One peek into a small part of the sky, one giant leap back in time...
The Hubble Deep Field (HDF) was assembled from 342 separate exposures taken with the Wide Field and Planetary Camera 2 (WFPC2) for ten consecutive days between December 18 and 28, 1995.
Representing a narrow "keyhole" view stretching to the visible horizon of the universe, the HDF image covers a speck of the sky only about the width of a dime located 75 feet away. Though the field is a very small sample of the heavens, it is considered representative of the typical distribution of galaxies in space because the universe, statistically, looks largely the same in all directions. Gazing into this small field, Hubble uncovered a bewildering assortment of at least 1,500 galaxies at various stages of evolution.
Most of the galaxies are so faint (nearly 30th magnitude or about four-billion times fainter than can be seen by the human eye) they have never before been seen by even the largest telescopes. Some fraction of the galaxies in this menagerie probably date back to nearly the beginning of the universe.
"The variety of galaxies we see is amazing. In time these Hubble data could turn out to be the double helix of galaxy formation. We are clearly seeing some of the galaxies as they were more than ten billion years ago, in the process of formation," said Robert Williams, Director of the Space Telescope Science Institute Baltimore, Maryland. "As the images have come up on our screens, we have not been able to keep from wondering if we might somehow be seeing our own origins in all of this. The past ten days have been an unbelievable experience."
Harry Ferguson, one of the HDF team astronomers added: "One of the great legacies of the Hubble Telescope will be these deep images of the sky showing galaxies to the faintest possible limits with the greatest possible clarity from here out to the very horizon of the universe."
The term "deep" in an astronomical sense means looking at the faintest objects in the universe. Because the most distant objects are also among the dimmest, the image is the equivalent of using a "time machine" to look into the past to witness the early formation of galaxies, perhaps less than one billion years after the universe's birth in the Big Bang.
The image data are so important (the astronomical equivalent of the Dead Sea Scrolls, one scientist quipped) they are being made available immediately to astronomers around the world to pursue research on the formation of galaxies and for probing basic questions about the structure and evolution of the universe.
Though months of detailed research and analysis lie ahead, HDF team astronomers believe they see evidence for a significant population of galaxies that existed when the universe was less than a billion years old.
The landmark research was carried out under Williams' direction, and using a significant fraction of his own director's discretionary time on the Space Telescope. He decided to conduct the Hubble Deep Field program to use Space Telescope's exquisite resolution and high sensitivity to push back the very limits of time and space.
Williams, and the STScI team he assembled to conduct the observations, hopes it will unlock clues to fundamental cosmological questions: Will the universe expand forever? How long ago did the first galaxies appear? How have galaxies evolved over the life history of the universe?
Essentially a narrow, deep "core sample" of sky, the HDF is analogous to a geologic core sample of the Earth's crust. Just as a terrestrial core sample is a history of events which took place as Earth's surface evolved, the HDF image contains information about the universe at many different stages in time. Unlike a geologic sample though, it is not clear what galaxies are nearby and therefore old, and what fraction are very distant and therefore existed when the universe was newborn. "It's like looking down a long tube and seeing all the galaxies along that line of sight. They're all stacked up against one another in this picture and the challenge now is to disentangle them," said Mark Dickinson of the HDF team.
PLANNING TO "DIP DEEP INTO THE DIPPER"
Nearly a year of preparation preceded the observation. The HDF team selected a piece of sky near the handle of the Big Dipper (part of the northern circumpolar constellation Ursa Major, the Great Bear). The field is far from the plane of our Galaxy and so is "uncluttered" of nearby objects, such as foreground stars. The field provides a "peephole" out of the galaxy that allows for a clear view all the way to the horizon of the universe.
Test exposures made in early 1995 with Hubble and the 4-meter telescope at Kitt Peak National Observatory also confirmed the field is devoid of large galaxy clusters, which would interfere with seeing farther and fainter objects. The target field is, by necessity, in the continuous viewing zone (CVZ) of Hubble's orbit, a special region where Hubble can view the sky without being blocked by Earth or interference from the Sun or Moon.
Staring at one spot in the sky for ten days, Hubble kept taking pictures one after another for the entire exposure time, accumulating data. Each exposure was typically 15 to 40 minutes long. Separate images were taken in ultraviolet, blue, red, and infrared light. By combining these separate images into a single color picture, astronomers will be able to infer -- at least statistically - the distances, ages, and composition of the galaxies in the HDF image.
Astronomers at STScI processed the frames, removing cosmic rays and other artifacts, and put them together into one final picture. Each time they add a picture, the view got deeper, revealing fainter objects. When they were done they had the deepest picture ever taken of the heavens.
Follow-up observations will be conducted by a variety of ground and space-base telescopes at other wavelengths of the electromagnetic spectrum, from X-ray through radio. An infrared camera scheduled to be installed in Hubble during the 1997 Servicing Mission will likely image the field to search for even farther primeval galaxies, whose light has been shifted to the infrared region of the spectrum by the expansion of the universe.
* Movie credit - STScI
* This text was adapted from an STScI press release caption.