High-precision Redshifts for Type Ia Supernovae with the Nancy Grace Roman Space Telescope P127 Prism

We present results from simulating slitless spectroscopic observations with the Nancy Grace Roman Space Telescope’s (Roman) Wide-Field Instrument P127 prism spanning from 0.75 to 1.8 μm. We quantify the efficiency of recovered Type Ia supernova (SN Ia) redshifts, as a function of P127 prism exposure time, to guide planning for future observing programs with the Roman prism. Generating the two-dimensional dispersed images and extracting one-dimensional spectra is done with the slitless spectroscopy package pyLINEAR along with custom-written software. From the analysis of 1698 simulated SN Ia P127 prism spectra, we show the efficiency of recovering SN redshifts to z ≲ 3.0, highlighting the exceptional sensitivity of the Roman P127 prism. Redshift recovery is assessed by setting a requirement of σ z = ( z − z true ) / ( 1 + z ) ≤ 0.01 . We find that 3 hr exposures are sufficient for meeting this requirement, for ≳50% of the sample of mock SNe Ia at z ≈ 2 and within ±5 days of rest-frame maximum light in the optical. We also show that a 1 hr integration of Roman can achieve the same precision in completeness to a depth of 24.4 ± 0.06 AB mag (or z ≲ 1). Implications for cosmological studies with Roman P127 prism spectra of SNe Ia are also discussed.