STUDY SUMMARY - IPMU

SUMMARY REPORT
WIDE FIELD FIBER-FED OPTICAL
MULTI-OBJECT SPECTROMETER (WFMOS)
2.0 Scientific Case and Requirements
2.1 Context
We can trace the motivation for an ambitious wide field, multi-fiber spectrograph such as
WFMOS to the early days of Gemini when the scientific promise of an f/6 Cassegrain focus on
Gemini-South was being enthusiastically promoted by various members of the UK, US, and Canadian
communities. The f/6 secondary and its associated wide field corrector were eventually
eliminated for financial reasons. However, the huge impact of galaxy surveys conducted using
multiplexed spectrographs on wide field 2.5–4 m telescopes such as SDSS and the AAT later
resurrected the need for an equivalent facility on an 8-m aperture telescope. The discovery of
dark energy in the late 1990s and the recognition that baryonic acoustic oscillations provide a
valuable tracer of the cosmic expansion, independently of supernovae, has given the issue further
impetus. The aim of this proposal is to make the strongest possible case for proceeding with the
construction of WFMOS, in collaboration with our Japanese colleagues.
Our starting point in this study was the valuable Feasibility Study (FS) conducted by a consortium
led by the Anglo-Australian Observatory. Gemini, Subaru, and the Gemini Board concluded
such a facility should be installed at the prime focus of the Subaru telescope where it
could share the optics destined for the (then proposed, now funded) HyperSuprime Camera
(HSC). The FS study proposed a range of ambitious surveys motivated by questions in both dark
energy and Galactic science. These questions, in turn, followed discussions by the Gemini community
at the Aspen meeting in June 2003.
We have revised the science strategy proposed in the FS and, as a consequence, significantly
improved the design of the instrument. Recognizing that HSC is now funded, we have put considerable
emphasis on harnessing the synergy between these two wide-field instruments. We
have also reassessed the role that WFMOS should play in dark energy studies. When the FS was
published, WFMOS was a rare example of a facility semi-dedicated to probing dark energy. Today,
there are a number of potential probes using various methods with at least three (AAT WiggleZ,
SDSS-3 BOSS and the Hobby Ebberley HETDEX) proposing a major spectroscopic effort
using baryonic acoustic oscillations. In the future there is optimism for one or two space missions
dedicated to studying dark energy. Likewise, experience and progress with Keck’s DEI-
MOS spectrograph and the ESO VLT’s FLAMES spectrograph has significantly shaped our
thinking of the optimum parameters for Galactic science.
One thing has not changed since the FS. Subaru remains the unique platform for an instrument
like WFMOS. Investigations by others have shown that such an ambitious wide field spectrograph
could not be accommodated at Keck or the VLT. Short of building a dedicated widefield
telescope, WFMOS on Subaru is the only route to achieving the science goals discussed in
this proposal.
The unique capabilities of the Subaru prime focus raise the important question of the attitude
of the Japanese community to WFMOS. Through active discussions and Japanese membership
of both WFMOS teams, this question has been brought into sharp focus over the past few years,
culminating in two major conferences—Panoramic Views of Galaxy Formation and Evolution
(Hayama, December 2007) and Cosmology Near and Far—Science with WFMOS (Kona, May
2008). Recognizing the unique synergy between HSC and WFMOS on their national telescope,
the Japanese community views WFMOS as a general resource whose scientific impact should
extend well beyond that envisaged by the Aspen process or the FS. To address this, we co-opted
a group of 5 Japanese extragalactic astronomers enthusiastic about using WFMOS for programs
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