The 1.6 Micron Bump as a Photometric Redshift Indicator

I describe the principle of using the 1. 6um 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 using only their infrared (JHKL) photometry. I then consider the applicability of the technique to the 3. 6--8um 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>~1. 5 and are dominated by stellar populations older than >~20Myr. Galaxies whose photometric fits indicate them to be at lower redshifts, zfit<~1. 5, or dominated by very young stellar populations, agefit<~20Myr, 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. 6um bump can provide a powerful way of estimating redshifts of distant galaxies in deep infrared imaging surveys that will soon be provided by SIRTF, and, eventually, by NGST.