Astro2020 Science White Paper: Construction of an L* Galaxy: the
  Transformative Power of Wide Fields for Revealing the Past, Present and
  Future of the Great Andromeda System
  The Great Andromeda Galaxy (M31) is the nexus of the near-far galaxy
evolution connection and a principal data point for near-field cosmology. Due
to its proximity (780 kpc), M31 can be resolved into individual stars like the
Milky Way (MW). Unlike the MW, we have the advantage of a global view of M31,
enabling M31 to be observed with techniques that also apply to more distant
galaxies. Moreover, recent evidence suggests that M31 may have survived a major
merger within the last several Gyr, shaping the morphology of its stellar halo
and triggering a starburst, while leaving the stellar disk largely intact. The
MW and M31 thus provide complementary opportunities for in-depth studies of the
disks, halos, and satellites of L* galaxies.
  Our understanding of the M31 system will be transformed in the 2020s if they
include wide field facilities for both photometry (HST-like sensitivity and
resolution) and spectroscopy (10-m class telescope, >1 sq. deg. field, highly
multiplexed, R~ 3000 to 6000). We focus here on the power of these facilities
to constrain the past, present, and future merger history of M31, via
chemo-dynamical analyses and star formation histories of phase-mixed stars
accreted at early times, as well as stars in surviving tidal debris features,
M31's extended disk, and intact satellite galaxies that will eventually be
tidally incorporated into the halo. This will yield an unprecedented view of
the hierarchical formation of the M31 system and the subhalos that built it
into the L* galaxy we observe today.
