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BIB-VERSION:: AST-PP-v1.0
ID:: epreps.stsci//prep1194
ENTRY:: March 9, 1998
TITLE:: The Hubble Deep Field
SUBTITLE::
AUTHOR:: Williams, Robert
AFFIL:: (1) Space Telescope Science Institute 3700 San Martin Drive Baltimore, MD 21218 USA
DATE:: November 1997
JOURNAL:: To appear in:
SUBMITTED::
ACCEPTED::
OTHER_ACCESS::
COPYRIGHT:: Copyright 1997 The Association of Universities for Research in Astronomy, Inc. All Rights Reserved.
LANGUAGE:: English
ABSTRACT::

Introduction:
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 c
ertain
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 Dressler 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.



The 0939+4713 observation was followed some months later by a GO program
of Dickinson (1995) to image for 32 orbits the strong radio galaxy 3C 324 at
redshift z = 1.2. The final combined image of 3C 324 and associated galaxies
was
very interesting in that it revealed numerous ellipticals, but unlike 0939+4713,
no spiral galaxies. Furthermore, the E's were measured to have a
r1/4 radial
light distribution characteristic of dynamically relaxed stellar systems. The
ellipticals thus showed evidence of already being old at this higher redshift,
whereas spiral galaxies were not yet present. The extent to which this
difference in the two clusters indicated basic evolutionary properties of
E's and S's was difficult to ascertain because of the small sample involved of
only two clusters. However, it did demonstrate the power of HST to
discern galaxy structure at high redshift.



END:: epreps.stsci//prep1194