Publications

Calibration uncertainties are the leading systematics in cosmological analyses using Type Ia supernovae (SNe Ia). For the Nancy Grace Roman Space Telescope (Roman), we quantify the impact of chromatic effects on SNe Ia photometry and derived cosmological parameters, using simulated light curves from the High-Latitude Time Domain Survey. We investigate two sources of wavelength-dependent bias: focal plane array (FPA)-dependent wavelength shifts arising from spatial variations across Roman's 18 detectors, and coherent wavelength shifts corresponding to the measured 0.06% uncertainty in absolute filter wavelength calibration. Using simulated SNe Ia light curves, we find that the FPA-dependent shifts -- which range from +6 to -80 Å introduce a redshift-dependent distance modulus bias that, if left uncorrected, propagates to Δw0∼−0.06 and Δwa=−0.236, which are larger than the forecast statistical uncertainties of σstat,w0=0.025 and σstat,wa=0.114, rendering the survey systematics-limited. We probe the impact of chromatic effects by employing detector-specific filter curves that recover unbiased cosmological constraints; to remain below the statistical noise floor, FPA wavelength shifts must be characterized to within 20%. In contrast, a coherent 0.06% offset in filter wavelength calibration -- ranging from -3 to -11 Å -- produces negligible redshift-dependent bias, with a minimal spread in wa (Δwa=−0.0004,σwa,sys=0.114), demonstrating that the achieved pre-launch calibration precision is sufficient for this systematic to remain subdominant. Our results establish that chromatic effects are a required component of SN Ia cosmology with Roman.

Submitted. 2026arXiv260708886P

In the coming years, the Vera Rubin Observatory's Legacy Survey of Space and Time (Rubin-LSST) and the Nancy Grace Roman Space Telescope's (Roman) High Latitude Time Domain Survey (HLTDS) are expected to discover more than a million Type Ia supernovae (SNe Ia), several orders of magnitude more than current samples and with a tighter control on systematic uncertainties. One of the largest systematic uncertainties in cosmological analyses with SNe Ia is the accuracy of the spectro-photometric model for SNe Ia time series data, which depends on the photometric calibration of the surveys. To quantify the impact of this uncertainty, we analyze simulated Rubin-LSST and HLTDS data, perturb the photometric zero-points and filter mean wavelengths, and propagate these systematics to spectral model recovery, estimated distances, and dark energy figure of merit (FoM) based on the w0waCDM model. Zero-point shifts of 5 mmag and filter mean wavelength shifts of 5 angstrom lead to a ∼50% decrease in the FoM relative to a statistical-only case when calibration uncertainties are propagated only through light-curve fitting. The same calibration shifts applied only during model training produce a smaller ∼13% degradation. Contrary to previous analyses, calibration uncertainties in light-curve fitting dominate over those from model training. Their effect during light-curve fitting varies smoothly with redshift and is nearly degenerate with cosmology, preventing mitigation through self-calibration. Finally, we show that the FoM dependence on the size of the calibration uncertainties (in the range of expected sizes) is roughly linear.

Accepted. 2026arXiv260419746A

Slitless spectroscopy enables efficient, large-area surveys without target preselection, yet it faces challenges from source blending, higher noise, and lost spatial─spectral information. We present an advanced, nonparametric, data-driven algorithm that leverages multiple dispersion angles to reconstruct three-dimensional flux distributions, providing low-resolution integral field unit capabilities from slitless data. By treating each pixel as an independent element, our method naturally handles source confusion without requiring prior assumptions regarding redshifts, templates, or model libraries. We validate the algorithm using simulated Roman Space Telescope wide-field slitless spectroscopy images that are equivalent to what is expected from the High-Latitude Time-Domain Survey. First, we demonstrate that a host-galaxy model reconstructed from multiple dispersion angles can be used to accurately subtract host light from a transient, recovering a Type Ia supernova spectrum with minimal bias. Second, we showcase a high-fidelity flux-cube reconstruction of a complex galaxy, successfully measuring the redshift and recovering continuum, emission, and absorption features. This approach highlights the potential of multi-dispersion-angle slitless data to provide spatially resolved spectral information in a nonparametric way, which is traditionally accessible only with integral field spectroscopy, opening a new window into large, unbiased, and spatially resolved studies of galaxy evolution.

Published in ApJ. DOI: 10.3847/1538-3881/ae7a48.

