About this ArticleK. Gilbert (kgilbert[at]stsci.edu)
The Nancy Grace Roman Space Telescope is planned for launch in late 2026. It will be a uniquely powerful survey facility, thanks to the large field of view (0.28 sq deg) and Hubble-like sensitivity and resolution of the Wide Field Instrument (WFI), as well as Roman’s highly efficient survey operations, which will yield survey speeds roughly 1000 times faster than achieved with Hubble. The Roman Space Telescope's WFI observing program will include both Core Community Surveys (CCSs) and General Astrophysics Surveys, defined by a community-led process and traditional peer-reviewed calls for proposals, respectively. In contrast to previous NASA flagship observatories, the majority of time in Roman’s first five years will be dedicated to the community surveys, which will include a High Latitude Wide Area Survey, a High Latitude Time Domain Survey, and a Galactic Bulge Time Domain Survey.
Roman’s Core Community Surveys will be defined by committees composed of community members who collectively reflect the range of astrophysics the community wants to see enabled by the Core Community Surveys. These CCS definition committees will be formed in mid-2023 and will be charged with working with the broader astronomical community to define Core Community Surveys that maximize the science enabled while meeting the Roman Mission’s science requirements in cosmology and exoplanet demographics. In order to meet those requirements, the two High Latitude surveys will collectively constrain the nature of dark energy using weak lensing, baryon acoustic oscillations, and type Ia supernovae, while the Galactic Bulge Time Domain Survey will constrain exoplanet demographics through the discovery and characterization of microlensing events (e.g., Spergel et al. 2015).
As the first step in the process of defining Roman’s Core Community Surveys, the Roman Mission has requested community input via two avenues. The first avenue was the submission of Roman CCS science pitches, which were intended to capture the full breadth of science investigations possible with Roman’s Core Community Surveys. The Roman CCS science pitch call requested one to two paragraphs describing science investigations that could be enabled by one of the Roman Core Community Surveys given an appropriate observational strategy. An associated questionnaire collected high-level input on the importance of various observational strategy choices (e.g. survey area, location, depth, and cadence) for the success of the science investigation described in the pitch. This article summarizes the more than 100 submissions received.
The second avenue is the submission of more technically-focused Roman CCS white papers, due by June 16, 2023. The white papers are intended to compile detailed descriptions of Core Community Survey science drivers, quantify the observational strategies that will enable and optimize different science investigations, and provide metrics or figures of merit that can be used by the CCS definition committees to assess whether an observational strategy will enable a particular investigation. All Roman CCS white papers and science pitch submissions will be provided to the CCS definition committees and used to explore the impact of various trades in observational strategy on a Core Community Survey’s overall science return and to identify areas where additional discussion with the community is needed.
To discover additional ways in which the community can engage with Roman and learn more about the community definition of Roman’s Core Community Surveys, see this previous Roman newsletter article from January 2023.
Format of science pitch submissions
The Roman CCS science pitches were intended to provide a low-barrier means for collecting potential Core Community Survey science investigations from as broad a cross-section of the astronomical community as possible. Accordingly, the science pitch submission form consisted of a small number of required elements, along with additional optional elements.
In addition to providing a title and one-to-two paragraph description of the science investigation, submitting authors were required to identify the relevant Roman Core Community Survey and choose a scientific category (or categories) that best fit their science case. They were also required to rank 14 observational strategy elements (such as final coadded depth, depth of each epoch, total survey area, and specific filter choices) as “very important,” “somewhat important,” or “not important.” Authors had the option of including a list of coauthors, providing additional scientific keywords, and providing details on the observational strategies needed for their science case via both guided choice and open response questions. Some additional detail on observational strategy was provided for 60% of submissions.
Respondents were also asked to provide information about their career stage, the type of institution they are affiliated with, and prior involvement in the Roman Mission. Despite these questions being optional, they received a nearly 100% response rate: all submitting authors provided some information, with the minimum response rate being 98% for the question about prior involvement in Roman.
Overview of submissions received
A total of 113 submitted Roman CCS science pitches described science that could be enabled by the Roman Core Community Surveys, ranging from solar system investigations to the large-scale structure of the universe. The scientific breadth of the submissions is demonstrated in Figure 2 and discussed in more detail below. The High Latitude Wide Area Survey was the focus of the greatest number of science pitches (52, or 46%). The Galactic Bulge Time Domain Survey was the focus of 41 science pitches (36%), and the High Latitude Time Domain Survey was the focus of 20 pitches (18%). Slightly more than half (52%) of the pitches were submitted by authors with no prior involvement in the Roman Mission, and nearly a third (29%) of the pitches were submitted by graduate students or postdoctoral researchers.
