The remarkable Hubble Deep Field (HDF) image has been produced during a period of great progress towards the goal of understanding the evolution of galaxies over cosmic time. The progress over the last five years has been driven in large measure by the introduction in the early 1990's of highly multiplexed spectrographs on existing 4-m class telescopes, by the commissioning of the Keck 10m telescope and by the attainment of the full capabilities of the HST/WFPC. A primary motivation for the HDF was to build on these advances through the deepest possible look at galaxies in the distant Universe.
In recent years, there have been several large and deep redshift surveys carried out on ground-based 4-m class telescopes (Table 1). Broadly speaking these contain several hundred galaxies from magnitude limited samples selected in various wavebands. Different programs have been based in different wavebands: the LDSS (Glazebrook et al. 1995) in the B-band, the CFRS (Le Fèvre et al. 1995 and references therein) in the I-band, and the Hawaii Deep program (Cowie et al. 1996) in the K-band with extensions in the other bands also. Most of the galaxies in the 4-m samples lie above broadly equivalent flux density limits of BAB ~ 24, IAB ~ 22.5 and KAB ~ 21.5. Cowie et al. (1996) have pushed another 0.5-1.0 magnitudes deeper with Keck on smaller samples. The surveys have generally achieved a reasonably high success rate in spectroscopic identification (around 80%) producing a high degree of statistical completeness in their sampling of the galaxy population.
|Program||Selection||Galaxies||Redshift range and median z at limit|
|Broadhurst et al. (1988)||B < 21.5||~400||0.03 < z < 0.9, < zlim > = 0.4|
|Colless et al. (1990)||B < 22.5|
|Glazebrook et al. (1995)||B < 24|
|CFRS (1995)||IAB < 22.5||730||0.1 < z < 1.3, < zlim > = 0.6|
|Hawaii Deep Survey||K < 20||390||0.1 < z < 1.7, < zlim > = 0.7|
|Cowie et al. (1996)||I < 23; B < 24.5|
In the context of the HDF and the spectroscopic work that has been done in it, these existing ground-based survey are noteworthy for the much larger area of sky covered. For instance the effective area of the CFRS is 110 arcmin2, almost 20 times larger than the HDF. Thus the spectroscopic surveys contain many more relatively bright galaxies (e.g., IAB < 22.5 for the CFRS) than HDF-based studies.
Despite, by the standards of the HDF, being rather bright, the galaxies sampled by the redshift surveys are nevertheless an important population for evolution studies. First, and in quite general terms, the redshift surveys reach down to the brightness level where the integrated surface brightness of the galaxy population per magnitude interval is maximized (see Fig. 1) and the surveys are therefore sampling a good fraction of the extragalactic background light that is produced by discrete objects. It should be recalled that in the idealized case of galaxies with fiat spectra (f v0), the production of heavy elements by a particular component of the galaxy population is proportional to the contribution of that population to the extragalactic background light (Lilly and Cowie 1987). Thus the population sampled by the redshift surveys is likely to be playing an important role in the overall history of star-formation in the Universe. Second, these surveys yield a median redshift < z > ~ 0.6 with the more luminous L* galaxies extending to redshifts z > 1. Thus these samples cover the evolution of luminous galaxies over the last 2/3 of the age of the Universe, and probe to a point not far from the peak in the global luminosity density that has been inferred from estimated redshifts in the HDF (Madau et al. 1996, Connolly et al. 1997). The median redshift is similar to that found in the HDF galaxies with measured redshifts.
Figure 1. The relative surface brightness per magnitude interval of the galaxy population as a function of AB magnitude at three wavelengths (offset vertically for clarity). The limiting magnitudes of the large systematic redshift surveys are shown as triangles. These surveys sample galaxies that contribute most of the extragalactic background light in discrete galaxies.