Photometric Redshift Using the Far-infrared Spectral Energy Distribution

A method for using the submillimeter band to determine photometric redshifts of luminous, high-z, dusty galaxies is presented. It is based on the observation that local ultraluminous infrared galaxies (ULIRGs) show an average spectral energy distribution (SED) that has a remarkably small dispersion for wavelengths λ > 50 μm. The shape of the long-wavelength part is independent of whether the local ULIRG possesses an active galactic nucleus or not. A local template is derived from a sample of ULIRG data presented by Klaas et al. in 2001. Using this template and the internal dispersion in its shape, it is shown that observations at two fixed submillimeter wavelengths, e.g., 450 and 850 μm, can be used to determine the photometric redshift of galaxies in the redshift range 1 < z < 5. The uncertainty in the redshift arising from the template SED amounts to Δz/z ~ 0.3, with an improvement at the higher redshift range. Implications for using this method together with the large instantaneous bandwidth of the Atacama Large Millimeter Array (ALMA) for deriving spectroscopic redshifts using CO emission is discussed.