The Hubble Deep Field

Great progress has been made in recent years in understanding the large-scale structure of the universe. Recall that it is only within this century that we have even come to realize the existence of other galaxies as separate entities. Until the 1920’s the universe did not extend beyond the Milky Way. Novae and Cepheid variables changed that picture, and for the past 75 years extragalactic astronomy has been one of the most active and fertile areas of science. The concept of an expanding universe and its beginning in a Big Bang all derive from the discovery of external galaxies. Distant galaxies are faint and have small angular sizes, therefore their study has remained the province of the largest telescopes. Before the launch of Hubble Space Telescope, ground-based telescopes had succeeded in detecting distant galaxies out to redshifts of z ~ 1, and in establishing certain of their characteristics. But, there was uncertainty as to how much further HST could push the study of distant galaxies given its modest 2.4m diameter mirror and the fact that the surface brightnesses of cosmologically distant objects decrease as (1 + z) 4 . For this reason, one of the early observations that was scheduled immediately after the first servicing mission of HST in December 1993 to repair spherical aberration was the re-imaging of the cluster of galaxies 0939+4713 at z = 0.4 that had been observed previously by Dressier et al. (1994) from both the ground and with the aberrated HST. The ten-orbit WFPC2 image demonstrated HST’s ability to resolve structure in distant galaxies, showing spiral and elliptical galaxies with a clarity approaching that achieved for the Coma cluster from the ground. Spirals are seen to be relatively abundant in 0939+4713, although they generally show an anomalous morphology.