The Nancy Grace Roman Space Telescope will carry out a wide-field imaging and slitless spectroscopic survey of Type Ia supernovae to improve our understanding of dark energy. Crucial to this endeavor is obtaining supernova spectra uncontaminated by light from their host galaxies. However, obtaining such spectra is made more difficult by the inherent problem in wide-field slitless spectroscopic surveys: the blending of spectra of close objects. The spectrum of a supernova will blend with the host galaxy, even from regions distant from the supernova on the sky. If not properly removed, this contamination will introduce systematic bias when the supernova spectra are later used to determine intrinsic supernova parameters and to infer the parameters of dark energy. To address this problem, we developed an algorithm that makes use of the spectroscopic observations of the host galaxy at all available observatory roll angles to reconstruct a 3D (2D spatial, 1D spectral) representation of the underlying host galaxy that accurately matches the 2D slitless spectrum of the host galaxy when projected to an arbitrary rotation angle. We call this "scene reconstruction." The projection of the reconstructed scene can be subtracted from an observation of a supernova to remove the contamination from the underlying host. Using simulated Roman data, we show that our method has extremely small systematic errors and significantly less random noise than if we subtracted a single perfectly aligned spectrum of the host obtained before or after the supernova was visible.

Published in ApJ. DOI: 10.3847/1538-4357/ae355e

The High-Latitude Time-Domain Survey (HLTDS) for the Nancy Grace Roman Space Telescope (Roman) will discover thousands of high redshift Type Ia supernovae (SNe Ia) to make generation-defining precision cosmological constraints on dark energy. To construct the Roman SN Hubble diagram, a strategy to obtain redshifts must be determined. While the nominal HLTDS will use only the Roman P127 prism spectral element, in this work we consider the utility of the Roman G150 grism for SNe Ia cosmology. We determine a general galaxy grism redshift recovery rate by simulating dispersed grism images and measuring redshifts with the Grizli software. We define a successfully measured redshift as having $\sigma_z = (|z - z_{\rm true}|)/(1+z)\leq 0.02$, signal-to-noise ratio $\geq 5$, and a single dominant peak in the $p(z)$, or a 6.5$\sigma$ detection of two or more emission lines, measuring a 50\% redshift recovery rate at magnitude \fifty\ and 90\% recovery at magnitude \ninety. To provide an approximate picture of the total number of spectroscopic redshifts that will be available for Roman SN cosmology, we also consider a Roman prism redshift efficiency for redshifts measured from SN spectra and a current ground-based telescope redshift efficiency for host-galaxies as a proxy for the Subaru Telescope Prime Focus Spectrograph. We apply these redshift efficiencies to SNIa catalog level simulations and predict that $\sim$6500 SNe will have a spectroscopic redshift from either the SN spectrum or host-galaxy. Second, we evaluate the size of potential systematics related to the modeling of the redshift efficiency, as either a function of host galaxy magnitude, magnitude and stellar mass, or magnitude and host galaxy color. We estimate the largest potential size of this systematic to be \womasssyst{} and \wamasssyst{} on $w_0$ and $w_a$ respectively. Lastly, we consider the effects of assuming different redshift sources on the HLTDS survey strategy optimization by measuring relative changes to the dark energy Figure of Merit (FoM).

Published in ApJ. DOI: 10.3847/1538-4357/ae42be

NASA’s Nancy Grace Roman Space Telescope (Roman) will provide an opportunity to study dark energy with unprecedented precision using several techniques, including measurements of Type Ia Supernovae (SNe Ia). The Roman High Latitude Time Domain Survey (HLTDS) will observe SNe Ia out to a higher redshift than ever before (z <= 3.0). We characterize this precision in terms of spatial, magnitude, and color dependences using repeatability of stellar fluxes when sky noise is subdominant, which must be below 1% to enable a number of calibration requirements. Achieving this level of flux precision requires attention to Roman's highly-structured, spatially-varying, undersampled PSF. In this work, we build a library of effective PSFs (ePSFs) compatible with the OpenUniverse HLTDS simulations. Using our library of ePSFs, we recover fractional flux between 0.6 - 1.2% photometric precision, finding that redder bands perform better by this metric. We also find that flux recovery is improved by up to 20% when a chip (sensor chip assembly; SCA) is divided into 8 sub-SCAs in order to account for the spatial variation of the PSF. We measure nonlinearity (magnitude dependence) at |s_{NL}| < 1.93 \times 10^{-3} per dex. Color dependence is inconclusive based on this analysis. Finally, we characterize the detection efficiency function of each OpenUniverse Roman filter, which will inform future studies.

Published in AJ. DOI: 10.3847/1538-3881/ae3714