The Hubble Deep Fields

Hubble Deep Field North (WFPC2)

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 first deep fields – Hubble Deep Field North and South - gave astronomers a peephole to the ancient Universe for the first time, and have caused a real revolution in modern astronomy.

Hubble Deep Field South (STIS)

Deep field observations are long-lasting observations of a particular region of the sky intended to reveal faint objects by collecting the light from them for an appropriately long time. The ‘deeper’ the observation is (i.e. longer exposure time), the fainter are the objects that become visible on the images. Astronomical objects can either look faint because their natural brightness is low, or because of their distance.

10 days in the making
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 analyzed 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.