The Signifance of The Deep Field

In December 1995 HST spent 10 days pointed toward a patch of sky some 2.7 arcminutes on a side, or about a tenth the diameter of the full Moon, near the Handle of the Big Dipper. The aim was to provide a "deep core sample of the universe" by securing an image of objects as faint as possible with the telescope. Although previous ground-based observations might have reached the 3Oth magnitude limit of this Hubble Deep Field (HDF), HST provided much better angular resolution.

The HDF sparked enormous interest and numerous investigations. Somewhat unusual in astronomy, everyone had immediate access to the same sample and it consisted of more than 1,500 galaxies at the farthest limits of the observable universe. Exactly what the sample had to say about cosmology and the evolution of galaxies led to a hot competition to produce the first interpretive papers, and many others have followed in their wake. The HDF also prompted numerous follow-up observations, not just with HST but also with other space observatories and big ground-based telescopes, including the Very Large Array radio telescope. To judge from the response of astronomers, the HDF is perhaps the HST's single most scientifically significant image to date. (Robert Smith)

Research area	Pre-HST knowledge (about 1990)	HST contribution (to about 2000)
Universe's rate of expansion	Research groups disagree by a factor of 2, yielding estimated ages of the universe between 10 and 20 billion years.	The Key Project value is Ho = 70km per second per megaparsec, good to 10 percent, suggestinq an age of 12-14 billion years. However, disagreement lingers among HST teams.
Gravitational lenses	A few examples are known.	Many small lenses were uncovered. Lenses have the potential to measure the curvature of space and the age of the universe.
Quasars	Quasars originated in the early universe. Some are surrounded by "fuzz," which is interpreted as the host galaxy. Quasars are thought to be powered by black holes.	HST clearly resolves a variety of galaxies hosting quasars. A disproportionate number are involved in mergers with other galaxies. These mergers may fuel the black holes.
Distant galaxies and galaxy evolution	Relatively little is known about galaxies more than a few billion light-years from Earth.	Deep imaging provides evidence for the evolution of galaxy morphology and the rate of star formation. Galaxies are seen to within a billion years of the Big Bang.
Supermassive black holes	Ground-based data are suggestive, but telescopes cannot see close enough to suspected black holes to prove absolutely that they exist.	HST surveys reveal that dark, compact objects are common in the cores of galaxies and greatly strengthen the case that they may be black holes.
Galactic bulge structure	Only the Milky Way's central bulge and those of nearby galaxies can be viewed in detail.	HST confirms large galactic bulges formed in the early universe along with elliptical galaxies. Smaller bulges can be "inflated" by ongoing starbirth fueled by disk instabilities or galaxy mergers.
Environments around protostars	Little is known about circumstellar environments. Jetlike features have been noted; stellar disks have been seen in the infrared.	HST helps show that protoplanetary disks are common. Circumstellar disks confine and direct the flow of jets.
Supernova 1987A	The nearest supernova seen in the last 400 years. An armada of telescopes watches changes following its detection in February 1987.	Only HST has the resolution to trace, at sub-light-year scale, yearly changes in the fireball debris and the circumstellar ring of enriched gas.
Pluto	Photometry during transit of Pluto's moon Charon yields an albedo map of Pluto's surface.	HST confirms an early map and clearly shows a variegated surface. Images resolve Pluto and its moon.