At their annual meeting, the Subaru Users community and their representative endorsed the IPMU-led proposal to build a multi-object spectrograph (PrimeFocusSpectrograph or PFS) as the next new instrument on Subaru. On Subaru, PFS will be the most powerful spectrograph in the world capable of simultaneously studying thousands of galaxies. The Subaru telescope is unique among the largest telescopes in the world because of its wide field of view, about a thousand times that of the Hubble Space Telescope. PFS will complement imaging survey with HyperSuprimeCam (HSC), a new digital camera with 0.9 billion pixels and heavier than 3 tons whose construction is well underway. The new proposal came from Hitoshi Murayama (IPMU, Tokyo & Berkeley) as the principal investigator aiming at deciphering mysterious dark energy that is believed to comprise 73% of the Universe today and to determine its fate. PFS would also allow for detailed studies of evolution of galaxies from their birth, the productive period of star formation, to their mature stage now. The proposal is based on a close collaboration of IPMU with ASIAA Taiwan, Caltech, Laboratory of Astrophysics Marseille, NASA Jet Propulsion Laboratory, Princeton University, the Hawaii Observatory of National Astronomical Observatory of Japan, and University of São Paolo.
Dark Energy was discovered in 1990s and completely changed our view of the Universe. It is ripping the Universe apart, accelerating its expansion. Depending on its precise nature, it may end the Universe by ripping the galaxies down to elementary particles. There are many observational projects around the world trying to determine the nature of dark energy, and hence the fate of the Universe. They are categorized as “Stage III” projects because of their superior precisions compared to previous generations of measurements. The world astrophysics community is engaged in preparing for the ultimate “Stage IV” projects, and the recent report from the US National Research Council “Astro 2010” emphasizes the critical importance of such projects.
PrimeFocusSpectrograph (PFS) on the Subaru telescope was proposed as a “Stage IV” project on dark energy to start data taking later this decade. The Subaru Users met at the National Astronomical Observatory in Mitaka, Tokyo, on January 19 and 20, and voted overwhelmingly to endorse the proposed instrument as the next-generation major facility instrument on their observatory. The Subaru telescope is exceptionally suited for this purpose because of its large aperture (8.2 m) that allows the study of faraway galaxies billions of light years away while having a very wide field of view (over a square degree, a thousand times as large as the Hubble Space Telescope) that allows a kind of “census” of galaxies to identify the trend of the Universe without being bogged down by the personality of each galaxy. It sits on the summit of Mauna Kea, Hawaii, one of the best astronomical sites in the world.
To measure properties of dark energy, one needs to measure the expansion history of the Universe precisely. Because light travels at a finite speed, one can measure the expansion rate of the past by looking far. Comparing the expansion rate at varying distances would reveal the expansion history. The way the Universe turned from deceleration to acceleration about some seven billion years ago will point to the precise nature of dark energy. The expansion itself is relatively easy to measure. The light emitted by a distant galaxy is stretched by the expansion of space and becomes redder, which can be measured by any decent spectrograph.
To measure the expansion history, we also need to know how far back in time the light was emitted from the galaxy, or equivalently, how far away it is. Measuring distances precisely on cosmological scales is very challenging. PFS will employ a special feature in the way galaxies are distributed throughout space that comes with a characteristic distance and can be used as a “standard ruler.” To use this technique called baryon acoustic oscillation (BAO), one has to study millions of galaxies, and needs a wide field of view. By building a spectrograph that can study several thousand galaxies at the same time, the group hopes to obtain a large enough sample to be able to use this technique without spending thousands of years of observation.
In addition to BAO, there are a number of other measurements to constrain the properties of dark energy using this instrument. Furthermore, this type of spectrograph with a large field of view and a massive multi-object capability will be unique among the largest telescopes in the world, allowing for unprecedented studies of formation and evolution of galaxies, as well as the assembly history of our own Milky Way galaxy.
The strength of this project comes from exploiting the data using HyperSuprimeCam (HSC), a 3-tonne digital camera with 900 million pixels, slated for the first light later this year. The combination of imaging using HSC and spectroscopy using PFS on Subaru is dubbed SuMIRe, Subaru Measurement of Images and Redshifts. The SuMIRe project is expected to repeat and exceed the tremendous success of Sloan Digital Sky Survey (SDSS) mounted on a 2.5 m telescope, but with a much deeper view of the Universe back to the era that formed early stars and supermassive blackholes.
