Impact of stellar population synthesis choices on forward modelling-based redshift distribution estimates

Context. The forward modelling of galaxy surveys has recently gathered interest as one of the primary methods to achieve the required precision on the estimate of the redshift distributions for stage IV surveys, allowing them to perform cosmological tests with unprecedented accuracy. One of the key aspects of forward modelling a galaxy survey is the connection between the physical properties drawn from a galaxy population model and the intrinsic galaxy spectral energy distributions (SEDs), achieved through stellar population synthesis (SPS) codes (e.g. FSPS). However, SPS requires a large number of detailed assumptions on the constituents of galaxies, for which the model choice or parameter values are currently uncertain. Aims. In this work, we perform a sensitivity study of the impact that the variations of the SED modelling choices have on the mean and scatter of the tomographic galaxy redshift distributions. Methods. We assumed the P ROSPECTOR - β model as the fiducial input galaxy population model and used its SPS parameters to build 9-bands ugriZYJHK s observed-frame magnitudes of a fiducial sample of galaxies. We then built samples of galaxy magnitudes by varying one SED modelling choice at a time. We modelled the colour-redshift relation of these galaxy samples using the self-organising map (SOM) approach that optimally groups similar redshifts galaxies by their multidimensional colours. We placed galaxies in the SOM cells according to their simulated observed-frame colours and used their cell assignment to build colour-selected tomographic bins. Finally, we compared each variant’s binned redshift distributions against the estimates obtained for the original P ROSPECTOR - β model. Results. We find that the SED components related to the initial mass function, as well as the active galactic nuclei, the gas physics, and the attenuation law substantially bias the mean and the scatter of the tomographic redshift distributions with respect to those estimated with the fiducial model. Conclusions. For the uncertainty of these choices currently present in the literature and regardless of the applied stellar mass function based re-weighting strategy, the bias in the mean and the scatter of the tomographic redshift distributions are greater than the precision requirements set by next-generation Stage IV galaxy surveys, such as the Vera C. Rubin Observatory’s Legacy Survey of Space and Time (LSST) and Euclid .