Roman Galactic Plane Survey (RGPS)

• The Roman Galactic Plane Survey is a multi-component, high-resolution NIR survey designed to map Galactic structure and stellar populations.
• It utilizes wide-field imaging, dynamic time-domain sampling, and deep spectroscopy to capture detailed views of crowded and extinguished regions.
• The survey generates legacy datasets that enable studies of star formation, proper motions, and synergies with facilities like LSST and SPHEREx.

The Roman Galactic Plane Survey (RGPS) is a community-designed, multi-component survey program executed with the Nancy Grace Roman Space Telescope, optimized to deliver high-resolution, wide-field near-infrared (NIR) imaging and slitless spectroscopy of the Galactic plane, bulge, and selected star-forming regions. Designed to address core science drivers in Galactic structure, stellar populations, time-domain astrophysics, and star formation, RGPS leverages Roman’s 2.4 m aperture and Wide Field Instrument (WFI) to obtain legacy-quality datasets that will underpin Milky Way research for decades.

1. Survey Architecture and Observing Strategy

The RGPS is structured as a ~700-hour General Astrophysics Survey selected for early definition through a community-driven process (Committee, 10 Nov 2025). It consists of three major program elements:

1.1 Wide-Field Imaging

• Area: 691.2 deg² predominantly within |b|<2°, with extensions covering the Carina warp, bulge/bar caps, and Serpens South.
• Filters: F129 (1.29 μm), F158 (1.58 μm), F184 (1.84 μm), F213 (2.13 μm); F106 (1.06 μm) for Serpens South.
• Exposure: Two 60 s resultants per filter per pointing, using LINEGAP2_5 dither for uniform spatial coverage.
• Phased visits: F129+F213 in Year 1; F158+F184 in Year 2, providing ≳2 yr baseline for cross-band proper motions.
• Scheduling: Wide-field pass 1 (F129+F213) is planned for February 2026 for l=–80° to +50°, requiring ~12 days; pass 2 during bulge season in 2028.

1.2 Time-Domain Science Fields

• Area: 19.1 deg² in six fields, including the full Nuclear Stellar Disk (NSD), Central Molecular Zone (CMZ), and major star-forming complexes.
• Filters: F062 (0.62 μm), F087, F106, F129, F158, F184, F213.
• Cadence: High (Δt≈11 min, 43 visits/8hr in one filter), medium (Δt=4–16 hr), and slow (Δt≈1 wk, 8 visits) cadences; annual single-epoch color visits.
• Specialization: This element enables investigation of rapid variables (e.g., compact binaries), multi-temporal sampling of YSO variability, and microlensing of free-floating planets with events down to ~1 d duration.

1.3 Deep-Field and Spectroscopic Element

• Area: 4.22 deg² across 15 fields, chosen to span a range of extinction (AK=0.6–1.9 mag), stellar densities (10⁷–10⁸ deg⁻²), and emission regions.
• Filters: Seven filters (F062–F213, excluding wide F146) with grism (R~481; 1.00–1.94 μm) and prism (R~80–180; 0.75–1.80 μm) spectroscopy, at two roll angles and 300 s exposures; BOXGAP8 dithers for imaging.
• Depth: Imaging exposures ×4 compared to wide field; 0.75 mag deeper.
• Special pointings: W40 for deep substellar IMF studies using F129, F158, F213 (1,000 s exposures), and prism spectroscopy.

Survey Element	Area (deg²)	Time (hr)	Filters	Dither & Cadence
Wide-Field Imaging	691.2	540.7	F129,F158,F184,F213 [+F106:Serpens]	LINEGAP2_5, single visit
Time-Domain Fields	19.1	129.5	F062–F213	LINEGAP2_5, multi-cadence
Deep+Spectroscopy	4.22	30.8	F062–F213 + grism/prism	BOXGAP8, single pointing

2. Instrumentation, Sensitivity, and Resolution

Roman's WFI instrument delivers exquisite spatial resolution and sensitivity, crucial for resolving highly crowded and extincted fields in the Galactic plane (Paladini et al., 2023, Committee, 10 Nov 2025):

• Pixel scale: ∼0.11″ pix⁻¹
• PSF FWHMs:
  • F129: 0.106″
  • F158: 0.128″
  • F184: 0.146″
  • F213: 0.169″
• 5σ Limiting Magnitudes (per 60 s exposure, AB system): AB ≈ 24 in JHK bands; deeper (≈24.7–25.5) in wide fields with repeated exposures (Paladini et al., 2023).
• Crowding limits: Completeness ≳90% (m≲24) for AK≲0.5 mag; ≤50% (m≳22) for AK≳2 mag due to source confusion.
• Astrometric precision: ≤0.5 mas yr⁻¹ proper motions to J≲22 mag (over 2 yr baseline in wide field); ≤0.2 mas in sparsely populated deep fields.

