A near field guide to Roman's wide-area surveys

Abstract

The Nancy Grace Roman Space Telescope currently plans to survey nearly 6000 square degrees of the sky, mainly in the High-Latitude Wide-Area Survey (HLWAS) and Galactic Plane Survey (GPS). Although these surveys are optimized for other science, they are also a treasure trove for studying the nearby universe. The foreground of the HLWAS includes 59 known stellar streams, 14 known satellite galaxies, and 9 globular clusters in the Milky Way, and an additional 63 galaxies within 10 Mpc spanning several orders of magnitude in stellar mass. The GPS includes an additional 38 globular clusters in its footprint. We summarize and visualize these populations and discuss some of the relevant characteristics of the planned Roman observations. We also examine the expected astrometric performance of the core surveys based on the anticipated time-baselines between observations, and point out the substantial improvement provided by longer time intervals between repeat observations. In particular, the plan for a 6-month revisit timescale in the HLWAS is a missed opportunity from the perspective of proper motions. These data will nonetheless be a powerful new resource for studying the Milky Way and its neighborhood.

Figures (14)

• Figure 1: The reach of Roman proper motions. Magnitudes have been converted to distances assuming K giants with absolute magnitude $G=2$, color $B_P-R_P = 1.5$ (top) and MSTO stars with absolute magnitude $G=4.5$, color $B_P-R_P = 1$ (bottom). As noted in the legend, dotted and dashed curves show typical proper motions for orbital and internal velocities; vertical arrows denote locations of satellites; points denote measured velocities in the MW. The pink shaded region is the reach of Gaia DR5. Yellow is the additional reach of combining the HLWAS with Gaia, assuming a final epoch with a 5 year baseline. Orange shows the additional gain possible for fields with HST coverage, which can date back as far as 2002. Solid green and blue lines estimate PM precisions for the GBTDS and HLTDS, respectively.
• Figure 2: Expected astrometric precision as a function of magnitude for different Roman surveys relevant to Near Field science. The precise configurations (e.g., exposure time, timing of epochs, pointing) used for these simulations are described in the text and McKinnon_2026. Here, we use the color information of an old ($12.7$ Gyr) main-sequence turnoff (MSTO) star when incorporating information from different Gaia and Roman filters. Gaia DR3-DR5 astrometric precisions are shown by the different blue lines, which extend to 21.5 mag in position and 20.7 mag in parallax and proper motion. Position uncertainties are given with respect to the Gaia reference epoch of J2016.0. The GBTDS and HLTDS are both expected to produce high quality parallaxes and PMs. The HLWAS, when observed with a 6 month time baseline using 2 epochs early in Roman's life (solid lines) do not yield scientifically useful PMs or parallaxes for faint stars ($G>21.5$ mag). Adding a late HLWAS epoch 5 years after the first (dashed lines) dramatically improves the astrometric precision by more than 3 order of magnitude for $G=21.5$ mag. When apriori distances are available such that parallaxes can be independently constrained, proper motion uncertainties for the 6 month baseline HLWAS can be improved (dotted lines), but this option will not be available for the majority of stars.
• Figure 3: Expected proper motion precision as a function of magnitude for an old MSTO star in the HLWAS Medium tier (i.e. dotted orange line in middle panel of Figure \ref{['fig:Roman_astrometry_errs']}) with a single additional epoch of HST (colored lines). As expected, an early HST epoch near 2002 leads to the largest time baseline with Roman, producing the most precise proper motions. We note that only a small-but-powerful fraction of the sky has access to these archival HST images.
• Figure 4: Roman core community survey footprints and streams (colored by distance) shown in Galactic Coordinates on a Gaia map of the Milky Way. Light-green shows the single-band (F158) wide-tier HLWAS survey. Darker green shows the 3-band (F106, F129, F158) medium-tier HLWAS. Stream data from galstreamsMateu2023.
• Figure 5: Roman core community survey footprints and streams, north (left) and south (right) galactic cap views. Colors are the same as Figure \ref{['fig:streams-allsky']}. Stream data from galstreamsMateu2023. Stream numbers correspond with Tables \ref{['tbl:streams_north']} (left) and \ref{['tbl:streams_south']} (right).