About this Article

With a diverse and growing users community (7,769 unique investigators to date), after twenty-nine years on orbit the Hubble Space Telescope (HST) continues to draw a high demand for observing time from astronomers across the world. The peer-reviewed proposal selection process is fundamental to maximizing the science return from this unique observatory in as fair a manner as possible. It is only thanks to the dedication, hard work, and integrity of all the Time Allocation Committee (TAC), review panel members, and external reviewers that this process is possible. We are deeply grateful to those who provide this important community service and to the many proposers whose creativity keeps HST's scientific demand high year after year. With this in mind, we present here the highlights from HST's Cycle 27 selection process.

We received a total of 1,019 proposals by the Phase Ⅰ deadline in April, including 92 in Archival Research and Archival Legacy categories, and 57 in the Theory category, requesting a total of 24,454 orbits. These proposals included investigators from 48 U.S. states (and the District of Columbia and the US Virgin Islands), and investigators from 40 countries. The international members of the proposal review panels and the TAC met in June to provide recommendations to the Director, who approved 181 proposals totaling 2,702 awarded orbits, which will start executing at the beginning of Cycle 27 in October. Up to 150 orbits will be available for Mid-Cycle GO programs targeting recently discovered, non-transient objects. Proposals may be submitted any time before September 27, 2019 for implementation in the November 1, 2019 to March 31, 2020 timeframe.

The Call for Proposals

The Cycle 27 Call for Proposals (CP) was released on January 14, 2019, announcing observing opportunities with HST’s current instrumentation: the Advanced Camera for Surveys (ACS), the Cosmic Origins Spectrograph (COS), the Fine Guidance Sensors (FGS), the Space Telescope Imaging Spectrograph (STIS), and the Wide Field Camera 3 (WFC3). The category of JWST Preparatory proposals (with a default proprietary period of zero months) to complement and enhance the scientific impact of future JWST observations was continued. Medium Proposals continued as a separate category for programs requesting between 35 and 74 orbits, to improve the success rate of programs in this historically challenging orbit range. As in previous cycles, as part of the HST proposal it was possible to request time on Chandra, Spitzer, XMM-Newton, and on NOAO and NRAO facilities. New this cycle was the opportunity to request high-cadence photometric monitoring by the Transiting Exoplanet Survey Satellite (TESS) for individual targets in HST programs.

The CP also announced opportunities to request funding for theoretical and archival research. To support the latter, the CP advertised that all non-exclusive access data for current HST instruments (ACS, COS, STIS, WFC3, FGS) have been made available as part of the Amazon Web Services (AWS) public dataset program; the CP included a new Archival Cloud Computation Studies category through which proposes could request to use these data via the AWS platform. The CP also encouraged archival proposals that mine the HST Source Catalog, combining tens of thousands of single visit-based WFC3, ACS, and WFPC2 source lists from the HST Legacy Archive into a single master catalog with roughly 100 million individual sources and the HST Spectroscopic Legacy Archive for high-level data products, containing "science grade" co-added spectra of all usable public data, combining exposures for each target from across visits.

Recognizing the unique and limited availability of HST's ultraviolet (UV) capabilities, the UV Initiative was continued to encourage the community (and the TAC and panelists) to increase the fraction of time and awards dedicated to wavelengths below 3200 Å. The UV initiative applied to all Small, Medium, Large, and Treasury GO Proposal (with the exception of SNAP), as well as Archival Proposals. The available UV instrument modes include ACS/SBC imaging, COS spectroscopy, STIS/MAMA imaging and spectroscopy, STIS/CCD spectroscopy (UV gratings only) and WFC3/UVIS imaging (UV filters only).

Finally, the CP highlighted the upcoming HST UV Legacy Library of Young Stars as Essential Standards (ULLYSES) program, which will serve as a UV spectroscopic reference sample of young high- and low-mass stars. Though specific targets have not yet been chosen, the community was encouraged consider to submit Cycle 27 proposals to supplement and complement the conceptual program. The CP also encouraged the community to submit proposals that address questions in fundamental physics, whether they are GO, AR, or theory proposals.

The review process

We recruited 156 members of the review panels and the TAC several months prior to the proposal deadline, and asking them to serve on one of the 14 panels organized by science category, consisting of two panels on cosmology and the intergalactic medium, three on galaxies and the circumgalactic medium, two panels covering active galactic nuclei and their hosts, two on stellar populations, three on stellar physics, two covering exoplanets, and one covering solar-system objects. With the exception of solar system, each panel has at least one "mirror" panel, covering similar topics and expertise, allowing proposals to be transferred as needed to avoid conflicts of interest within a given panel.

A total of 1,019 proposals were received electronically via the Astronomer's Proposal Tool by the Phase Ⅰ deadline of April 5, 2019 and each was sorted by science category and organized into the review panels described above. Each review panel subsequently received between 30 and 70 Small (<35 orbits) and Medium (35 to 74 orbits) proposals to grade in preparation for the in-person discussion in June.

