The Prime Focus Spectrograph (PFS), a proposed multi-object spectrograph for a major astronomical observation program on the Subaru Telescope to address nature of dark energy, formation and evolution of galaxies, and assembly history of our own Milky Way and Andromeda galaxies, has successfully passed the Conceptual Design Review by an international review committee to move ahead to the next phase of preliminary design and construction.

The review panel consisted of an international group of astronomers and instrumentalists. The report praises the conceptual design of the proposed instrument, "PFS is a very exciting instrument concept which will deliver unique cutting-edge science well into the next decade." Their recommendation for the next step was unequivocal: "We conclude that the PFS design meets the standards appropriate to a Conceptual Design Review and recommend that the project should proceed to the next phase."

The PFS is designed by an international consortium led by the Institute for the Physics and Mathematics of the universe (IPMU), a research center at the University of Tokyo, Japan.

It consists of Academia Sinica Institute for Astronomy and Astrophysics (ASIAA, Taiwan), California Institute for Technology (Caltech, California), Jet Propulsion Laboratory of NASA (JPL, California), Johns Hopkins University (JHU, Maryland), Laboratoire d'Astrophysique Marseille (LAM, France), Princeton University (New Jersey), Universidad Sao Paulo (USP), and Laboratorio Nacional de Astrofisica (LNA, Brazil).

The review panel commented on the strength of the group, "The Panel was impressed both with the broad technical strengths of the collaboration and the technical detail of the presentations."

Prof. Hitoshi Murayama, the Principal Investigator (PI) of the PFS project, the director of the IPMU and also a MacAdams Professor of Physics at the University of California Berkeley and Lawrence Berkeley National Laboratory of the United States, responded to the report by the review panel, "I'm ecstatic. We can now move forward with this ambitious project and start building this ambitious instrument. I'm excited about the unprecedented assault on the nature of dark energy with this unique instrument."

Multi-object spectroscopy has been recognized as an important step for the future of astronomy and astrophysics worldwide. For example, the 2010 National Academy studies in the United States on the coming decade of astronomy stated "Massively multiplexed spectrographs in intermediate-class and large-aperture ground-based telescopes would also play an important role" to understand physics of the universe.

The review panel is chaired by Prof. Ray Sharples, Director of the Centre for Advanced Instrumentation of Durham University, England. The rest of the panel consists of Prof. David Crampton, head of the instrumentation group at the Dominion Astrophysical Observatory/Herzberg Institute of Astrophysics (HIA) in Victoria, Canada, Prof. Karl Glazebrook, University Distinguished Professor of Swinburne University, Australia, Prof. Satoshi Miyazaki, Head of the Advanced Technology Center at the National Astronomical Observatory of Japan, and Prof. Tomonori Usuda, Associate Director of the Subaru Telescope, Hawaii.

Science
The PFS project aims at three major scientific objectives. One of them is cosmology, in particular the nature of the so-called dark energy that is causing an accelerating expansion of the universe. Its discovery earned 2011 Nobel Prize in Physics to Profs. Saul Perlmutter, Brian Schmidt, and Adam Riess. However the discovery led to major new puzzles: why is the universe accelerating and where does it lead us? The accelerating expansion is tearing the universe apart, and distant galaxies will eventually disappear from our view even with the most giant telescopes.

The dark energy is apparently an infinite source of energy that permeates the whole cosmos uniformly. If the energy multiplies too quickly, the dark energy may rip the universe at some point with an infinite speed and the universe will end with a Big Rip. The PFS aims at measuring the history of acceleration with an unprecedented accuracy, forecasting the fate of the universe and the number of its remaining years.

The second scientific objective is to reveal the earliest moments of the baby galaxies and how they evolved to the present mature Universe. It is well known that the universe went through the dark ages, after the initial fireball of the Big Bang fizzled but before the cosmic dawn when the first stars were born.

The way the primordial gas and first stars assembled into galaxies and their central supermassive black holes is still not understood. Furthermore, the galaxies underwent the period of active star formation, which mysterious shut off billions of years ago. The PFS instrument is specifically designed to study the history of galaxies in an uninterrupted way from their births to their midlife.

The third scientific objective is to understand the history of our own galaxy: the Milky Way. Combining with the data from the Gaia satellite that will be launched next year by the European Space Agency (ESA), the PFS instrument can map out locations and motions of millions of stars, revealing where they came from and how they were sucked in by the gravitational pull of the smaller Milky Way in the past.

A similar study will be conducted on the nearby Andromeda galaxy, which is expected to reveal for the first time the history of our local neighborhood within a few million light-years.

