In 2021 a review of possible early definition surveys was completed by an Early-Definition Astrophysics Survey Assessment Committee. The committee found that there was sufficient justification to define an Early-Definition General Astrophysics Survey through a community-led process. 

The top-ranked concept was a survey of the Galactic Plane. Accordingly, a community-defined Roman Galactic Plane General Astrophysics Survey, of up to approximately 700 hours, has been defined by a committee of community members utilizing community input in an analogous method to the definition of Roman's Core Community Surveys by the Core Community Survey definition committees.

There are no mission level science requirements on a Roman Galactic Plane Survey (RGPS), leaving the full parameter space available for defining the observational strategies (filters, depth, cadence, etc.) in a way that will enable a broad range of astrophysical investigations with Roman data of the Galactic Plane.

The definition committee provided their recommendation to the Roman Observatory Time Allocation Committee (ROTAC) in this report.

The definition committee recommended a survey consisting of three elements: (1) a wide-field science element (691 deg², 541 hrs) covering the Galactic plane — Galactic latitude |b|<2° and Galactic longitude l=+50°.1 to 281° — in four filters (F129, F159, F184, and F213) with higher latitude extensions for the bulge, the Serpens South/W40 star formation region, and Carina; (2) a time-domain science element (19 deg², 130 hrs) of six fields — including the full Nuclear Stellar Disk (NSD) and Central Molecular Zone (CMZ) — with coverage in seven filters and repeat observations in one or more filters with cadences from ~11 minutes to weeks; and, (3) a deep-field/spectroscopic science element (4 deg², 30 hrs) consisting of fifteen Roman pointings — with a wide range of extinction, diffuse emission, stellar density and populations—using longer exposure times in seven filters in addition to grism and prism observations.

Wide-field science: The majority of time allocated for the RGPS (541 hr/77% of the total time) is recommended for a 691 deg² Wide Field Imaging (WFI) survey of the Galactic plane, |b|<2° and Galactic longitude l=+50°.1 to 281°, in four filters—F129 (“J”), F158 (“H”), F184 (“H/K”), and F213 (“K”)—with ~sixty-second exposures with a gap-filling dither (LINEGAP2). The recommended wide-field survey also has three vertical extensions: (a) higher latitude coverage in the lower extinction region of the Galactic bulge/bar (2°< |b|<6°) in the three filters F129, F158, and F213, (b) coverage of the nearby—d=440 pc—high-mass star formation region Serpens South/W40 with l=26°.5 to 30° and b=+2° to +4°.5 deg in five filters (F106 F129, F158, F184, and F213), and (c) coverage of the Carina “tangency” direction (l=293° to 281°) from b=–2°.5 to +2° to account for the warping of the Galactic disk in this direction. The single-exposure sensitivity limits range from mag_AB=23–24, with saturation limits of mag_AB =13-14. For regions with an extinction of A_K=1.4 mag (A_V~18 mag) — the mean level of extinction in the inner Galactic plane — a red clump giant will be detected from 3 to 190 kpc, a solar type star out to 20 kpc, and an M0 dwarf to the distance of Galactic center.

This component of the survey is expected to yield a catalog of approximately 20 billion sources and the highest angular resolution panoramic view of the Galaxy ever obtained. These data will provide unique opportunities for discovery in numerous sub-fields of Galactic astronomy.

The definition committee requested that the wide-field F129 and F213 filter coverage of the whole wide-field survey footprint be obtained as early as possible in the allotted two-year time frame, with the F106, F158, and F184 to be obtained as late as possible. This time spacing will allow for cross-band proper motion measurements with precision better than 0.5 mas/yr for the brightest sources, sufficient for population selections, cluster identification and characterization, and Galactic dynamics investigations. These data may also be combined with past optical/infrared programs, e.g., HST, Gaia, 2MASS, UKIDSS-GPS, VVV, etc, or future Roman Space Telescope programs (plus JASIMINE and Gaia NIR) to allow for long-term proper motion measurements and variability of stellar and diffuse sources.

Time-Domain Science: Six subregions of the survey, totalling 19.1 deg² (130 hr/18.5% of total time), are recommended for all-filter coverage — F062 through F213, excluding the wide F146 filter — and time-domain investigations. These include two 2.1 deg² regions (six Roman pointings each) on either side of the Galactic Bulge Time-Domain Survey (GBTDS) Galactic Center field, spanning the full Nuclear Stellar Disk and Central Molecular Zone. These fields would be observed in F213 for eight hours at high cadence (11.3 min, 43 visits), an hourly cadence with F213 with increasing intervals (eight visits), and weekly subsequent monitoring (eight visits) in both F129 and F213. The high cadence strategy will be repeated for three regions of similar area containing — but extending beyond — three well-known star formation complexes: Carina (l=287°.5) NGC 6334/NGC 6357 (l=352°.1), and W43 (l=30°.6). All but one of these regions will also be monitored by the Rubin Observatory at shorter wavelengths and longer cadences; the northernmost W43 field, which would be monitored in F184 rather than F213, will be covered as part of the Subaru Galactic Plane Survey. Hourly monitoring of the full Serpens South/W40 area with increasing cadence is also recommended. Taken together, these observations are expected to uncover populations of compact binaries, YSO variability, and other classes of variable sources in very different environments. The second year F129 and F213 observations may be combined with early wide-field coverage to constrain longer-term source variability.

Deep-field/Spectroscopic science: Fifteen single Roman pointings, totalling 4.2 deg² (31 hrs/4.4% of total time) are also recommended. These include fourteen deep—four times nominal exposure time—observations in all filters (except F146) as well as two 300-second observations at different roll angles using both the grism and prism. These fields span the full Galactic longitude range of the survey, with extinctions ranging from A_K=0.6 to 1.9 mag (A_V=7.6 to 24.3 mag), expected source densities of 8 to 160 million sources/deg², and a wide range of expected diffuse emission. The data from these observations would provide higher S/N data to validate the wide-field data, would detect an additional ~40% of sources per field (where source crowding allows), would provide full spectral energy distribution information, and would provide data to test the use of the grism and prism in a range of Galactic environments. A fifteenth pointing towards W40 would go even deeper in F129, F158, and F213, and prism observations to probe the substellar content of this nearby star forming region.