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The forward-modelling of galaxy surveys has recently gathered interest as one of the primary methods to achieve the precision on the estimate of the redshift distributions required by stage IV surveys. One of the key aspects of forward-modelling is the connection between the physical properties of galaxies and their intrinsic spectral energy distributions (SEDs), achieved through stellar population synthesis (SPS) codes, e. g. FSPS. However, SPS requires many detailed assumptions about the galaxy constituents, for which the model choice or parameters are currently uncertain. In this work, we perform a sensitivity study of the impact that the SED modelling choices variations have on the mean and scatter of the tomographic galaxy redshift distributions. We use the Prospector- model and its SPS parameters to build observed magnitudes of a fiducial sample of galaxies. We then build new samples by varying one SED modelling choice at a time. We model the colour-redshift relation of these galaxy samples using the KiDS-VIKING remapped version (McCullough et al. 2023) of the Masters et al. (2015) SOM. We place galaxies in the SOM cells according to the simulated galaxy colours. We then build color-selected tomographic bins and compare each variant's binned redshift distributions against the estimates obtained for the fiducial model. We find that the SED components related to the IMF, AGN, gas physics, and attenuation law substantially bias the mean and the scatter of the tomographic redshift distributions with respect to those estimated with the fiducial model. For the uncertainty of these choices currently present in the literature, and regardless of any stellar mass function based reweighting strategy applied, the bias in the mean and the scatter of the tomographic redshift distributions is larger than the precision requirements set by Stage IV galaxy surveys, e. g. LSST and Euclid.
Tortorelli et al. (Thu,) studied this question.