The 1.6 micron bump as a photometric redshift indicator

I describe the principle of using the 1.6 mum H- spectral feature as a photometric redshift indicator and demonstrate that the technique holds promise by successfully recovering the redshifts of a small sample of z = 0-1 galaxies by using only their infrared (JHKL) photometry. I then consider the applicability of the technique to the 3.6-8 mum Space Infrared Telescope Facility (SIRTF)filter set and investigate the systematic errors that could arise in photometric redshifts from random photometric errors or from a mismatch between target galaxies and fitting templates in metallicity, star formation history, and amount of interstellar dust. It appears that SIRTF near-IR data alone should be sufficient to estimate redshift of most galaxies that are at z greater than or similar to 1.5 and are dominated by stellar populations older than greater than or similar to20 Myr. Galaxies whose photometric fits indicate them to be at lower redshifts, z(fit) less than or similar to 1.5, or dominated by very young stellar populations, age(fit) less than or similar to 20 Myr, suffer from severe degeneracies in photometric redshift, and a reliable photometric determination of their redshifts will have to include either IR observations at shorter wavelengths (H and K) or optical data. Overall, it appears that with care and caveats the 1.6 mum bump can provide a powerful way of estimating red-shifts of distant galaxies in deep infrared imaging surveys that will soon be provided by SIRTF and eventually by Next Generation Space Telescope.