STUDY SUMMARY - IPMU

SUMMARY REPORT
WIDE FIELD FIBER-FED OPTICAL
MULTI-OBJECT SPECTROMETER (WFMOS)
2.4 Galactic Archaeology
2.4.1 Near-Field Cosmology Comes of Age
Understanding the formation and evolution of galaxies remains one of the key questions in
astrophysics. The complex processes that govern growth over cosmic history are imprinted on
the Galactic structure see around us. WFMOS will be the premier instrument in the next decade
capable of connecting a detailed inventory of the Milky Way galaxy and its assembly history
with the rapid progress being made in probing the nature of the Universe on large scales and at
high redshift.
The questions posed by this area of ‘near-field cosmology’ are as fundamental as those posed
in unraveling the mystery of dark energy. What can our Galaxy tell us about the validity of the
standard cosmological model and therefore the nature of dark matter and dark energy on small
scales What role does dark matter play in the assembly history of galaxies and what processes
govern the interaction between dark matter and baryonic gas Are these consistent with predictions
of the standard model of cold, non-interacting dark matter What can the fine structure of
the stellar distribution tell us about the distribution and nature of dark matter itself What was the
sequence and process of formation of the Galaxy and how does our understanding of the history
of the primary sub-components of the Milky Way (bulge, disk and halo and surviving satellites)
influence our picture of how normal galaxies evolved Finally, from studies of neighboring systems,
can we be sure our inferences from the Milky Way are applicable more generally
The key to progress lies in developing a detailed picture of the fossil record of past events,
surveying stars to determine their spatial, kinematic and chemical properties. As galaxies are
dynamically and chemically inhomogeneous, only by using the higher dimensionality afforded
by combining kinematic and chemical measures can a full understanding of their constituent
parts be determined. With its enormous multiplex advantage over an unprecedented wide field,
WFMOS offers unique opportunities in this area. As introduced in the Feasibility Study (FS),
information of two kinds is required. A deep Low-Resolution (LR) survey will chart the structural
composition of the Galaxy using kinematic information and metallicities derived from absorption
line indices. A higher-resolution (HR) survey will use ‘chemical tagging’ to identify
sub-components as well as to unravel their detailed evolutionary history.
A number of developments have occurred in the subject since the FS was published in 2005.
We have taken full account of these, both in defining our science requirements for the instrument
and in fixing our proposed survey parameters. Foremost, we have placed significant emphasis on
synergies with the upcoming Gaia mission; this remarkable satellite will provide parallax and
proper motion information to limits well-matched to those of our WFMOS surveys. The combination
will deliver, for the first time, six-dimensional velocity phase space essential for identifying
galactic components. Secondly, we have taken into account the early progress made in surveying
nearby galaxies, both satellites of the Milky Way and the neighboring M31 and M33
systems. Much work is needed to extend these preliminary investigations that are crucial to determine
whether the conclusions drawn from our Milky Way apply generally to most massive
spirals.
Finally, our team has been directly involved in progress made with Keck’s DEIMOS and
ESO’s FLAMES spectrographs. The latter, in particular, has demonstrated clearly the optimal
resolution for chemo-dynamical fingerprinting as a means to understanding the structure and
evolution of nearby dwarf galaxies. As a result, our technical design and survey strategy have
been significantly streamlined from those proposed in the FS.
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