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J.P. Gardner, S. Satyapal (NASA/GSFC)
The Hubble Deep Field datasets include the deepest observations ever made in the ultraviolet, optical and near-infrared, reaching depths comparable to that expected for NGST spectroscopy. We present the source counts, galaxy sizes and isophotal filling factors of the HDF-South images. The observed integrated galaxy counts reach nearly 1000 galaxies per square arcminute. We extend these counts to fainter levels and further into the infrared using galaxy count models. It was determined from the HDF-N and other deep WFPC2 imaging that fainter galaxies are smaller. This trend continues to AB=29 in the high resolution HDF-S STIS image, where galaxies have a typical half-light radius of 0.1 arcseconds. We have run extensive Monte Carlo simulations of the detection of galaxies in the HDF-S, and show that the small measured sizes are not due to selection effects until >29mag. We compare observed sizes in the optical and near-infrared using the HDF-S NICMOS image. We discuss the implications of these data for the design of the NGST near-infrared spectrograph. We compare the effects of resolution and the confusion limit of various designs, as well as the multiplexing advantages of either multi-object or full-field spectroscopy.
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