By observing through four different color filters, the Hubble Deep Field provides a unique way to distinguish very distant galaxies from nearby galaxies. Hydrogen both within the distant galaxies, and in intergalactic space, absorbs some of the ultraviolet light from the very distant galaxies. The cumulative effect of the absorption is to make these galaxies essentially vanish in the ultraviolet. The Hubble Deep Field will be used to perform a statistical study of the distribution of galaxies on the sky. This is an essential test of models for the structure of the universe and galaxy formation theories.
Current observations show that galaxies tend to cluster around other galaxies.
However, the faintest galaxies are almost randomly distributed on the sky. The Hubble Deep Field will push such studies to fainter limits. 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. This information, combined with actual distances derived from ground-based spectroscopic observations, will provide a new test for theories of spiral galaxies.