Where Are the Fossils of the First Galaxies? I. Local Volume Maps and Properties of the Undetected Dwarfs

We present a new method for generating initial conditions for ΛCDM N -body simulations which provides the dynamical range necessary to follow the evolution and distribution of the fossils of the first galaxies on Local Volume, 5–10 Mpc, scales. The initial distribution of particles represents the position, velocity, and mass distribution of the dark and luminous halos extracted from pre-reionization simulations. We confirm previous results that ultra-faint dwarfs have properties compatible with being well-preserved fossils of the first galaxies. However, because the brightest pre-reionization dwarfs form preferentially in biased regions, they most likely merge into non-fossil halos with circular velocities >20–30 km s −1 . Hence, we find that the maximum luminosity of true fossils in the Milky Way is L V < 10 6 L ☉ , casting doubts on the interpretation that some classical dSphs are true fossils. In addition, we argue that most ultra-faints at small galactocentric distance, R < 50 kpc, had their stellar properties modified by tides, while a large population of fossils is still undetected due to their extremely low surface brightness log (Σ V ) < −1.4. We estimate that the region outside R 50 (∼400 kpc) up to 1 Mpc from the Milky Way contains about a hundred true fossils of the first galaxies with V -band luminosity 10 3 –10 5 L ☉ and half-light radii, r hl ∼ 100–1000 pc.