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

TL;DR

This study uses simulations and empirical models to analyze the mass spectrum and metallicity distribution of dwarf galaxies contributing to the Milky Way's stellar halo, revealing dominant contributors and implications for galaxy formation history.

Contribution

It provides new insights into the mass and metallicity contributions of dwarf satellites to the Milky Way's stellar halo, highlighting the role of relatively massive dwarfs and challenging the assumption of a purely quiescent accretion history.

Findings

Massive dwarfs (10^8-10^10 M_sun) dominate accreted stellar mass.

Ultra-faint dwarfs contribute less than 1% to the stellar halo.

Dwarfs with 10^5-10^8 M_sun supply 40-80% of very metal-poor stars.

Abstract

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_sun) 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 ~ 10^8-10^10 M_sun. Halos with more quiescent accretion histories tend to have lower mass progenitors (10^8-10^9 M_sun), and lower overall accreted stellar masses. Ultra-faint mass (M_star < 10^5 M_sun) 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 10^5 < M_star/M_sun < 10^8 provide a substantial amount of the very metal-poor stellar material (~40-80 %), and even relatively metal-rich dwarfs with M_star > 10^8 M_sun 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.