A diverse suite of science investigations enabled by each Core Community Survey
The science pitches related to each of the three Core Community Surveys span an exciting and diverse array of topics, highlighting the vast scientific potential of each of the surveys. A representative listing of the breadth of the science investigations is provided here for each Core Community Survey. A full listing of the science pitches received is available on the Roman Mission’s Goddard Space Flight Center website.
The Galactic Bulge Time Domain Survey
Science investigations that could be enabled by the Galactic Bulge Time Domain Survey range from solar system science to extragalactic astronomy. The majority of pitches described gains that could be made in our understanding of stellar astrophysics. Pitches highlighted the potential for increasing our understanding of a wide variety of variable stars and time-varying stellar phenomena, including eclipsing binaries, X-ray binaries, cataclysmic variables, rare hot pulsating stars, and stellar flares. Pitches also highlighted the potential of using astroseismology to measure stellar masses, radii, and granulation in smaller stars (e.g., with radii less than twice that of the Sun) and for stars with transiting exoplanets.
A significant fraction of pitches focused on the gains that can be made in our knowledge of exoplanetary systems and free-floating planets (through microlensing, transits, transit timing variations, and measuring exoplanetary phase curves) as well as compact objects (e.g., white dwarfs, neutron stars, and stellar mass black holes), and discussed the possibility of finding the first exomoon. A few pitches discussed the extragalactic science potential of the survey, including the possibility of identifying quasars and supernovae (SNe) behind the Galactic Bulge.
In the regime of solar system science investigations, pitches discussed the potential for measuring the light curves of small solar system objects (including near-earth objects, main-belt asteroids, and trans-Neptunian objects) in order to characterize their rotation periods, binarity rate, shape distribution, reflectivity, and surface roughness.
The relative importance of various observational strategies for the science pitches related to the Galactic Bulge Time Domain Survey are shown in Figure 3. Nearly all elements of observational strategy captured in these (required) responses were very or somewhat important for the majority of the science pitches. The elements related to cadence were ranked as very important for the highest number of science pitches (≳50%). The survey area, location(s), and specific filters used were ranked as either very or somewhat important for the vast majority of the science pitches (90%, 80%, and 78%, respectively). In contrast, dithering strategies and the final coadded depth of the survey were ranked as not important for the majority of submissions.
The High Latitude Time Domain Survey
Science investigations that could be enabled by the High Latitude Time Domain Survey range from detecting solar system planetary analogs to building statistical samples of rare and exotic SNe in the very early universe. The majority of pitches highlighted the variety of stellar-based transients that could be detected and studied over a wide range of redshift given appropriate cadences and filter choices (including all types of SNe, strongly lensed SNe, fast blue optical transients, kilonovae, tidal disruption events, and high redshift transients, including SNe deriving from Pop III stars in the early universe). A few pitches highlighted potential science investigations within the Milky Way, including detecting cool gas giants around nearby stars and measuring the stellar pulsation of stars near the tip of the red giant branch to determine their distance and identify the edge of the MW’s stellar halo.
Pitches also described the potential for a wide range of extragalactic science investigations, including detecting black holes over a large range of mass and dynamical state (including stellar mass black holes, massive black hole binaries, and low-mass super massive black holes), performing variability studies of active galactic nuclei (AGN; including identifying low-luminosity AGN at intermediate and high redshifts and studying the evolution with redshift of AGN dust), and building statistical samples of high redshift (z>10) galaxies.
The relative importance of various observational strategies for the science pitches related to the High Latitude Time Domain survey are shown in Figure 4. While all elements of the observational strategy shown in Figure 4 were ranked as very or somewhat important for the majority of the science pitches, the total survey area and most of the observational strategies related to cadence were ranked as very important for the vast majority of the science pitch submissions. Few pitches identified slitless spectroscopy strategies (number of orientations and relative orientations; not shown) as very or somewhat important to their science case (20% for both).
The High Latitude Wide Area Survey
Science investigations that could be enabled by the High Latitude Wide Area Survey range from characterizing the orbits and surface compositions of small solar system bodies such as asteroids, comets, and trans-Neptunian objects to measuring Cosmic Infrared Background fluctuations to detect radiation from the earliest phases of the universe. The majority of pitches highlighted the potential for increasing our understanding of galaxies and their evolution, as well as the evolution of super massive black holes and AGN. Pitches discussed the potential for studying the environmental dependence of galaxy quenching at low to moderate redshift, the formation of bulges as a function of redshift and as a function of the presence of mergers and galaxy-galaxy interactions, the characteristics of ‘primordial’ or ‘proto’ clusters at z>1.5, and the co-evolution of black holes and galaxies. Other pitches highlighted the potential for increasing our understanding of the high redshift universe, including through the search for z>10 galaxies, and the discovery of the earliest quasars and AGN (to z~10).
