The Effect of Dust on Photometric Redshift Measurement; A Self-consistent Technique

Abstract

A new method is developed for estimating photometric redshifts to galaxies, using realistic template SEDs, extending over four decades in wavelength (i.e. from 0.05 micron to 1 mm). The template SEDs are constructed for four different spectral types of galaxies (elliptical, spiral, irregular and starburst), satisfying the following characteristics: a). they are normalised to produce the observed colours of galaxies at z ~ 0; b). incorporate the chemo-photometric spectral evolution of galaxies of different types, in agreement with the observations; c). allow treatment of dust contribution and its evolution with redshift, consistent with the spectral evolution model; d). include absorption and re-emission of radiation by dust and hence, realistic estimates of the far-infrared radiation; e). include correction for inter-galactic absorption by Lyman continuum and Lyman forest. Using these template SEDs, the photometric redshifts are estimated to an accuracy of ~ 0.11. The effect on the estimated photometric redshifts, due to assumptions in the evolutionary population synthesis models, are investigated and discussed. The simultaneous and self-consistent modelling of both the photometric and chemical evolution of galaxies and the effect of dust, makes this technique particularly useful for high redshift galaxies. Moreover, since the template SEDs (at different redshifts) cover the range from UV to far-infrared and sub-mm wavelengths, one could consistently use the optimised SEDs to estimate the contribution from individual galaxies to these wavelengths, where no observational data are available.