To decrease the burden on the panelists, each was only assigned about two-thirds of the proposals in their panel. We collected these grades a few weeks before the meeting, and sorted into a preliminary rank order within each panel. Proposals ranking in the bottom 40% were not discussed further in the TAC process unless raised for discussion by a non-conflicted panelist. The Large and Treasury proposals (>75 orbits) were reviewed by the TAC members for discussion in their meeting following the panel reviews.

The review panels met over three days in Baltimore, MD, to deliberate and re-grade the proposals, and produce a final rank order for the proposals in the top 60% in each panel. Members of the TAC were also assigned to these panels to serve as non-voting chairs, guiding the discussion and carrying forward opinions (should they be necessary) from the panels to the TAC. Each panel was provided a nominal orbit allocation to help guide decisions, especially for proposals critically ranked at or near the potential award boundary.

The Medium proposals were ranked amongst the Small proposals, allowing a gauge of their relative importance in the competition for the pool set aside for the Medium category, nominally 650 orbits. A panel could also choose to identify Medium proposals to award from its nominal allocation, ensuring the proposal's success, albeit at the expense of a large fraction of that panel's awardable time. Panelists were also asked to review the Large and Treasury proposals pertinent to their panel's science category. Comments on the Large proposals were provided to the panel chairs for the TAC review.

Immediately following the panel review, the TAC met for an additional two days to review the panels' recommendations, and to decide the final rank orders for the Large and Treasury programs, within those respective orbit pools. Dr. Rachel Somerville (Flatiron Institute, NY) served as chair of the Cycle 27 TAC; Prof. Corinne Charbonnel of University of Geneva, Prof. Mary Putman of Columbia University, and Prof. Mark Voit of Michigan State University served as TAC members at large. The Institute Director completed the final review of the TAC recommendations in the week following the TAC meeting, and the Cycle 27 results were announced shortly thereafter.

View accepted proposals for Cycle 27.

Ensuring an impartial review

We continue to strive for impartiality and fairness in the HST review process. Conflicts of interest for each reviewer are identified based on institution and publication record, and mirror panels are used to avoid conflicts when possible. Once the proposals are initially distributed to the panel, each panelist must identify any remaining strong conflicts of interest, including competing proposals, mentorship relationships, and close collaborations. Panelists are not permitted to grade proposals for which they are conflicted, and for strong conflicts, e.g., any from which they themselves would directly benefit, panelists are not permitted to participate in the discussion.

Additionally, the Institute has taken steps to address the unconscious biases of the HST TAC process, which has resulted in small but statistically significant over-representation of male PIs relative to female PIs in each of the last 26 HST cycles. As with the Cycle 26 TAC, a dual-anonymous system was used to review the proposals, in which the panelists did not know the identities of the proposers while reviewing the proposals.

Results

Of the 1,019 proposals submitted (requesting 24,454 prime orbits), 181 proposals were approved, including 10 Large, Treasury and Pure-Parallel Programs, 15 Medium Programs, 3 Archival Legacy Programs, and 7 Snapshot Programs. In addition, 2 joint HST/NOAO programs, 1 joint HST/Chandra program, 1 joint XMM-Newton program, 2 joint HST/NRAO programs, and 1 TESS coordinated program were awarded time. Additionally, 12 JWST Initiative and 9 Fundamental Physics proposals were successful.

The 2,700 available orbits were broken down into approximately 600 orbits for Large and Treasury proposals, 600 orbits for Medium proposals, and 1,500 orbits for Small proposals. The TAC also recommended approval of 813 Snapshot observations. The over-subscription ratio for GO programs was approximately 9.1:1 by orbits and 5.6:1 by proposals, which is higher than in previous cycles due to the lower number of available orbits. The over-subscription for Archival and Theory proposals was 4.1:1 and 9.5:1, respectively, which is also higher than in Cycle 24.

With 181 of 1,019 proposals accepted, the average HST Cycle 27 acceptance rate was 17.8%, somewhat lower than the acceptance rate from Cycle 24. (Owing to the available Cycle 26 small orbits being allocated in Cycle 25, Cycle 24 represents the last "normal" size allocation pool and is thus the most relevant for comparison here.) The estimated oversubscription of Archival and Theory proposals by nominal funding was 4.7:1 compared to 4.0:1 from Cycle 24. PIs from ESA member countries lead 21% of the accepted Cycle 27 programs, slightly lower than in Cycle 27. The success rate of Medium proposals was 8.1% by proposal (14 out of 172), for a total of 678 orbits, representing a decrease with respect to Cycle 24 (with a success rate of 15%).

