# Photometric Supernovae Redshift Systematics Requirements

## TL;DR

This paper analyzes the precision needed in photometric supernova redshift measurements to avoid biasing cosmological parameters, emphasizing the importance of spectroscopic followup and controlling outliers.

## Contribution

It quantifies the systematic control requirements for photometric supernova redshifts to ensure unbiased dark energy measurements.

## Key findings

- Redshift systematics must be constrained at a few times 10^{-3}.
- Spectroscopic followup is effectively required for robust results.
- Outliers need to be controlled at the subpercent level.

## Abstract

Imaging surveys will find many tens to hundreds of thousands of Type Ia supernovae in the next decade, and measure their light curves. In addition to a need for characterizing their types and subtypes, a redshift is required to place them on a Hubble diagram to map the cosmological expansion. We investigate the requirements on redshift systematics control in order not to bias cosmological results, in particular dark energy parameter estimation. We find that additive and multiplicative systematics must be constrained at the few$\,\times 10^{-3}$ level, effectively requiring spectroscopic followup for robust use of photometric supernovae. Catastrophic outliers need control at the subpercent level. We also investigate sculpting the spectroscopic sample.