The NEWFIRM Medium-Band Survey: Filter Definitions and First Results

Deep near-infrared imaging surveys allow us to select and study distant galaxies in the rest-frame optical, and have transformed our understanding of the early Universe. As the vast majority of K- or IRAC-selected galaxies are too faint for spectroscopy, the interpretation of these surveys relies almost exclusively on photometric redshifts determined from fitting templates to the broadband photometry. The best-achieved accuracy of these redshifts, Delta z/(1 + z) greater than or similar to 0: 06 at z > 1.5, is sufficient for determining the broad characteristics of the galaxy population but not for measuring accurate rest-frame colors, stellar population parameters, or the local galaxy density. We have started a near-infrared imaging survey with the NEWFIRM camera on the Kitt Peak 4-m telescope to greatly improve the accuracy of photometric redshifts in the range 1:5 less than or similar to z less than or similar to 3: 5. The survey uses five medium-bandwidth filters, which provide crude spectra over the wavelength range 1-1.8 mu m for all objects in the 27: 60 x 27: 60 NEWFIRM field. In this first paper, we illustrate the technique by showing medium-band NEWFIRM photometry of several galaxies at 1: 7 < z < 2.7 from the recent near-infrared spectroscopic sample of Kriek et al. The filters unambiguously pinpoint the location of the redshifted Balmer break in these galaxies, enabling very accurate red-shift measurements. The full survey will provide similar data for similar to 8000 faint K- selected galaxies at z > 1.5 in the COSMOS and AEGIS fields. The filter set also enables efficient selection of exotic objects such as high-redshift quasars, galaxies dominated by emission lines, and very cool brown dwarfs; we show that late T and candidate Y dwarfs could be identified using only two of the filters.