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Roman Galactic Plane Survey Definition Committee

Updated 13 November 2025

Roman Galactic Plane Survey Definition Committee defines the survey, balancing deep NIR imaging, time-domain monitoring, and slitless spectroscopy for the inner Milky Way.
It integrates multi-wavelength synergies (Gaia, Rubin/LSST, etc.) and a community-driven process to optimize survey design and science deliverables.
The committee establishes technical standards and rigorous observing strategies to achieve high-precision astrometry and extensive stellar mapping.

The Roman Galactic Plane Survey Definition Committee (RGPS-DC) is the formal body charged with designing, through a transparent and community-driven process, the Roman Galactic Plane Survey (RGPS)—a 700-hour General Astrophysics Survey using the Nancy Grace Roman Space Telescope. The RGPS-DC coordinates scientific, technical, and collaborative aspects for early implementation, balancing deep near-infrared (NIR) imaging, time-domain monitoring, and slitless spectroscopy of the inner Milky Way, and integrating synergies with Gaia, Rubin/LSST, and other multi-wavelength surveys. Its policies, composition, recommendations, and science deliverables are outlined below, reflecting consensus and design iterations from 2024–2025 (Committee, 10 Nov 2025, Sanderson et al., 22 Apr 2024).

1. Committee Charter, Authority, and Membership

The RGPS-DC was appointed by the NASA Roman Senior Project Scientist (RSPS) following recommendations from the Early Definition Astrophysics Survey Committee and in response to extensive community input (Committee, 10 Nov 2025, Sanderson et al., 22 Apr 2024). Its core mandate is to define the RGPS scientifically and operationally for early execution as a General Astrophysics Survey.

Membership was selected to encompass all major domains required for Galactic-plane science:

Galactic structure/dynamics specialists (bulge, bar, disk, nucleus)
Stellar populations and star formation experts
Time-domain and astrometry scientists
Instrumentation/operations (e.g., WFI performance, crowded-field photometry)
Synergy liaisons (Gaia, Rubin/LSST, Euclid, radio/X-ray facilities)

The recommended size is ~10–15 core members, including two co-chairs (Galactic structure, time-domain), and assigned leads for filters/cadence, proper motions, and external collaborations. The governance model features biweekly telecons, two in-person workshops, and focused sub-teams (science drivers, technical feasibility, pipelines, synergies/outreach) reporting into the committee, with a nine-month window for definition and consensus-building.

2. Science Objectives and Rationale

The RGPS-DC defined top-tier science goals directly aligned with broad stakeholder input (Committee, 10 Nov 2025, Sanderson et al., 22 Apr 2024, Paladini et al., 2023):

3D mapping of the inner Galaxy: Proper motions in dusty regions (σ_μ ≲ 0.5 mas/yr for bright sources), complementing Gaia, enabling kinematic characterization of bulge, bar, and disk.
Stellar population census: Multiband photometry (~20 billion sources) within varying extinction/crowding regimes, tracing the stellar initial mass function down to brown dwarfs and fragmentation limits.
Structure of Galactic bar and spiral arms: Variable-star distances (Cepheids, RR Lyrae) out to 8–10 kpc, and trigonometric parallaxes.
High-resolution dust mapping: SED fitting in multiple NIR filters to resolve extinction curves and 3D dust distribution (ΔA_K ≲ 0.05 mag).
Time-domain phenomenology: Monitoring young clusters, compact binaries, microlensing events, and eruptive transients with cadences from ~11 min to weeks.
Synergies: Providing overlap fields for calibration and contextual studies with LSST/Rubin, Euclid, radio and X-ray facilities, and ground-based NIR surveys.

3. Survey Program Design: Elements, Metrics, and Techniques

The RGPS, as recommended by RGPS-DC, comprises three major interlocked elements (Committee, 10 Nov 2025):

3.1 Wide-Field Science Element

Coverage: 691 deg² contiguous strip, Galactic latitude |b|<2°, longitude ℓ=+50.1° to –79°, with key extensions for bulge/bar (80 deg²), Serpens South/W40 (8.75 deg²), and Carina warp (54 deg²).
Filters: F129, F158, F184, F213 (JHK-equivalent, plus broad H/K); bulge extension in three (F129, F158, F213); Serpens in five (adds F106).
Depth: Single exposure M_AB ≃ 24 mag (F129/F158/F184), 23 mag (F213), saturation ~14 mag; deep pilot fields at +0.75 mag (4× standard exposure).
Proper Motions: Split epochs (F129/F213 Year 1, F158/F184 Year 2), yielding σ_μ ≲ 0.5 mas/yr for bright sources with 2-year baseline.
Catalogs: ~20 billion sources, multiband photometry, PMs, extinction maps, cluster and YSO selections.

3.2 Time-Domain Science Element

Fields: Six, totaling 19.1 deg² (2.06 deg² each, Serpens South 8.75 deg²), including the full Nuclear Stellar Disk (NSD), Central Molecular Zone (CMZ), Carina, NGC 6334/6357, W43.
Cadence: High-cadence (11.3 min, F213/F184), hourly monitoring, weekly visits. 43 visits per 8 hr (high-cadence), eight hourly, eight weekly.
Science: YSO variability, compact binaries, microlensing (free-floating planets, black holes), eruptive transients, pulsating variables; expected recovery >60% for P_orb < 20 min, detection of ellipsoidal binaries to P ~ 3 h.

