Fishing for the Optimal Roman High Latitude Time Domain Survey: Cosmological Constraints for 1,000 Possible Surveys

TL;DR

This paper evaluates 1,000 potential survey configurations for the Roman Space Telescope's High Latitude Time Domain Survey to optimize dark energy constraints, considering various parameters, calibration uncertainties, and survey strategies.

Contribution

It provides a comprehensive simulation-based analysis of survey parameters affecting cosmological constraints, recommending optimal strategies and introducing interlaced cadences to improve efficiency.

Findings

Optimal survey configuration involves ~20% prism time and 30-40% wide imaging.

Interlaced cadences reduce overheads and increase filter coverage.

Simulated light curves and covariance matrices are released for community use.

Abstract

The upcoming Nancy Grace Roman Space Telescope is set to conduct a generation-defining SN Ia cosmology measurement with its High Latitude Time Domain Survey (HLTDS). However, between optical elements, exposure times, cadences, and survey areas, there are many survey parameters to consider. This work was part of a Roman Project Infrastructure Team effort to help the Core Community Survey (CCS) Committee finalize the HLTDS recommendation to the Roman Observations Time Allocation Committee. We simulate 1,000 surveys, with and without a conservative (volume-limited) version of the Vera C. Rubin Observatory Deep Drilling Field SNe Ia, and compute Fisher-matrix-analysis Dark Energy Task Force Figures of Merit (FoM, based on w0-wa constraints) for each. We investigate which survey parameters correlate with FoM, as well as the dependence of the FoM values on calibration uncertainties and the SN dispersion model. The exact optimum depends on the assumed dispersion model and whether Rubin DDF SNe Ia are also considered, but ~20% time in prism, ~30--40% time in Wide imaging and the remainder in Deep imaging seems most promising. We also advocate for "interlaced" cadences where not every filter is used in every cadence step to reduce overheads while maintaining a good cadence and increasing the number of filters compared to the Rose et al. (2021) reference survey (the prism has proportionately lower overheads and can be used for each cadence step). We show simulated light curves and spectra for the baseline HLTDS CCS recommendation and release distance-modulus covariance matrices for all surveys to the community.