Photometric redshift using the far - infrared SED

A method for using the sub-millimeter band to determine photometric red-shifts of luminous high-z dusty galaxies is presented. It is based on the observation that local ultra-luminous IR galaxies show an average spectral energy distribution which has a remarkably small dispersion for wavelengths λ > 50µm. The shape of the long wavelength part is independent on whether the local ULIRG possesses an AGN or not. A local template is derived from a sample of ULIRG data presented by Klaas et al. (2001). Using this template and the internal dispersion in its shape, it is shown that observations at two fixed submm wavelengths , e.g. 450µm and 850µm, can be used to determine the photometric red-shift of galaxies in the redshift range 1 < z < 5. The uncertainty in the redshift arising from the template SED amounts to ∆z/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.