The Subaru Advisory Committee (SAC), which represents the Subaru Users community, issued the following statement. “Subaru can maintain its position as one of the top telescope facilities in the world by having both a wide-field imager and a wide-field spectrograph. The PFS instrument concept was initially developed primarily for a BAO survey, but after consideration of the instrument specifications, it was realized that PFS could have much broader scientific impact, in areas such as galactic archaeology and galaxy/AGN evolution. Thus, with the conditions listed below, SAC recommends further development of the PFS project as a next-generation Subaru instrument.”
The PFS collaboration currently consists of researchers and engineers from Caltech, NASA/Jet Propulsion Laboratory, Princeton University, Laboratory of Astrophysics at Marseille, Royal Observatory Edinburgh and the UK coalition, University of São Paolo and the Brazilian coalition, Academia Sinica Institute of Astronomy and Astrophysics (ASIAA) and the Taiwan coalition, the NAOJ Hawaii Observatory, and IPMU.
Members of the collaboration are overjoyed by this endorsement. The PI Hitoshi Murayama says “PFS would provide a unique opportunity not only to measure the dark energy properties at an unprecedented precision, but also to study events of the Universe when it was still a baby. Given the endorsement by the Subaru Users community, the collaboration will work together to build an impressive instrument to expand the frontier in our understanding of the Universe. I view this endorsement analogous to a half way between CD-0 and CD-1 approval by the US Department of Energy. I am so excited and enthusiastic about its prospect. Wouldn’t it be amazing to know whether the Universe has an end?”
Richard Ellis, Steele Professor of Astronomy from Caltech has been hoping for an instrument like PFS on an 8m-class telescope for many years. He is very happy to see the endorsement and says, "I welcome the decision from the Subaru Users' meeting to support the construction of the Subaru Prime Focus Spectrograph (PFS). Realizing PFS is the dream of many of us at Caltech and in the Keck community, and we will be pleased to work with our Japanese and other colleagues to realize such an opportunity. This is great news!"
David Spergel, Charles A. Young Professor of Astronomy from Princeton University, has been in the HSC collaboration working with scientists in Japan and Taiwan. He adds "We are delighted and encouraged by the support PFS has in the Subaru community. This will be a unique instrument on one of the best telescopes in the world, and we very much look forward to working closely with Japanese astronomers to make it happen."
From the UK group, John Peacock has been mapping the Universe for some time. He points out “SuMIRe-PFS is the next step we needed to make in the grand project of mapping the Universe. The 2dFGRS and SDSS pictures taught us so many new things about the distribution of matter in the Universe today; but to understand where this structure originated, we have to look at the distribution of faint galaxies at early times. Only the light grasp of Subaru lets us do this, and it is fantastic news that Japanese astronomers are willing for their world-leading telescope to be used for this project.”
Laerte Sodre Jr, Professor of Astronomy at University of São Paolo, leads a group in Brazil with expertise in optical fibers, a critical element of the project. He proclaims “I and several other colleagues in Brazil are excited with the possibility that the Brazilian community can join the PFS/SuMIRe project. Personally, I am very happy to be a member of this project, and look forward to build a rich and scientifically productive collaboration between our community and IPMU and the other partners of the PFS/SuMIRe project.”
Paul Ho, the director of ASIAA, which has been making critical contributions to the HSC collaboration, the imaging component of the SuMIRe project. He expressed his enthusiasm as “We at the ASIAA are delighted that the Subaru users community and the science advisory committee have recommended the PFS to be the next instrument for Subaru. We look forward to working with both IPMU and NAOJ to build this new instrument which will enormously increase the speed at which we can measure the redshifts of distant systems over wide fields. We consider the PFS, as in the case of the HSC, to be our continuing collaborations between ASIAA and the Subaru project.”
The conditions put forward by SAC are
• PFS must satisfy instrument specifications agreed by the Japanese community.
• A firm management structure should be built in Japan to develop PFS, including the assignment of a Japanese project manager.
• SAC representative(s) should participate in important decision-making stages about international collaboration.
• There must be a framework for young Japanese students/researchers to get involved in the PFS instrumentation.
The next step for the collaboration is formulation of the management structure and securing necessary resources, and meeting the conditions put forward by the SAC. The study of the scientific merit of the PFS instrument was lead by Masahiro Takada, Associate Professor of IPMU. The outcome of the study led to this endorsement by SAC.
The work by PI Hitoshi Murayama is funded by the FIRST (Funding program for world Innovative R&d on Science and Technology) program from the Cabinet Office of Japan.