extensive operational control of the spacecraft. Indeed, conducting this project depends not so much on the ATD/NTOT’s performance as on other factors. Prime among these are the extra operational costs. Nevertheless, if the ATD/NTOT is viewed as a demonstration of technological capability, such an innovative use of a spacecraft represents an interesting challenge.


The area in which the baseline ATD/NTOT has the greatest promise for making new astronomical discoveries is in deep infrared surveys of the early universe. Several possible observing campaigns are described below.

Galaxies in the Early Universe

Several lines of evidence suggest that the epoch of galaxy formation lies beyond a redshift (z) of about 1. For example, the co-moving number density of both quasars and their absorption-line systems changes dramatically for redshifts greater than approximately 1.5. The nature of galaxies associated with distant radio sources, and the nature of the radio sources themselves, also change significantly for redshifts beyond z ~ 1.

This has been beautifully exemplified in HST images of the radio source 3C 324 at z ~ 1.2 (Figure 5.1). The dramatic examples of interacting and merging galaxies in this image are also seen at lower redshifts, but it is becoming clear that not all galaxies have evolved significantly below z ~ 1. Dwarf galaxies at lower redshifts show evidence of substantial evolution, primarily through bursts of star formation. But more massive galaxies appear to be different; the available data suggest that the bulk of their formation occurred at redshifts greater than about 0.8 to 1. As a class, massive galaxies appear to have undergone little evolution between that time and the present day. Thus the formation or assemblage of these objects must have occurred at a higher redshift.

Unfortunately, extensive studies of such objects with the HST will prove to be very difficult. Because they are expected to have high sensitivity from below 0.4 microns to ~2 microns, the HST’s second-and third-generation imaging instruments, such as NICMOS and HACE, respectively, will clearly provide substantial gains for studying distant galaxies. However, several factors mitigate against the HST for a comprehensive study of galaxies at redshifts of z ~ 1 to 5.

At these redshifts, the optical and near-infrared images correspond largely to wavelengths in the rest-frame ultraviolet. The brightness of a galaxy in the ultraviolet, however, depends dramatically on the amount of star formation and on the quantity and distribution of dust. Surface brightness dimming (through the (1 + z)4 factor) also has a major impact at these redshifts. The contrast of distant galaxies against the foreground therefore decreases rapidly at redshifts of z > 1. While quite model dependent, the dilution factor owing to foreground galaxies could be as large as 100:1.

The cumulative effect of all these factors is large and is exacerbated by the HST’s small collecting area. Extremely luminous star-burst galaxies with strong near-ultraviolet and ultraviolet fluxes will likely be identified and studied with the HST, but a more complete census of high-redshift objects will require a telescope with greater infrared sensitivity. Typical “young” galaxies are likely to have broadly peaked energy distributions around 0.4 to 0.6 microns, and so the study of these objects is best carried out in the wavelength range 0.5(1 + z) microns, or ~ 1 to 5 microns for such objects in the redshift range 1 < z < 6.

The sensitivity of most large, ground-based telescopes is limited for wavelengths greater than 2 microns by their own extremely high thermal background. Only telescopes that are both optimized for the infrared and located in Antarctica have a chance of reaching the limits of the atmospheric background, which even in Antarctica is considerable compared to, for example, the zodiacal background seen from space. Adaptive optics will clearly help these telescopes by increasing the contrast between small sources and the background, but the background remains a dominant source of noise, especially for any extended components of the sources.

Cryogenic space telescopes, such as the European Space Agency’s soon-to-be-launched Infrared Space Observatory (ISO) and NASA’s proposed Space Infrared Telescope Facility (SIRTF), will be free from any thermal background at the wavelength regions being considered here, but they will face another problem. At the faint magnitudes required for study of these distant galaxies, source confusion is already a serious concern. At a median

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