The Science Case for Multi-Object Spectroscopy on the European ELT
  This White Paper presents the scientific motivations for a multi-object
spectrograph (MOS) on the European Extremely Large Telescope (E-ELT). The MOS
case draws on all fields of contemporary astronomy, from extra-solar planets,
to the study of the halo of the Milky Way and its satellites, and from resolved
stellar populations in nearby galaxies out to observations of the earliest
'first-light' structures in the partially-reionised Universe. The material
presented here results from thorough discussions within the community over the
past four years, building on the past competitive studies to agree a common
strategy toward realising a MOS capability on the E-ELT. The cases have been
distilled to a set of common requirements which will be used to define the
MOSAIC instrument, entailing two observational modes ('high multiplex' and
'high definition'). When combined with the unprecedented sensitivity of the
E-ELT, MOSAIC will be the world's leading MOS facility. In analysing the
requirements we also identify a high-multiplex MOS for the longer-term plans
for the E-ELT, with an even greater multiplex (>1000 targets) to enable studies
of large-scale structures in the high-redshift Universe. Following the green
light for the construction of the E-ELT the MOS community, structured through
the MOSAIC consortium, is eager to realise a MOS on the E-ELT as soon as
possible. We argue that several of the most compelling cases for ELT science,
in highly competitive areas of modern astronomy, demand such a capability. For
example, MOS observations in the early stages of E-ELT operations will be
essential for follow-up of sources identified by the James Webb Space Telescope
(JWST). In particular, multi-object adaptive optics and accurate sky
subtraction with fibres have both recently been demonstrated on sky, making
fast-track development of MOSAIC feasible.
