Prime Focus Spectrograph on Subaru Telescope to begin scientific operations in February       
 

Researchers have finished equipping the Subaru Telescope with a new special “compound eye” culminating several years of effort. This new instrument features approximately 2,400 fibers scattered across the extremely wide field of view available at the Subaru Telescope’s primary focus, allowing for simultaneous spectroscopic observation of thousands of celestial objects. This unrivaled capability will help researchers precisely understand the formation and evolution of galaxies and the Universe once it begins scientific operations in February 2025.

With the ultra-wide field of view of approximately 1.3 degrees in diameter at its prime focus and with its large light-gathering power thanks to its 8m main mirror, the Subaru telescopes offers superb power to conduct very large surveys of galaxies drilling very deep into the history of our universe. From February 2025 onwards, the new flagship instrument ‘Prime Focus Spectrograph’ (PFS) of the Subaru Telescope is now becoming available to the scientific community and will begin with the observations of its main survey program. PFS will use about 2,400 fibers to collect light from celestial objects and to simultaneously obtain spectra across the entire visible light range and part of the near-infrared band. Throughout the survey, astronomers will collect millions of spectra that will allow them to then determine distances to galaxies, measure precise radial velocities and study their detailed physical properties.

For nearly 15 years, the development of PFS has been undertaken by an international collaboration of over 20 research institutions spanning Japan, the United States, France, Brazil, Taiwan, Germany, and China. The Max Planck Institutes for Astrophysics (MPA) and Extraterrestrial Physics (MPE) joined the collaboration in 2014 and 2016, respectively, contributing financially to the development of key parts of the instruments, the development of the fiber assignment software, as well as the design of the scientific programs. After overcoming the difficult period due to the COVID-19 pandemic and various technical challenges of this very ambitions instrument, PFS is now finally ready to begin operations.

“We are thrilled to begin survey observations with PFS,” says Eiichiro Komatsu, director at the MPA. “All of the planned programs, including Cosmology, Galaxy Formation and Evolution, and Galactic Archaeology, will yield exciting new results.”

Using this powerful instrument, the international team will invest a total of 360 nights of telescope time over the next six years to create a detailed 3D map of the universe and to understand its evolution in time. They aim to uncover the nature of dark energy, which is driving the accelerated expansion of the universe. In addition, spectroscopic surveys of hundreds of thousands of galaxies will reveal the physical processes of galaxy formation and evolution over the 13.8 billion-year history of the universe. Furthermore, through spectroscopically observing also hundreds of thousands of stars in the Milky Way, the Andromeda and local dwarf galaxies, astronomers will determine the strength of gravity, thus exploring the nature of dark matter and the physical processes that have governed the growth of these galaxies.

“Optimally allocating PFS’s fibers to objects across possibly dozens of sky revisits turned out to be a very interesting combinatorial challenge,” says Maximilian Fabricius from MPE, who led the development of the fiber allocation software together with Martin Reinecke (MPA). “We had to optimize hundreds of thousands of variables simultaneously to take all observational and instrumental constraints into account. Using a network flow approach and linear programming, we achieved a 10–20% overall improvement in observing efficiency compared to traditional methods. I’m excited to see this working in real life as the survey begins this year.”