The earliest stars and their relics in the Milky Way

We have implemented a simple model to identify the likely sites of the first stars and galaxies in the high-resolution simulations of the formation of galactic dark matter halos of the Aquarius Project. The first star in a galaxy like the Milky Way formed around redshift z = 35; by z = 10, the young galaxy contained up to ∼ 3 × 10 dark matter haloes capable of forming stars by molecular hydrogen cooling. These minihaloes were strongly clustered and feedback may have severely limited the actual number of Population III stars that formed. By the present day, the remnants of the first stars would be strongly concentrated to the centre of the main halo. If a second generation of long-lived stars formed near the first (Population II.5), we would expect to find half of them within 30hkpc of the galactic centre and a significant fraction in satellites where they may be singled out by their anomalous metallicity patterns. The first halo in which gas could cool by atomic hydrogen line radiation formed at z = 25; by z = 10, the number of such ‘first galaxies’ had increased to ∼ 300. Feedback might have decreased the number of first galaxies at the time they undergo significant star formation, but not the number that survive to the present because near neighbours merge. Half of all the “first galaxies” that form before z = 10 merge with the main halo before z ∼ 3 and most lose a significant fraction of their mass. However, today there should still be more than 20 remnants orbiting within the central ∼ 30hkpc of the Milky Way. These satellites have circular velocities of a few kilometers per second or more, comparable to those of known Milky Way dwarfs. They are a promising hunting ground for the remnants of the earliest epoch of star formation.