the entire astronomical community by taking the deepest optical picture of the universe. The research was done by aiming Hubble at a single piece of the northern sky for 10 days (150 consecutive orbits) in December, 1995. Images from the Hubble Deep Field project were made available to the astronomers around the world shortly after completion of the observation.

A few thousand previously unseen galaxies are visible in the original "deepest ever" view of the universe, called the Hubble Deep Field (later named the HDF-North). In addition to the classical spiral and elliptical forms, the variety of other galaxy shapes and colors seen in the image are important clues to understanding the evolution of the universe. Some of the galaxies may have formed less than one billion years after the Big Bang.

Hubble took a second deep look in the Southern Hemisphere in October of 1998 — the HDF-South — to see if a similar result would be obtained. Each of the Hubble Deep Fields shows hundreds of galaxies in an area of the sky that is as small as the size of President Roosevelt's eye on a dime held at arm's length.

18. Why was a second Deep Field taken?

The HDF-N covers a very small fraction of the sky. It would have taken 27 million fields and well over 500,000 years to use Hubble's Wide Field and Planetary Camera 2 to survey the entire sky to the depths of the HDF. Instead, astronomers rely on several thin "looking-through-a-soda-straw" views across the cosmos to infer the history of star and galaxy formation.

Taking a second Deep Field helped astronomers to confirm that the HDF-N is representative, and that it is not unusual in some way. The two HDFs are, in fact, consistent with the common assumption of astronomers that the universe should look largely the same in any direction.

19. How were the two Hubble Deep Field sites chosen?

Each of the Hubble Deep Fields represents a "carefully selected random spot on the sky." To allow the Hubble Space Telescope to peer deeply into the sky, astronomers selected a special region of Hubble's orbit where Hubble could view the sky without being blocked by Earth or experiencing interference from the Sun or Moon. The field also had to be far away from the plane of our own galaxy to avoid being cluttered with objects within the Milky Way. Finally, the field needed to have nearby "guide stars," which are used to keep Hubble pointed at the field. These criteria led to the selection of a spot of sky near the handle of the Big Dipper, in the Northern Hemisphere, and a spot of the sky in the constellation Tucana, in the Southern Hemisphere.

20. What is the importance of the HDFs?

When produced, the HDFs contained the faintest galaxies we'd ever been able to see over a large range of distances. Since seemingly "empty" spots were chosen, most of the galaxies in the Deep Fields lie billions of light-years away.

The images show some galaxies in their early stages of formation, appearing in peculiar shapes never previously seen by astronomers. The variety of new galaxy shapes and colors seen in the HDFs, along with the classical elliptical and spiral shapes, are important clues to understanding the evolution of the universe. Some of the galaxies may have formed less than one billion years after the Big Bang. The HDFs are important because they can help answer such questions as:

  • How many galaxies are there in the universe?
    The Hubble Deep Field images were used to count galaxies ten times as faint as the deepest existing ground-based optical observations and nearly twice as faint as previous Hubble images.

  • How did large-scale structure evolve in the universe?
    The Hubble Deep Field images were used to perform a statistical study of the distribution of galaxies in the sky. This is an essential test of models for the structure of the universe and galaxy formation theories. The Hubble Deep Field images pushed such studies to fainter limits.

  • How were galaxies assembled?
    Detailed studies of the ages and chemical compositions of stars in our own galaxy suggest that it has led a relatively quiet existence, forming stars at a rate of a few suns a year for the last 10 billion years. Other spiral galaxies seem to have similar histories.

    If this is typical evolution for spiral galaxies, then predictions can be made for what they should have looked like at half their present age — their size, color and abundance. This information, combined with actual distances derived from ground-based spectroscopic observations, will provide a new test for theories of spiral galaxies.

    The other major class of galaxies seen in the nearby universe is the elliptical type — football-shaped aggregates of stars that appear to be very old and stopped forming stars long ago. There is currently debate about when such galaxies formed and whether they formed through collisions of other types of galaxies or through collapse of a pristine cloud of primordial gas in the very early universe.

    The Hubble Deep Field images, along with other deep Hubble images, provide a snapshot through time, which can be used to search for distant elliptical galaxies or primeval galaxies that might later evolve into elliptical galaxies.

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Q&A: Galaxies