Title: It's the little things: ultra-faint satellites of isolated dwarf galaxies

Abstract:
In the currently favored cosmological paradigm, all galaxies are embedded within dark matter 
halos that are themselves filled with smaller dark matter clumps in a hierarchical manner. 
Observationally verifying the existence of these small clumps is one of the most important goals 
in modern cosmology. We expect that the smallest should be largely free of stars because the 
ambient ionizing background prevents them from accreting much gas. But below what mass do 
we expect all subhalos to be completely dark? We run ultra-high resolution zoom-in simulations 
of isolated dwarf galaxies with GIZMO (Hopkins 2014), a state-of-the-art code that utilizes the 
highly sophisticated FIRE (Feedback in Realistic Environments) recipes (Hopkins et al. 2014) 
for converting gas into stars and capturing the energy fed back from those stars into the 
surrounding medium. Using these simulations, we predict that ultra-faint satellite galaxies (M* ~ 
3,000 Msun) should exist around isolated dwarf galaxies (M* ~ 1e6 Msun). These satellites 
survive the ionizing background radiation by forming their stars in the first billion years after the 
Big Bang. Importantly, we show that these tiny galaxies should be observable in the local 
universe. If they are detected, it would provide evidence that dark matter substructure is truly 
hierarchical, as predicted in the standard paradigm.