Simulating Roman+Gaia Combined Astrometry, Parallaxes, and Proper Motions

TL;DR

The study tackles Gaia’s faint-magnitude astrometry limits by introducing a publicly available tool to simulate Gaia+Roman end-to-end astrometry, leveraging Gaia priors and Roman PSF-based position uncertainties to predict uncertainties in position, parallax, and proper motion. The approach centers on a Bayesian framework with the posterior covariance $\pmb\Sigma_{v,i}$ for each star, combined with realistic Roman uncertainties modeled through $\sigma_{\mathrm{pos}} = k/\mathrm{SNR}_{\mathrm{flux}}$ and related expressions. Applying the tool to the core Roman surveys (GPS, HLTDS, GBTDS, HLWAS) shows substantial PM/parallax gains to fainter magnitudes (e.g., $G>21.5$), with results strongly dependent on observing cadence and baselines, especially for HLWAS. The work provides a practical planning resource for Roman proposals and paves the way for integrating future datasets from other facilities to maximize end-to-end astrometric science in the Local Group and beyond.

Abstract

The next generation of high-precision astrometry is rapidly approaching thanks to ongoing and upcoming missions like Euclid, LSST, and RST. We present a new tool (available at https://github.com/KevinMcK95/gaia_roman_astrometry) to simulate the astrometric precision that will be achieved when combining Gaia data with Roman images. We construct realistic Roman position uncertainties as a function of filter, magnitude, and exposure time, which are combined with Gaia precisions and user-defined Roman observing strategies to predict the expected uncertainty in position, parallax, and proper motion (PM). We also simulate the core Roman surveys to assess their end-of-mission astrometric capabilities, finding that the High Latitude and Galactic Bulge Time Domain Surveys will deliver Gaia-DR3-quality PMs down to G=26.5 mag and G=29.0 mag, respectively. Due to its modest number of repeat observations, we find that the astrometry of the High Latitude Wide Area Survey (HLWAS) is very sensitive to particular choices in observing strategies. We compare possible HLWAS strategies to highlight the impact of parallax effects and conclude that a multi-year Roman-only baseline is required for useful PM uncertainties (<100 mas/yr). This simulation tool is actively being used for ongoing Roman proposal writing to ensure astrometric requirements for science goals will be met. Subsequent work will expand this tool to include simulated observations from other telescopes to plan for a future where all surveys and datasets are harnessed together.

Figures (12)

• Figure 1: Examples of parallax offsets swept out over the course of a year for different RA, Dec coordinates (colored lines). The large points on each ellipse show the parallax offset at the start of the Julian year (e.g. J2027.0).
• Figure 2: Position uncertainty (per coordinate) as a function of magnitude for different Roman filters for a $\sim193$ second exposure with a "medium" background level in the HLWAS Medium Field 1. A guiding thick blue line (with arbitrary y values) breaks down different expected trends for different source brightness regimes. Before using the curves in this figure, we add (in quadrature), a $1\%~\mathrm{pixelwidth}\approx 1.1$ mas noise floor to all of the position uncertainties (black dotted line), as motivated in the text. We have computed these curves for all Roman filters, all exposure times, and all available pandeia background choices at the "medium" background level.
• Figure 3: Example of astrometric precision after combining Gaia with Roman observations given the strategy described in the text. The solid lines show the results for Gaia DR4 while the dashed lines show the results for Gaia DR5. The Gaia positions extend to $G=21.5$ mag, but the Gaia parallaxes and PMs only go as deep as $G=20.7$ mag, which explains why there are some discontinuities at these magnitudes. In all cases, we find that the Gaia+Roman results are at least as precise as the corresponding Gaia data release alone. As we push to fainter magnitudes ($G>18$ mag), the combined Gaia+Roman astrometry is a significant improvement over Gaia-alone. As expected, the difference between the Roman+Gaia DR4 versus DR5 is negligible for $G>21.5$ mag, because the same Roman information is providing all the constraints to these too-faint-for-Gaia sources.
• Figure 4: Position uncertainty at different epochs for the Roman+Gaia DR4 data shown in Figure \ref{['fig:example_gaia+roman_astrometry']} as a function of magnitude (top) and time (bottom). The blue and orange lines in the top panel show the results at J2016.0, while the colored lines show the results at J2028.0, J2031.0, and J2031.5. As expected, the position uncertainty for $G>21.5$ mag stars is much smaller at the Roman epochs than at J2016.0. The bottom panel shows the position uncertainty as a function of time for different magnitude sources, with vertical dashed lines showing the epochs of the different position measurements used in the simulation (either Gaia at J2016 or Roman for later times). The oscillating wiggles in the bottom panel within each year come from parallax uncertainty propagating to position uncertainty, while the general V-shape is due to PM uncertainties affecting past and future position extrapolations. The large leap in position uncertainty in the bottom panel is the transition to $G>21.5$ mag, where sources are too faint to have Gaia-measured positions at J2016.0.
• Figure 5: Example of astrometric precision after combining the Roman+Gaia DR4 results in Figure \ref{['fig:example_gaia+roman_astrometry']} with additional HST observations at different epochs. The colored lines show the result of adding a single epoch of HST data with 4 dithers, assuming all magnitudes reach a per-exposure position uncertainty of $1\%$ of an HSTACS/WFC pixel (0.5 mas). Adding a single additional HST epoch does not significantly improve the parallax precision, but it does lead to large improvements for the position and PM uncertainties.