Cosmology Requirements on Supernova Photometric Redshift Systematics for Rubin LSST and Roman Space Telescope

TL;DR

This paper evaluates the systematic errors in photometric redshifts of supernovae for upcoming surveys like LSST and Roman, emphasizing the need for spectroscopic follow-up to ensure accurate cosmological measurements.

Contribution

It quantifies the redshift systematic control requirements and highlights the importance of spectroscopic surveys for robust supernova cosmology.

Findings

Photometric redshift systematics can bias dark energy inference.

Spectroscopic follow-up at low redshift improves cosmological robustness.

Including Roman infrared bands enhances high-redshift constraints.

Abstract

Some million Type Ia supernovae (SN) will be discovered and monitored during upcoming wide area time domain surveys such as the Vera C. Rubin Observatory Legacy Survey of Space and Time (LSST). For cosmological use, accurate redshifts are needed among other characteristics; however the vast majority of the SN will not have spectroscopic redshifts, even for their host galaxies, only photometric redshifts. We assess the redshift systematic control necessary for robust cosmology. Based on the photometric vs true redshift relation generated by machine learning applied to a simulation of 500,000 galaxies as observed with LSST quality, we quantify requirements on systematics in the mean relation and in the outlier fraction and deviance so as not to bias dark energy cosmological inference. Certain redshift ranges are particularly sensitive, motivating spectroscopic followup of SN at $z\lesssim0.2$ and around $z\approx0.5$-0.6. Including Nancy Grace Roman Space Telescope near infrared bands in the simulation, we reanalyze the constraints, finding improvements at high redshift but little at the low redshifts where systematics lead to strong cosmology bias. We identify a complete spectroscopic survey of SN host galaxies for $z\lesssim0.2$ as a highly favored element for robust SN cosmology.