ESA Science & Technology05-Jul-2005 10:18:36

Beyond The Milky Way

The Deep Fields

One of the main scientific justifications for building Hubble was to measure the size and age of the Universe and test theories about its origin. Images of faint galaxies give 'fossil' clues as to how the Universe looked in the remote past and how it may have evolved with time. The Deep Fields gave astronomers the first really clear look back to the time when galaxies were forming.

The idea for the Hubble Deep Fields originated in results from the first deep images taken after the repair in 1993. These images showed many galaxies, which were often quite unlike those we see in the local Universe and could not otherwise be studied using conventional ground-based telescopes.

The first Deep Field, the Hubble Deep Field North (HDF-N), was observed over 10 consecutive days during Christmas 1995. The resulting image consisted of 342 separate exposures, with a total exposure time of more than 100 hours, compared with typical Hubble exposures of a few hours. The observed region of sky in Ursa Major was carefully selected to be as empty as possible so that Hubble would look far beyond the stars of our own Milky Way and out past nearby galaxies.

The results were astonishing! Almost 3000 galaxies were seen in the image. Scientists analysed the image statistically and found that the HDF had seen back to the very young Universe where the bulk of the galaxies had not, as yet, had time to form stars. Or, as the popular press dramatically reported, 'Hubble sees back to Big Bang'...

These very remote galaxies also seemed to be smaller and more irregular than those nearby. This was taken as a clear indication that galaxies form by gravitational coalescence of smaller parts.

In 1996 it was decided to observe a second Deep Field, the Hubble Deep Field South (HDF-S), to assess whether the HDF-N was indeed a special area and thus not representative of the Universe as a whole. This time the field also contained a quasar, which was used as a cosmological lighthouse and provided valuable information about the matter between the quasar and the Earth.

After the Hubble observations of HDF-N and -S, other ground- and space-based instruments targeted the same patches of sky for long periods. Some of the most interesting results seem to emerge from these fruitful synergies between instruments of different sizes, in different environments and with sensitivity to different wavelengths.

Stefano Cristiani
Space Telescope-European Coordinating Facility (ST-ECF)

"In my view the Hubble Deep Fields are some of the images that have made the greatest impact on observational cosmology so far. These impressive dips into the depths of space and time have allowed astronomers to glimpse the first steps of galaxy formation more than 10 billion years ago and are without doubt some of the great legacies of the Hubble Space Telescope."

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