Redshift estimation from low-resolution prism SEDs with an NGST MOS

We discuss the utility of a low resolution prism as a component of a Multi-Object Spectrometer for NASA's proposed Next Generation Space Telescope (NGST). Low resolution prism spectroscopy permits simultaneous observation of the 0.6-5micron wavelength regime at R~50. To such data we can apply the modern techniques in spectral energy distribution (SED) fitting to determine source redshifts, sometimes called ``photometric redshifts''. Low resolution prism observations of galaxy SED's provide a significant advantage over multi-filter observations for any realistic observing strategy, and we have performed extensive simulations to quantify it. The advantage of the prism is largest in the regime of faint (K_AB>30) objects at high redshift (z>4), the primary discovery space of NGST. Many important objects will be too faint for follow up at higher spectral resolution, so prism observations are the optimal technique to study them. Prism observations also reduce the contamination of high redshift samples by lower redshift interlopers.