Overview

The Nancy Grace Roman Space Telescope’s Wide Field Instrument’s observing program will include both Core Community Surveys, defined by a community-led process, and General Astrophysics Surveys, for which a minimum of 25% of Roman’s observing time will be reserved in the first five years and which will primarily be selected via traditional peer-reviewed calls for proposals.

In 2021 the Roman Project released a Request for Information to the science community to solicit comments on (a) whether to select an Early-Definition General Astrophysics Survey, and (b) to outline and submit survey concepts that would demonstrably benefit from selection as an Early-Definition General Astrophysics Survey. Twenty complete submissions were received with over 340 unique authors. A review of these submissions has been 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, will be 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, leaving the full parameter space available to define 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.

Galactic Plane Survey Definition Committee Membership

A Galactic Plane General Astrophysics Survey Definition Committee charged with providing a executable survey to the Roman mission. The membership of that committee is:

• Bob Benjamin, U Wisconsin-Whitewater (Co-chair)
• Rachel Street, Las Cumbres Observatory (Co-chair)
• Rachael Beaton, STScI
• Sean Carey, Caltech/IPAC
• Kishalay De, MIT
• Janet Drew, University College London
• Matthew De Furio, UT Austin
• Thomas Kupfer, Universität Hamburg
• Dante Minniti, Universidad Andrés Bello
• Roberta Paladini, Caltech/IPAC
• Eddie Schlafly, STScI
• Catherine Zucker, CfA | Harvard & Smithsonian

Roman’s Galactic Plane Survey

A Roman General Astrophysics Survey of the Galactic Plane that features wide-field mapping, time-domain observations, and deep-field/spectroscopic measurements. These three elements yield a wide range of science including, but not limited to, Galactic structure, extinction, molecular clouds, stellar populations and characterization, insights into dynamical behavior via proper motions, stellar variability, and microlensing events.

The recommended program by the Galactic Plane Survey Definition Team consists of three elements:

• Wide-field mapping - 691 deg², 541 hours
  • Covering a large, continuous portion of the Galactic plane (Galactic latitude |b|<2 deg and Galactic longitude l = 50.1 - 281 deg) with higher latitude extensions for the bulge, Serpens South/W40 star formation region, and Carina
  • Four filters: F129, F159, F184, F213
• Time-domain - 19.1 deg², 130 hours
  • Six fields including the full Nuclear Stellar Disk (NSD) and Central Molecular Zone (CMZ)
  • Seven filters: F062, F087, F106, F129, F158, F184, F213
  • Repeat observations in one or more filters with cadences from ~11 minutes to weeks
• Deep-field/Spectroscopic - 4.8 deg², 30 hours
  • Fifteen pointings over a wide range of extinction, diffuse emission, stellar density, and populations
  • Seven filters: F062, F087, F106, F129, F158, F184, F213
  • Longer exposure times (with respect to time-domain element) and grism and prism observations

Wide Field Science

The majority of time allocated for the Roman Galactic Plane Survey (541 hours; 77% of the total time) is recommended for a 691 deg² wide-field imaging survey of the Galactic plane, |b| < 2 deg and Galactic longitude l = +50.1 deg to 281 deg, in four filters (F129 (“J”), F158 (“H”), F184 (“H/K”), and F213 (“K”)) with ~60-second exposures with a gap-filling dither. The recommended survey also has three vertical extensions:

1. higher latitude coverage in the lower extinction region of the Galactic bulge/bar (2 deg < |b| < 6 deg) in the three filters F129, F158, and F213, coverage of the nearby (440 pc) high-mass star formation region Serpens South/W40 with l = 26.5 deg to 30 deg and b = +2 deg to +4.5 deg in five filters (F106 F129, F158, F184, and F213), and
2. coverage of the Carina “tangency” direction (l = 293 deg to 281 deg) from b = –2.5 deg to +2 deg to account for the warping of the Galactic disk in this direction.

The single-exposure sensitivity limits will span mag_AB = 23–24, with saturation limits of mag_AB = 13–14. For regions with an extinction of A_K = 1.4 (A_V ~ 18), 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. 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 requests 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 observations 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 hours; 18.5% of total time), are recommended for time-domain investigations with all filters F062 through F213, excluding the wide F146 filter. 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 deg), NGC 6334/NGC6357 (l = 352.1 deg), and W43 (l = 30.6 deg). 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 4.8 deg² (31 hours; 4.4% of total time) are also recommended. These pointings 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 AK = 0.6 to 1.9 (AV = 7.6 to 24.3), expected source densities of 8 to 160 million sources per deg², and a wide range of expected diffuse emission. These observations would provide higher signal-to-noise data to validate the wide-field portion of the survey, detections of an additional ~40% of sources per field (where source crowding allows), full spectral energy distribution information, and 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 include prism observations to probe the substellar content of this nearby star forming region.

White Paper and Science Pitches

Two different categories were solicited by the project to inform the definition of the Galactic Plane Survey.

The science pitch option were intended to provide those with limited time, or a kernel of an idea, with a means to convey that information to the committee that will define the survey. This option aimed to capture, from a large cross-section of the community, the full breadth of science investigations possible with a Roman Galactic Plane Survey, as well as high level information on what aspects of survey design are most important for these investigations. Science pitches are intended to be short, one to two paragraph descriptions describing science investigations that could be enabled by the Galactic Plane Survey.

White papers describing in more detail science investigations that could be enabled by the Galactic Plane Survey were also requested. White papers provide the Roman Mission, the definition committee, and the astronomical community with a detailed understanding of the observational strategy space over which trades need to be considered and consensus built.