2016ApJ...818...10G

We present the largest number of Milky Way sized dark matter halos simulated at very high mass (∼10⁴M_☉/particle) and temporal resolution (5 Myr/snapshot) done to date, quadrupling what is currently available in the literature. This initial suite consists of the first 24 halos of the Caterpillar Project whose project goal of 60-70 halos will be made public when complete. We do not bias our halo selection by the size of the Lagrangian volume. We resolve ∼20,000 gravitationally bound subhalos within the virial radius of each host halo. Improvements were made upon current state-of-the-art halo finders to better identify substructure at such high resolutions, and on average we recover ∼4 subhalos in each host halo above 10⁸ M_☉ which would have otherwise not been found. The density profiles of relaxed host halos are reasonably fit by Einasto profiles (α = 0.169±0.023) with dependence on the assembly history of a given halo. Averaging over all halos, the substructure mass fraction is f_m,subs = 0.121 ± 0.041, and mass function slope is dN/dM ∝ M1.88 ± 0.10^. We find concentration-dependent scatter in the normalizations at fixed halo mass. Our detailed contamination study of 264 low-resolution halos has resulted in unprecedentedly large high-resolution regions around our host halos for our fiducial resolution (sphere of radius ∼ 1.4 ± 0.4 Mpc). This suite will allow detailed studies of low mass dwarf galaxies out to large galactocentric radii and the very first stellar systems at high redshift (z ≥ 15).