3.3 Deep-Field/Spectroscopic Science Element

Fields: Fifteen pointings (4.22 deg²), spanning A_K = 0.6–1.9 mag, stellar densities 8–160 million/deg², diffuse emission regimes.
Imaging: Four times nominal exposure (BOXGAP8, 977/999 s per filter), seven broad filters (F062–F213) and very deep imaging for IMF studies in W40.
Spectroscopy: Grism (R ~ 480) and prism (R ~ 80–180) at two roll angles, 300 s exposures; tests slitless performance in high-density, high-extinction fields.
Deliverables: Deep catalogs, slitless spectra for 10³–10⁴ sources/field, precise extinction mapping.

Table: RGPS-DC Survey Elements Overview

Element	Area (deg²)	Filters	Cadence / Depth
Wide-field	691	F129,F158,F184,F213	Typical M_AB 23–24, PM σ_μ≲0.5 mas/yr
Time-domain	19.1	Up to 7 (field dependent)	11.3 min to weekly
Deep/spec	4.22	F062–F213, Grism/Prism	4× exposure; slitless spectra

4. Technical Implementation: Observing, Calibration, and Data Products

The RGPS-DC established technical strategies maximizing area/depth trade-offs and leveraging Roman's Wide Field Instrument (WFI) parameters (Committee, 10 Nov 2025, Paladini et al., 2023):

Dithering: LINEGAP2_5 (wide-field mapping), BOXGAP8 (deep imaging), randomized sub-pixel dithers for crowded fields.
Readout: Typical imaging uses IM_60_6_S (60 s, 6 resultants); deep imaging MA_1000_16; spectroscopy SP_300_16 (289 s, 16 resultants).
Exposure Time & Mapping Rate: Wide-field imaging: ~4.9 deg²/hr/filter; deep imaging +0.75 mag; time-domain fields mapped for cadence (total 130 hr).
Sensitivity/Saturation/Confusion: F129–F184 m_AB ≈ 23–24 (60 s), F213 m_AB ≈ 23. Deep imaging/spectroscopy fields probe substellar IMF to ≈1 M_J (photometry) and ≈3 M_J (spectroscopy).
Proper Motion/PM Precision: Two epochs, σ_μ ≈ 1 mas/yr at m_AB=23, floor at bright end 0.4 mas/yr.
Extinction Modeling: Composite 2D AK maps to AK ≲ 2.5 mag; transmission via exp(–0.4 A_band).
Photometric/Astrometric Products: Crowded field PSF modeling, forced photometry, catalogs with completeness/crowding flags, 3D reddening/extinction maps.
Data Release: Calibrated images, catalogs, time-series light curves, spectra, delivered via STScI archives, with a dedicated portal for pipeline feedback and community engagement.

5. Synergies, External Collaborations, and Survey Expansion

The RGPS-DC actively coordinated overlap and maximized scientific return through complementarity with ongoing surveys (Committee, 10 Nov 2025, Sanderson et al., 22 Apr 2024, Kruszyńska et al., 20 Jun 2024):

Gaia: Roman NIR astrometry penetrates confusion-limited regions where Gaia saturates at G ≳17; proper motions cross-calibrated in overlapping fields.
Rubin/LSST: LSST maps the Plane in six bands over 10 yr; Roman NIR imaging anchors LSST deblending, extends photometry into regions of extinction.
Euclid: Roman's deeper, high-resolution imaging bridges to Euclid's bulk-cosmology fields.
Ground NIR Surveys: Roman is 5–7 mag deeper at 0.1″, ground surveys extend context and long baselines.
Radio/X-ray: Legacy overlap (e.g., Chandra Galactic Bulge), identification of compact-object binaries.
Spectroscopy (Subaru, VLT): Scheduling of large-multiplex NIR spectroscopy in fields of interest, synchronizing with Roman definition.
Data Fusion and Cross-Calibration: Global astrometric alignment, photometric color–color relations, joint light-curve SED modeling for microlensing, variable stars, extinction, and lens mass estimation.

Expansion paths include further epochs for improved variability and kinematics, spatial footprints beyond baseline (warp, outer disk), deeper imaging in targeted zones, and additional filter programs via guest investigator calls.

6. Survey Definition Process and Community Engagement

The RGPS-DC implemented a nine-month, milestone-driven definition pipeline (Committee, 10 Nov 2025, Sanderson et al., 22 Apr 2024):

Community solicitation: RfI and white paper call yielded 340+ authors across 20 whitepapers, 32 science pitches.
Iterative workshops: Kickoff, public comment on draft parameters (sky map, filters, cadence, time budget), technical breakout sessions.
Process milestones: Early draft (Month 3), straw-man survey design (Month 7), community open review (4 weeks), final report (Month 9) with error budgets, data rights, recommended programs.
Technical Q&A: Portal for ancillary requests, mailing list for feedback.
Early Science Verification: Dense field validation for dithering/PSF/crowding.
Data products and public access: Progressive releases hosted by STScI, integration into MAST/VO-compliant systems, value-added catalogs, collaborative development of Level 1–3 pipelines.

7. Legacy, Impact, and Future Directions

The RGPS, as implemented by RGPS-DC, will deliver a lasting legacy dataset: ~20 billion sources with deep NIR photometry, multi-epoch proper motions, extinction mapping, 3D structure, and time-series data for transients and binaries across the dusty inner Milky Way (Committee, 10 Nov 2025). Slitless spectroscopy results will test techniques and enable faint-source stellar population studies. The program forms a foundational resource for ongoing and future Galactic astronomy, supporting identification and follow-up of gravitational-wave EM counterparts, exoplanet demographics, dust physics, cluster formation, and variable-source astrophysics.

A plausible implication of the RGPS-DC model is the establishment of a blueprint for large-scale, community-defined surveys in complex astrophysical environments, with rigorous engagement, detailed science-driver mapping, and cross-survey technical coordination leading to optimized, consensus-driven observing designs with broad legacy value.