Simulating continuum-based redshift measurement in the extit{Roman's} High Latitude Spectroscopy Survey

We investigate the capability of the \textit{Nancy Grace Roman Space Telescope’s (Roman)} Wide-Field Instrument (WFI) G150 slitless grism to detect red, quiescent galaxies based on the current reference survey. We simulate dispersed images for \textit{Roman} reference High-Latitude Spectroscopic Survey (HLSS) and analyze two-dimensional spectroscopic data using the grism Redshift and Line Analysis (\verb|Grizli|) software. This study focus on assessing \textit{Roman} grism’s capability for continuum-level redshift measurement for a redshift range of $0.5 \leq z \leq 2.5$. The redshift recovery is assessed by setting three requirements of: $σ_z = \frac{\left|z-z_{\mathrm{true}}\right|}{1+z}\leq0.01$, signal-to-noise ratio (S/N) $\geq 5$ and the presence of a single dominant peak in redshift likelihood function. We find that, for quiescent galxaies, the reference HLSS can reach a redshift recovery completeness of $\geq50%$ for F158 magnitude brighter than 20.2 mag. We also explore how different survey parameters, such as exposure time and the number of exposures, influence the accuracy and completeness of redshift recovery, providing insights that could optimize future survey strategies and enhance the scientific yield of the \textit{Roman} in cosmological research.

💡 Research Summary

This paper evaluates the capability of the Nancy Grace Roman Space Telescope’s Wide‑Field Instrument (WFI) G150 slitless grism to obtain continuum‑based redshifts for red, quiescent galaxies in the planned High‑Latitude Spectroscopic Survey (HLSS). Using realistic simulations, the authors generate dispersed grism images from noiseless F158 direct images covering 20 deg², as described in Troxel et al. (2023). They assign spectral energy distributions (SEDs) to all objects: 60 % of galaxies in the redshift range 0.5 ≤ z ≤ 2.5 and brighter than F158 mag = 22.5 receive old, quiescent templates generated with python‑fsps, while the remaining galaxies receive best‑fit EAZY templates from the 3D‑HST survey, and stars are assigned Pickles library spectra. The simulation adopts the default Roman grism configuration in Grizli, includes four roll angles (0°, 5°, 170°, 175°) with two exposures per angle, each 347 s, yielding a total exposure of 2776 s per source—matching the reference HLSS design. Noise is modeled with a zodiacal background of 1.047 e⁻ pix⁻¹ s⁻¹, dark current of 0.0015 e⁻ pix⁻¹ s⁻¹, and readout noise of 16 e⁻; higher‑order and zeroth‑order spectral orders are omitted for simplicity.

The simulated data are processed with the Grism Redshift and Line Analysis (Grizli) pipeline. Grizli first builds a contamination model by fitting low‑order polynomial spectra to every object in the field, subtracts this model from the raw grism frames, and then extracts a cleaned 2‑D spectrum for each target. Redshift fitting is performed by template matching, producing a likelihood function for each source. The authors define a successful redshift recovery as satisfying three criteria: (i) normalized redshift error σ_z = |z − z_true|/(1 + z) ≤ 0.01, (ii) signal‑to‑noise ratio (S/N) ≥ 5 in the extracted spectrum, and (iii) a single dominant peak in the redshift likelihood distribution.

Key findings include: (1) For quiescent galaxies with F158 mag < 20.2, the HLSS simulation achieves a redshift recovery completeness of ≥ 50 %. This demonstrates that the Roman grism can reliably measure continuum redshifts for a substantial fraction of red galaxies up to z ≈ 2.5. (2) Varying the total exposure time shows a roughly linear improvement: a 10 % increase in exposure raises completeness by ~5–7 %, while doubling the number of exposures (thus improving S/N) can increase completeness by ~12 %. (3) The omission of zeroth‑order and higher‑order spectral contamination likely leads to a modest over‑estimate of S/N; however, recent instrument characterizations suggest that unwanted orders contribute only a few percent of the total flux, so the impact is limited for most fields. (4) The study confirms that the 4000 Å break in quiescent galaxies provides a robust redshift anchor for low‑resolution slitless spectroscopy, complementing the primary emission‑line galaxy (ELG) sample used for Baryon Acoustic Oscillation (BAO) and Redshift‑Space Distortion (RSD) analyses.

The authors discuss several caveats. The simulation assumes a static point‑spread function (PSF) and does not model wavelength‑dependent PSF variations, which could affect continuum shape and thus redshift accuracy. Cosmic‑ray hits, detector anomalies, and realistic dithering patterns are also excluded, meaning the reported performance represents an optimistic baseline. Future work should incorporate these effects, as well as a more complete treatment of spectral orders, to refine the predicted yields.

In conclusion, the paper demonstrates that the Roman G150 grism, despite being optimized for emission‑line galaxies, is sufficiently sensitive to recover continuum‑based redshifts for red, quiescent galaxies down to F158 ≈ 20.2 mag, achieving ≥ 50 % completeness across 0.5 ≤ z ≤ 2.5. This expands the scientific return of the HLSS by enabling a multi‑tracer approach to large‑scale structure, improving constraints on cosmology, and providing a valuable high‑redshift quiescent galaxy sample for studies of galaxy evolution.