WFC3 remained the most requested instrument, with 40.1% of the awarded time utilizing this instrument in its various modes on primary targets (10.4% WFC3/IR imaging, 9.4% WFC3/IR spectroscopy, 18.7% WFC3/UVIS imaging, and 1.6% WFC3/UVIS spectroscopy). COS is the second most requested instrument, being awarded 27.7% of available orbits, with most of the time going to the FUV (23.2%) and NUV (4.2%) spectroscopy. ACS was awarded 19.9% of the allocation, with the bulk (19.1%) of the time going to the WFC imaging mode. STIS was awarded 12.3% of available orbits, of the awarded time almost evenly split across the spectroscopic modes. The success rate for the proposals under the UV Initiative was 45% by proposal (10 out of 40 for archival research and 71 out of 388 for GO), and 51% by orbit (1,373 orbits out of 3,686 requested).

The Cycle 27 orbit distribution per science category is as follows: 12.3% for Exoplanets (relative to 15.5% submitted), 28.7% for Galaxies and the CGM (of 35.3% submitted), 20.3% for Stellar Physics (of 17.6% submitted), 20% for Stellar Populations (of 12.9% submitted), 8.9% for Cosmology and IGM (of 9.2% submitted), 2.6% for Solar System (of 1.6% submitted), and 7.2% for Black Holes (of 7.8% submitted).

Acknowledgments

We thank all of the HST TAC members, review panelists, and external reviewers for their service on the HST Cycle 27 TAC. Numerous Institute personnel contributed to the support of review process.

Science Policies Group astronomers Katey Alatalo, Alessandra Aloisi, Christine Chen, Andy Fruchter, Claus Leitherer, Amaya Moro-Martín, Molly Peeples, Neill Reid, and Lou Strolger were responsible for selecting the panelists—distributing the workload among the panelists according to expertise, but taking into account the conflicts—coordinating policy, providing oversight during the review process, and checking for duplications within the recommended proposal pool. 

Technical Manager Brett Blacker received, organized, and distributed the proposals, oversaw the proposal database, announced the results, and prepared the statistical summaries and figures provided here.

The TAC logistics were devised and coordinated by Sherita Hanna, with Brett Blacker and Crystal Mannfolk providing technical assistance. Additional administrative support came from Robin Auer, Jody Charles, Martha Devaud, Brian Fincham, Flory Hill, Tracy Lamb, Jennifer McFalls, Susan Mucklow, Brenda Oleszczuk, Karyn Poletis, Jean-Baptiste Regnard, Karen Sealover, Michele Sharko, Darlene Spencer, Rolanda Taylor, Ana Maria Valenzuela, and Chris Zumbrun.

Panel support was provided by Trisha Ashley, Brett Blacker, Roger Cohen, Hannah Drew-Moyer, David French, Svea Hernandez, Tuomas Kangas, Mattia Libralato, Matthew Maclay, Camellia Magness, Crystal Mannfolk, Nathan Miles, Laura Prichard, Jenna Ryon, Elena Sacchi, Hannah Wakeford, and Tom Wilson. Keira Brooks, Lauren Chambers, John Debes, Theresa Dower, Bryan Hilbert, Samantha Hoffmann, Elizabeth Nance, James Paranilam, Blair Porterfield, Marc Rafelski, Catherine Riggs, Paule Sonnentrucker, Greg Snyder, Leslie Stabile, Lou Strolger, and Brian Williams served as Levelers.

Instrument expertise was provided by Roberto Avila and Marco Chiaberge for ACS; Cristina Oliveira and Gisella de Rosa for COS; Ed Nelan and Matt Lallo for FGS/OTA; John Debes, Joleen Carlberg, and TalaWanda Monroe for STIS; and Sylvia Baggett and Linda Dressel for WFC3. Denise Taylor provided expertise on scheduling issues.

IT support was provided by Romeo Gourgue, Jonathan Hanlon, Ryan Levine, Thomas Marufu, Corey Richardson, Patrick Taylor, Calvin Tullos, Jeff Wagner, and Sam Weinstock. Maria Bertch, Jeff Bucklew, Alex Framerini, Craig Hollinshead, Lauretta Nagel, Joel Richon, and Tom Comer provided support for the online reviewing tool. Leilani Felicitas, Kim Heavener, Lisa Kleinwort, Sherry Lewis, Ninel Serbreni, and Carolyn Walton provided support from the Business Resources Center. Ann Field and Pam Jeffries provided support from the Office of Public Outreach, and Zak Concannon and Steve Stout provided assistance from the Copy Center.

Finally, catering was provided by Café Azafran; we thank Salha Balala, Irena Stein, Mark Demshack, and Mikaele Porter for their support. From Facilities we thank Laura Bucklew, Bermond Few, Rob Franklin, Tiffany Lallo, Damon Levine, Greg Pabst, Trevor Thompson, Mike Venturella, and Grover Williams, without whose support the Cycle 27 review would not have been able to take place.

16 orbits are from calibration pool