The Eating Habits of Milky Way-mass Halos: Destroyed Dwarf Satellites and the Metallicity Distribution of Accreted Stars

We study the mass spectrum of destroyed dwarfs that contribute to the accreted stellar mass of Milky Way (MW)-mass (M_vir ∼ 10^12.1 M_⊙) halos using a suite of 45 zoom-in dissipationless simulations. Empirical models are employed to relate (peak) subhalo mass to dwarf stellar mass, and we use constraints from z = 0 observations and hydrodynamical simulations to estimate the metallicity distribution of the accreted stellar material. The dominant contributors to the accreted stellar mass are relatively massive dwarfs with M_star ∼ 10⁸-10¹⁰M_⊙. Halos with more quiescent accretion histories tend to have lower mass progenitors (10⁸-10⁹ M_⊙), and lower overall accreted stellar masses. Ultra-faint mass (M_star < 10⁵ M_⊙) dwarfs contribute a negligible amount (≪1%) to the accreted stellar mass and, despite having low average metallicities, supply a small fraction (∼2%-5%) of the very metal-poor stars with [Fe/H] < -2. Dwarfs with masses 10⁵ < M_star/M_⊙ < 10⁸ provide a substantial amount of the very metal-poor stellar material (∼40%-80%), and even relatively metal-rich dwarfs with M_star > 10⁸ M_⊙ can contribute a considerable fraction (∼20%-60%) of metal-poor stars if their metallicity distributions have significant metal-poor tails. Finally, we find that the generic assumption of a quiescent assembly history for the MW halo seems to be in tension with the mass spectrum of its surviving dwarfs. We suggest that the MW could be a “transient fossil” a quiescent halo with a recent accretion event(s) that disguises the preceding formation history of the halo.