Organized Chaos: Scatter in the relation between stellar mass and halo mass in small galaxies

Shea Garrison-Kimmel; James S. Bullock; Michael Boylan-Kolchin; Emma Bardwell

doi:10.1093/mnras/stw2564

paper

Organized Chaos: Scatter in the relation between stellar mass and halo mass in small galaxies

arXiv:1603.04855 · doi:10.1093/mnras/stw2564

Abstract

We use Local Group galaxy counts together with the ELVIS N-body simulations to explore the relationship between the scatter and slope in the stellar mass vs. halo mass relation at low masses, $M_{⋆} ≃ 1 0^{5} - 1 0^{8} M_{⊙}$ . Assuming models with log-normal scatter about a median relation of the form $M_{⋆} \propto M_{halo}^{α}$ , the preferred log-slope steepens from $α ≃ 1.8$ in the limit of zero scatter to $α ≃ 2.6$ in the case of $2$ dex of scatter in $M_{⋆}$ at fixed halo mass. We provide fitting functions for the best-fit relations as a function of scatter, including cases where the relation becomes increasingly stochastic with decreasing mass. We show that if the scatter at fixed halo mass is large enough ( $≳ 1$ dex) and if the median relation is steep enough ( $α ≳ 2$ ), then the "too-big-to-fail" problem seen in the Local Group can be self-consistently eliminated in about $\sim 5 - 10%$ of realizations. This scenario requires that the most massive subhalos host unobservable ultra-faint dwarfs fairly often; we discuss potentially observable signatures of these systems. Finally, we compare our derived constraints to recent high-resolution simulations of dwarf galaxy formation in the literature. Though simulation-to-simulation scatter in $M_{⋆}$ at fixed $M_{halo}$ is large among separate authors ( $\sim 2$ dex), individual codes produce relations with much less scatter and usually give relations that would over-produce local galaxy counts.

15 pages, 6 figures, 1 table. Accepted for publication into MNRAS

Organized Chaos: Scatter in the relation between stellar mass and halo mass in small galaxies

Abstract

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