The review panel stated "that PFS had a strong and timely science case in all three areas of cosmology, galactic archaeology and galaxy evolution/AGNs." This comment pleased the co-leader of the science team, Richard Ellis from Caltech, Steele Professor of Astronomy and a winner of the Gold Medal from Royal Astronomical Society in 2011, "I've always felt for more than a decade that multi-object spectroscopy was the key to the next step for understanding the history of the universe.

"I'm thrilled that the instrument of my dreams is now coming into reality." The other co-leader, Prof. Masahiro Takada of IPMU concurs, "The PFS will build on the imaging survey using the HyperSuprimeCam (HSC) instrument with 0.9-billion pixels and 1.5 square degrees field of view that will have its first light this May. The combination of imaging and spectroscopic survey will surely repeat the amazing success of the Sloan Digital Sky Survey on a 2.5m Apache Point telescope on the much larger 8.2m Subaru Telescope into much earlier moments in the cosmic evolution."

On comparison, the famed Hubble Space Telescope has approximately a thousand times smaller field of view. The large field of view is particularly suited for large-scale surveys, cosmic census to reveal the history and the fate of the universe. In this respect, "the Subaru Telescope is truly unique among the largest telescopes in the world," says Prof. Hideki Takami, Director of the Subaru Telescope.

Prof. David Spergel, the chair of the Department of Astrophysical Sciences of Princeton, is excited about this project, "The science case for this project is so strong, it was easy to obtain big support from our University. We've been in the HSC project already for some time, and the PFS will reap every drop of information out of the vast data from the HSC imaging survey. The combination will move us into an incredible frontier."

Instrument
The proposed PFS instrument will consist of robotic fiber positioners, that will control 2400 fiber optic cables to aim at galaxies billions of light-years away within less than a minute at the accuracy of ten microns; four precision spectrographs that cover wavelengths of light from near ultraviolet to near infrared for uninterrupted coverage of high-redshift galaxies; and a complex hardware of microlenses, optical fibers, mechanical devices, and software for system control and data reduction.

The review panel said "The panel found the technical design as presented is feasible and should be able to deliver the science program" and "the overall cost estimates (of approximately US$60M) appear reasonable" for the proposed instrument. Approximately US$40M of the necessary resources is already in the hands of the consortium.

"This is a complex project of components supplied by many partners that should work together at a demanding accuracy," says Prof. Olivier Le Fevre, Astronomer at Aix-Marseille University, former director of Laboratoire d'Astrophysique Marseille, and a member of the consortium".

"With the positive outcome from the conceptual design review, we can now allocate highly competent engineers from our 60-member team." The Marseille team is responsible for the design of challenging optics in the spectrographs.

The director Prof. Paul Ho of ASIAA, who is also a professor at the Harvard-Smithsonian Center for Astrophysics, is equally excited, "We have an excellent working relationship with the Japanese astronomical community. We have built a filter exchanger and tested the CCDs, the critical elements of the HSC. I'm very pleased that the PFS will allow us to exploit the vast amount of data from the HSC to their full potential."

The team from ASIAA will build the metrology system to aim the optical fibers at the galaxies identified by the HSC imaging survey.

Prof. Laerte Sodre Jr from Universidade de Sao Paulo, in collaboration with the Laboratorio Nacional de Astrofisica, is keen to provide the fiber system critical for the proposed instrument, "I am overwhelmed by the wealth of science that can be addressed by the PFS instrument on the Subaru Telescope. Ensuring quality of 2400 fibers will be a challenging task we aspire to accomplish on time."

Next Steps
The consortium plans to develop more detailed requirements on the instrument and its preliminary design by the end of the year. The procurement of major components and the construction of the instrument will start in the second half of the year. The first light is slated for 2017.

Prof. Tim Heckman of JHU, Dr. A. Hermann Pfund Professor and the director of the Center for Astrophysical Sciences, stated, "I have a religious belief that this instrument will bring us to the next decade in our attack on the mysteries of the universe. It is totally unique in the world, and the panel report validates that we know what we are doing."

The group JHU will build cameras and dewars for the spectrographs together with the Princeton team. Dr. Michael Seiffert from JPL will build a prototype of the robotic fiber positioners in the next few months.

He remarked "It is a great honor to participate in the PFS project, which we believe will be one of the most powerful astronomical instruments ever built. After our successful review, PFS is on the path to provide new insight into the content and geometry of our universe." The review panel concluded, "We congratulate the PI and the Project Team on having brought the project to its current status in such a short period of time."