The Local Group: The Ultimate Deep Field
  Near-field cosmology -- using detailed observations of the Local Group and
its environs to study wide-ranging questions in galaxy formation and dark
matter physics -- has become a mature and rich field over the past decade.
There are lingering concerns, however, that the relatively small size of the
present-day Local Group ($\sim 2$ Mpc diameter) imposes insurmountable
sample-variance uncertainties, limiting its broader utility. We consider the
region spanned by the Local Group's progenitors at earlier times and show that
it reaches $3' \approx 7$ co-moving Mpc in linear size (a volume of $\approx
350\,{\rm Mpc}^3$) at $z=7$. This size at early cosmic epochs is large enough
to be representative in terms of the matter density and counts of dark matter
halos with $M_{\rm vir}(z=7) \lesssim 2\times 10^{9}\,M_{\odot}$. The Local
Group's stellar fossil record traces the cosmic evolution of galaxies with
$10^{3} \lesssim M_{\star}(z=0) / M_{\odot} \lesssim 10^{9}$ (reaching
$M_{1500} > -9$ at $z\sim7$) over a region that is comparable to or larger than
the Hubble Ultra-Deep Field (HUDF) for the entire history of the Universe. It
is highly complementary to the HUDF, as it probes much fainter galaxies but
does not contain the intrinsically rarer, brighter sources that are detectable
in the HUDF. Archaeological studies in the Local Group also provide the ability
to trace the evolution of individual galaxies across time as opposed to
evaluating statistical connections between temporally distinct populations. In
the JWST era, resolved stellar populations will probe regions larger than the
HUDF and any deep JWST fields, further enhancing the value of near-field
cosmology.
