The Milky Way is the only large galaxy where individual stars can be resolved down to the central few parsecs. Existing large-scale photometric, spectroscopic, and astrometric surveys have fostered a rich understanding of the Galaxy as a spatially, chemically, and kinematically complex structure, with ample evidence of interactions, past mergers, and secular evolution and substructure in star formation. A recent abundance of large-scale ground-based spectroscopic surveys, and measurements of parallaxes and proper motions by the Gaia mission, have super-charged these investigations. A wide area Galactic survey with the Nancy Grace Roman Space Telescope could characterize most of the stellar content of our Galaxy and will provide unique information on both the history of our Galaxy's formation, and the on-going process of star formation in vastly different environments, as Roman is uniquely suited to deal with the confusion and extinction prevalent in the plane of the Galaxy.
A Nancy Grace Roman Space Telescope Galactic Plane survey was one of five programs specifically endorsed by the Science Definition Team (SDT) in the WFIRST Interim Report (Green et al 2012), and has recently been approved as a Community Defined program to be undertaken in the first few years of the operations. The Nancy Grace Roman Space Telescope significantly exceeds the capability of the existing efforts in three critical areas: (a) astrometric precision, (b) survey sensitivity, and (c) mapping speed. This opens new avenues in studies of stellar astrophysics, star formation and Galactic structure.
The goal of this workshop is to initiate a community discussion on the opportunities and trade-offs that will need to be considered in creating a Community Defined survey of the Galactic Plane. An ideal survey would cover a wide area, provide both a baseline for future time-dependent investigations, and establish the value of different filters, cadences, and strategies for future Galactic programs.
The high angular resolution of Roman will enable studies of previously unresolved stellar populations. That includes globular clusters in the Galactic plane and bulge, stellar clusters in star forming regions, and the entire nuclear region of the Galaxy. The Nobel prize-winning study of stellar motions near Sgr A*, the HST Galactic center survey and SWEEPS program, and the ESO VLT GALACTICNUCLEUS survey are all pertinent examples highlighting the relevance of such a dataset.
The sensitivity of Roman will provide the deepest infrared Galactic plane survey by at least two magnitudes. Red clump giants and young stellar objects can be surveyed out to a greater volume of the disk allowing the rewriting of Galactic structure, particularly the spiral arms and the central Galaxy where source confusion has blocked progress.
The greater depth will likewise enable studies of the stellar initial mass function down to lower mass limits in sites across the Galaxy, and provide significantly more sources for the construction of 3D dust maps. The combination of depth and angular resolution will also yield a novel catalog of galaxies and galaxy clusters beyond the Galactic disk. The mapping speed of Roman will allow for a significant fraction of the stars in the Galaxy to be covered in a uniform way, a crucial requirement for studies of Galactic structure.
Finally, a Roman single-pass survey of the Galactic Plane early in the mission would enable subsequent passes later on, largely surpassing what can be obtained by simply combining Roman and, e.g., 2MASS, with a 25-year baseline, therefore bolstering the characterization of stellar proper motions in regions inaccessible to Gaia, notably in the complex orbital structure of the Galactic bar(s) and nucleus. This will produce new insights on the inside-out evolution and central luminous/dark matter distributions of the Galaxy, and enable proper motion selection of stellar populations.
There will also be synergies with shorter wavelength monitoring of the Galactic plane by the Vera C. Rubin Observatory, with the spectral information obtained by SPHEREx and SDSS-V Milky Way Mapper, and with a proposed future Gaia-NIR mission