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 similar to 10(12.1) M-circle dot) 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 similar to 10(8)-10(10)M(circle dot). Halos with more quiescent accretion histories tend to have lower mass progenitors (10(8)-10(9)M(circle dot)), and lower overall accreted stellar masses. Ultra-faint mass (M-star < 10(5)M(circle dot)) dwarfs contribute a negligible amount (<< 1%) to the accreted stellar mass and, despite having low average metallicities, supply a small fraction (similar to 2%-5%) of the very metal-poor stars with [Fe/H] < -2. Dwarfs with masses 10(5) < M-star/M-circle dot < 10(8) provide a substantial amount of the very metal-poor stellar material (similar to 40%-80%), and even relatively metal-rich dwarfs with M-star > 10(8) M-circle dot can contribute a considerable fraction (similar to 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.