We present the largest number of Milky Way sized dark matter halos simulated at very high mass (∼ 10 4 M /particle) and temporal resolution (5 Myrs/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 a 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 stateof-the-art halo finders to better identify substructure at such high resolutions, and on average we recover ∼4 subhalos in each host halo above 10 8 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 ∝ M −1.88±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).

Recent discovery of many dwarf satellite galaxies in the direction of the Small and Large Magellanic Clouds (SMC and LMC) provokes questions of their origins, and what they can reveal about galaxy evolution theory. Here, we predict the satellite stellar mass function of Magellanic Cloud-mass host galaxies using abundance matching and reionization models applied to the Caterpillar simulations. Specifically focusing on the volume within 50 kpc of the LMC, we predict a mean of 4-8 satellites with stellar mass M * > 10 4 M , and 3-4 satellites with 80 < M * ≤ 3000 M . Surprisingly, all 12 currently known satellite candidates have stellar masses of 80 < M * ≤ 3000 M . Reconciling the dearth of large satellites and profusion of small satellites is challenging and may require a combination of a major modification of the M * − M halo relationship (steep, but with an abrupt flattening at 10 3 M ), late reionization for the Local Group (z reion 9 preferred), and/or strong tidal stripping. We can more robustly predict that ∼ 53% of satellites within this volume were accreted together with the LMC and SMC, and ∼ 47% were only ever Milky Way satellites. Observing satellites of isolated LMC-sized field galaxies is essential to placing the LMC in context, and to better constrain the M * − M halo relationship. Modeling known LMC-sized galaxies within 8 Mpc, we predict 1-6 (2-12) satellites with M * > 10 5 M (M * > 10 4 M ) within the virial volume of each, and 1-3 (1-7) within a single 1.5• diameter field of view, making their discovery likely.

A recent surge in the discovery of new ultrafaint dwarf satellites of the Milky Way has inspired the idea of searching for faint satellites, 10 3 M < M * < 10 6 M , around less massive field galaxies in the Local Group. Such satellites would be subject to weaker environmental influences than Milky Way satellites, and could lead to new insights on low mass galaxy formation. In this paper, we predict the number of luminous satellites expected around field dwarf galaxies by applying several abundance matching models and a reionization model to the dark-matter only Caterpillar simulation suite. For three of the four abundance matching models used, we find a > 99% chance that at least one satellite with stellar mass M * > 10 5 M exists around the combined five Local Group field dwarf galaxies with the largest stellar mass. When considering satellites with M * > 10 4 M , we predict a combined 5 − 25 satellites for the five largest field dwarfs, and 10 − 50 for the whole Local Group field dwarf population. Because of the relatively small number of predicted dwarfs, and their extended spatial distribution, a large fraction each Local Group dwarf's virial volume will need to be surveyed to guarantee discoveries. We compute the predicted number of satellites in a given field of view of specific Local Group galaxies, as a function of minimum satellite luminosity, and explicitly obtain such values for the Solitary Local dwarfs survey. Uncertainties in abundance matching and reionization models are large, implying that comprehensive searches could lead to refinements of both models.

We investigate the effects of self-interacting dark matter (SIDM) on the tidal stripping and evaporation of satellite galaxies in a Milky Way-like host. We use a suite of five zoom-in, dark-matter-only simulations, two with velocity-independent SIDM crosssections, two with velocity-dependent SIDM cross-sections, and one cold dark matter (CDM) simulation for comparison. After carefully assigning stellar mass to satellites at infall, we find that stars are stripped at a higher rate in SIDM than in CDM. In contrast, the total bound dark matter mass-loss rate is minimally affected, with subhalo evaporation having negligible effects on satellites for viable SIDM models. Centrally located stars in SIDM haloes disperse out to larger radii as cores grow. Consequently, the half-light radius of satellites increases, stars become more vulnerable to tidal stripping, and the stellar mass function is suppressed. We find that the ratio of core radius to tidal radius accurately predicts the relative strength of enhanced SIDM stellar stripping. Velocity-independent SIDM models show a modest increase in the stellar stripping effect with satellite mass, whereas velocity-dependent SIDM models show a large increase in this effect towards lower masses, making observations of ultrafaint dwarfs prime targets for distinguishing between and constraining SIDM models. Due to small cores in the largest satellites of velocity-dependent SIDM, no identifiable imprint is left on the all-sky properties of the stellar halo. While our results focus on SIDM, the main physical mechanism of enhanced tidal stripping of stars apply similarly to satellites with cores formed via other means.

The highly r-process-enhanced (r-II) metal-poor halo stars we observe today could play a key role in understanding early ultra-faint dwarf galaxies (UFDs), the smallest building blocks of the Milky Way. If a significant fraction of metal-poor r-II halo stars originated in the UFDs that merged to help form the Milky Way, observations of r-II stars could help us study these now-destroyed systems and probe the formation history of our Galaxy. To conduct our initial investigation into this possible connection, we use high-resolution cosmological simulations of Milky Way-mass galaxies from the Caterpillar suite in combination with a simple, empirically motivated treatment of r-process enrichment. We determine the fraction of metal-poor halo stars that could have formed from highly r-process-enhanced gas in now-destroyed low-mass UFDs, the simulated r-II fraction, and compare it to the "as observed" r-II fraction. We find that the simulated fraction, f r−II,sim ∼1%-2%, can account for around half of the "as observed" fraction, f r−II,obs ∼2%-4%. The "as observed" fraction likely overrepresents the fraction of r-II stars due to incomplete sampling, though, meaning f r−II,sim likely accounts for more than half of the true f r−II,obs . Further considering some parameter variations and scatter between individual simulations, the simulated fraction can account for around 20%-80% of the "as observed" fraction.