The missing dwarf galaxies of the Local Group
  We study the Local Group (LG) dwarf galaxy population predicted by the
\apostle $\Lambda$CDM cosmological hydrodynamics simulations. These indicate
that: (i)~the total mass within $3$ Mpc of the Milky Way-Andromeda midpoint
($M_{\rm 3Mpc}$) typically exceeds $\sim 3$ times the sum of the virial masses
($M_{\rm 200crit}$) of the two primaries and (ii)~the dwarf galaxy formation
efficiency per unit mass is uniform throughout the volume. This suggests that
the satellite population within the virial radii of the Milky Way and Andromeda
should make up fewer than one third of all LG dwarfs within $3$ Mpc. This is
consistent with the fraction of observed LG galaxies with stellar mass
$M_*>10^7\,M_{\odot}$ that are satellites ($12$ out of $42$; i.e., $28$ per
cent). For the \apostle galaxy mass-halo mass relation, the total number of
such galaxies further suggests a LG mass of $M_{\rm 3 Mpc}\sim 10^{13} \,
M_{\odot}$. At lower galaxy masses, however, the observed satellite fraction is
substantially higher ($42$ per cent for $M_*>10^5\,M_{\odot}$). If this is due
to incompleteness in the field sample, then $\sim 50$ dwarf galaxies at least
as massive as the Draco dwarf spheroidal must be missing from the current LG
{\it field} dwarf inventory. The incompleteness interpretation is supported by
the pronounced flattening of the LG luminosity function below $M_*\sim 10^7\,
M_{\odot}$, and by the scarcity of low-surface brightness LG field galaxies
compared to satellites. The simulations indicate that most missing dwarfs
should lie near the virial boundaries of the two LG primaries, and predict a
trove of nearby dwarfs that await discovery by upcoming wide-field imaging
surveys.
