ELVIS: Exploring the Local Volume in Simulations
  We introduce a set of high-resolution dissipationless simulations that model
the Local Group (LG) in a cosmological context: Exploring the Local Volume in
Simulations (ELVIS). The suite contains 48 Galaxy-size halos, each within
high-resolution volumes that span 2-5 Mpc in size, and each resolving thousands
of systems with masses below the atomic cooling limit. Half of the ELVIS galaxy
halos are in paired configurations similar to the Milky Way (MW) and M31; the
other half are isolated, mass-matched analogs. We find no difference in the
abundance or kinematics of substructure within the virial radii of isolated
versus paired hosts. On Mpc scales, however, LG-like pairs average almost twice
as many companions and the velocity field is kinematically hotter and more
complex. We present a refined abundance matching relation between stellar mass
and halo mass that reproduces the observed satellite stellar mass functions of
the MW and M31 down to the regime where incompleteness is an issue, $M_\star
\sim 5\times 10^5 \, M_\odot$. Within a larger region spanning approximately 3
Mpc, the same relation predicts that there should be $\sim$ 1000 galaxies with
$M_\star > 10^{3}\,M_\odot$ awaiting discovery. We show that up to 50% of halos
within 1 Mpc of the MW or M31 could be systems that have previously been within
the virial radius of either giant. By associating never-accreted halos with
gas-rich dwarfs, we show that there are plausibly 50 undiscovered dwarf
galaxies with HI masses $> 10^5\,M_\odot$ within the Local Volume. The radial
velocity distribution of these predicted gas-rich dwarfs can be used to inform
follow-up searches based on ultra-compact high-velocity clouds found in the
ALFALFA survey.