Key parameter relationships include:

• $$m_{\text{AB}} = -2.5\log_{10}(F_\nu/3631\,\text{Jy})$$
• Limiting magnitude as a function of exposure:

$$m_{\rm lim} \approx ZP - 2.5\log_{10}\!\Bigl(\frac{S/N}{\sqrt{t_{\rm exp}}}\Bigr)$$

Completeness in high-density regions is modeled as:

$$C(m,A_K) \approx \exp\left[-\frac{(m-m_0(A_K))^2}{2\sigma^2(A_K)}\right]$$

3. Science Goals and Methodological Trade-Offs

Core RGPS science objectives span Galactic mapping, stellar populations, star formation, kinematics, and time-domain astrophysics (Committee, 10 Nov 2025, Paladini et al., 2023):

• 3D structure: Resolve bulge, bar, spiral arms, and Galactic warp/flare down to infrared extinction AK ≲ 5 mag.
• Stellar populations: Map the IMF from clusters to the field, conduct a census of embedded YSOs (>10⁶ detected), and discover clusters to augment the current census by ×10.
• Proper motions and astrometry: Enable studies in regions inaccessible to Gaia (σ_μ ≲ 0.5 mas yr⁻¹; J ≲ 22 over 2 yrs).
• Variable stars and transients: Identify RR Lyrae (∼5×10⁴), Classical Cepheids (∼10³), eclipsing binaries (∼10⁴), and CVs, and monitor rapid outbursts.
• Microlensing: Free-floating planet, black hole, and stellar remnant microlensing: ∼100 events yr⁻¹ in RGPS fields with 90% recovery for t_E ≳1 d (Kruszyńska et al., 20 Jun 2024, Terry et al., 2023).

The survey is explicitly designed to allow trade-offs and optimization according to science priorities:

• Depth vs. area: Deeper exposures in F213 probe high-extinction regions but limit areal coverage.
• Filter allocation: Inclusion of F087 (for metallicities) or F184 (for dust penetration) increases program time.
• Cadence vs. program overhead: Higher temporal sampling for proper motions and rapid variability imparts significant overhead.
• Spectroscopy feasibility: Deep fields test whether slitless spectra (R~100–500) can be reliably extracted in crowded conditions.

4. Synergies with Other Surveys

The RGPS is coordinated for maximal synergy with major time-domain and spectroscopic surveys (Kruszyńska et al., 20 Jun 2024, Committee, 10 Nov 2025):

• Rubin LSST: Full RGPS footprint overlaps with LSST's Galactic Plane coverage for optical-NIR SED and variability studies. Near-contemporaneous (≤24 hr) pointings enable robust separation of extinction vs. temperature-driven variability and direct cross-calibration.
• SPHEREx: 0.75–5 μm low-res spectra supplement Roman photometry for SED fitting and extinction law measurement.
• SDSS-V MW Mapper, GaiaNIR (future), NEOCam: RGPS will bridge wavelengths, spatial resolutions, and extinction regimes inaccessible to any one mission.
• LSST-RGPS joint astrometry/photometry: For stars with σLSST ≈ 0.01 mag and σ_Roman ≈ 0.005 mag, the combined epoch photometric precision is ~0.011 mag; proper motions of σμ ≈ 0.02 mas yr⁻¹ and parallaxes σ_π ≈ 0.05 mas are achievable for S/N≫100 sources over ΔT_total ≈ 10 yr.

5. Special Focus: Galactic Center Field

A dedicated WFI field at Sgr A* (0.281 deg²) samples ≈3.3×10⁶ stars to F146 ≲24 mag with mean density ≈3,300 stars arcmin⁻² (Terry et al., 2023). High-cadence (Δt≈15 min; “optimal”) or minimal (≳12 hr) visit strategies deliver:

• Astrometry: Single-image centroid error σ_ast ≈1–1.6 mas; stacked epoch precision down to 0.04–0.06 mas for optimal cadence.
• Proper motions: Final precision of ≲25 μas yr⁻¹ (minimal) and ≲3 μas yr⁻¹ (optimal).
• Science yields (5 yrs, optimal): ∼25,000 microlensing events, ∼25 FFPs, ∼33 bound Earth-mass planets, ∼35–75 black hole/neutron star lenses, 8,500–28,000 transiting exoplanets, ∼350 star–compact object binaries, and time-resolved flares from Sgr A* and YSOs.
• Galactic dynamics: Mapping of NSC and NSD velocity/rotation fields, mass modeling, and measurement of tidal features in dense clusters.

A minimal cadence recovers most high-mass events but sacrifices sensitivity to short-duration phenomena, low-mass microlenses, and rapid transits.

6. Data Products, Legacy, and Operational Considerations

RGPS is expected to yield:

• Multi-band mosaics (AB≈24 mag in JHK, 5σ), source catalogs with ≳20 billion entries, and time series for millions of variables.
• Astrometric catalogs: Proper motions and parallaxes bridging the Gaia extinction gap.
• Slitless spectra: R∼100–500 for >10⁷ sources in deep fields.
• Operational trade-offs: ~1.2% zero-coverage area (dithering), calibration fields for PSF/stability, 27.9% single-pass regions.
• Scheduling leverages WFI flexibility and bulge season optimization.
• Synergy and data combination requirements dictate survey footprint, filter selection, and epoch timing.

All design elements are grounded in community consensus after a structured process involving proposal calls, workshops, and multi-phase review (Committee, 10 Nov 2025). The RGPS definition preserves adaptability for expanding area, depth, filter complement, and multi-epoch coverage, ensuring both immediate scientific return and a durable foundation for future Galactic investigations.