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Abstract: Proposed cosmological surveys will make use of photometric redshifts ofgalaxies that are significantly fainter than any complete spectroscopicredshift surveys that exist to train the photo-z methods. We investigate thephoto-z biases that result from known differences between the faint and brightpopulations: a rise in AGN activity toward higher redshift, and a metallicitydifference between intrinsically luminous and faint early-type galaxies. Wefind that even very small mismatches between the mean photometric target andthe training set can induce photo-z biases large enough to corrupt derivedcosmological parameters significantly. A metallicity shift of ~0.003dex in anold population, or contamination of any galaxy spectrum with ~0.2% AGN flux, issufficient to induce a 10^-3 bias in photo-z. These results highlight thedanger in extrapolating the behavior of bright galaxies to a fainterpopulation, and the desirability of a spectroscopic training set that spans allof the characteristics of the photo-z targets, i.e. extending to the 25th magor fainter galaxies that will be used in future surveys.



Author: C. J. MacDonald, Gary Bernstein

Source: https://arxiv.org/



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