Tracing the first stars and galaxies of the Milky Way

We use 30 high-resolution dark matter haloes of the Caterpillar simulation suite to probe the first stars and galaxies of Milky Way-mass systems. We quantify the environment of the high-z progenitors of the Milky Way and connect them to the properties of the host and satellites today. We identify the formation sites of the first generation of Population III (Pop III) stars (z similar to 25) and first galaxies (z similar to 22) with several different models based on a minimum halo mass. This includes a simple model for radiative feedback, the primary limitation of the model. Through this method we find approximately 23 000 +/- 5000 Pop III potentially star-forming sites per Milky Way-mass host, though this number is drastically reduced to similar to 550 star-forming sites if feedback is included. The majority of these haloes identified form in isolation (96 per cent at z = 15) and are not subject to external enrichment by neighbouring haloes (median separation similar to 1 kpc at z = 15), though half merge with a system larger than themselves within 1.5 Gyr. Using particle tagging, we additionally trace the Pop III remnant population to z = 0 and find an order of magnitude scatter in their number density at small (i.e. r < 5 kpc) and large (i.e. r > 50 kpc) galactocentric radii. We provide fitting functions for determining the number of progenitor minihalo and atomic cooling halo systems that present-day satellite galaxies might have accreted since their formation. We determine that observed dwarf galaxies with stellar masses below 10(4.6) M-circle dot are unlikely to have merged with any other star-forming systems.