A significant fraction of pitches discussed science investigations related to applications of gravitational lensing, including the discovery of many low-mass strong lensing systems, the ability to make precise measurements of dark matter substructure as a function of redshift, and the potential to use measurements of the Einstein radius for tens of thousands of lenses to constrain the expansion history of universe. Pitches also discussed other cosmological studies, including characterizing large-scale structure in the early universe at the scales of groups, filaments, and the galaxies within these structures, and using gravitational lensing of the Cosmic Microwave Background by high redshift Lyman-break galaxies to constrain cosmological parameters.
Other prominent themes for the High Latitude Wide Area Survey were investigating stellar physics, characterizing resolved stellar populations, and reconstructing the detailed assembly history of the MW and other nearby galaxies. Pitches that were focused on science investigations within the MW highlighted the potential for discovering and characterizing substellar populations (brown dwarfs, free-floating planetary mass objects), increasing our understanding of the low-end of the stellar initial mass function, and discovering and characterizing substructure and satellite galaxies in the MW’s halo, thereby increasing our understanding of the MW’s merger history.
Within the local universe, pitches highlighted the potential for studying the galaxies, dark matter, and intracluster light of nearby massive clusters, measuring the shape and structure of galaxies’ dark matter halos through studying their globular cluster populations, and unveiling minor merger histories over a range of host galaxy mass through the detection and characterization of stellar streams and halos. A few pitches also discussed opportunities to improve the cosmic distance ladder by obtaining improved and/or independent distance estimates to nearby galaxies using surface brightness fluctuations, long-period variable stars, and the magnitude of the tip of the red giant branch.
The relative importance of various observational strategies for the science pitches related to the High Latitude Wide Area Survey are shown in Figure 5. The final depth, area, location, and choice and number of filters were very or somewhat important for the vast majority of science investigations, while strategies related to observational cadence were ranked as very or somewhat important for fewer than one-third of the science pitch submissions. Subpixel dithering (to increase sampling of the point spread function) and large dithers (for fully covering the gaps between the detectors) where somewhat or very important for 65% and 73% of the science pitches. Few pitches identified slitless spectroscopy strategies (number of orientations and relative orientations; not shown) as very or somewhat important to their science case (23% and 19%, respectively).
Common threads and challenges
There are common themes across the science pitch submissions for all three Core Community Surveys, as well as clear challenges to be addressed during the definition of the Core Community Surveys.
One common discussion amongst science pitches for all three Core Community Surveys was the unique scientific benefits that would come from choosing sky locations that will enable the realization of synergies with a host of current and upcoming facilities. While the focus of these discussions was primarily on synergies with facilities or surveys obtaining imaging data at optical and near infrared (NIR) wavelengths, there was also mention of synergies with optical and NIR spectroscopic surveys and radio and X-ray facilities and surveys. Maximizing synergies with other facilities and large surveys will require careful consideration of survey location and area, and for some science investigations, will also include other observational strategies such as depth, cadence, and filter choice.
A common theme, as well as a common challenge for Core Community Survey definition, is the community’s interest in using the full suite of filters available on Roman’s WFI. Figure 6 shows the compilation of answers to the (optional) question “Which imaging filters does your science investigation require?” for each of the Core Community Surveys. In total, 60% of respondents provided an answer to this question, with all filters being in demand for each of the Core Community Surveys. Clearly, further community discussion will be needed to determine the filter suites that will maximize the science that can be achieved with each Core Community Survey while balancing the survey’s depth, area, and/or cadence with the total observing time.
Another common challenge across the three Core Community Surveys which is evident from the science pitches will be identifying the survey area and location(s) which will maximize the science return. The total survey area was ranked as very or somewhat important for the vast majority of science pitches (94%), although the required survey area differs between individual investigations.
A total of 62% of submissions across the three Core Community Surveys provided a response to the (optional) question “Is it preferable for the survey area to be contiguous or split into well separated areas on the sky?” 53% of the total respondents said it didn’t matter, 33% chose contiguous, and 14% chose split into well separated areas. While the exact percentages differ by survey, for all three Core Community Surveys there are science investigations that would benefit from a contiguous survey area, as well as a smaller number of investigations that would benefit from a survey area that includes well separated regions on the sky.
For some investigations, the exact location of the surveyed area is critical. For example, a number of Galactic Bulge Time Domain survey pitches described science investigations that would be uniquely enabled by the inclusion of a field located at the Galactic Center, and several science pitches for both the High Latitude surveys called for including specific astronomical objects within the surveyed area, or for full or partial areal overlap with other ground- or space-based surveys.
The above tensions highlight the need for continued community input, including more detailed and nuanced discussion of the range of survey strategies that will enable a given science investigation, and the impact of different trades in observational strategy on a given science investigation. The currently open Roman CCS white paper call is an opportunity for the astronomical community to provide the CCS definition committees with the information they need to understand and evaluate these impacts as they strive to define surveys that will provide the greatest science return. There will also be future opportunities for the community to engage with the CCS definition committees, including dedicated community workshops.