Photometric Redshift Biases from Galaxy Evolution
  Proposed cosmological surveys will make use of photometric redshifts of
galaxies that are significantly fainter than any complete spectroscopic
redshift surveys that exist to train the photo-z methods. We investigate the
photo-z biases that result from known differences between the faint and bright
populations: a rise in AGN activity toward higher redshift, and a metallicity
difference between intrinsically luminous and faint early-type galaxies. We
find that even very small mismatches between the mean photometric target and
the training set can induce photo-z biases large enough to corrupt derived
cosmological parameters significantly. A metallicity shift of ~0.003dex in an
old population, or contamination of any galaxy spectrum with ~0.2% AGN flux, is
sufficient to induce a 10^-3 bias in photo-z. These results highlight the
danger in extrapolating the behavior of bright galaxies to a fainter
population, and the desirability of a spectroscopic training set that spans all
of the characteristics of the photo-z targets, i.e. extending to the 25th mag
or fainter galaxies that will be used in future surveys.
