Too big to fail in the Local Group

Abstract

We compare the dynamical masses of dwarf galaxies in the Local Group (LG) to those of haloes in the ELVIS (Exploring the Local Volume in Simulations) suite of Λ cold dark matter simulations. We enumerate unaccounted-for, dense haloes (V_(ma)x ≳ 25 km s^(−1)) that became massive enough to have formed stars in the presence of an ionizing background (V_(peak) > 30 km s^(−1)). Within 300 kpc of the Milky Way, the number of these objects ranges from 2 to 25 over our full sample. Moreover, this 'too big to fail' count grows when extended to the outer regions of the LG: there are 12–40 unaccounted-for massive haloes in the outskirts of the LG, a region that should be largely unaffected by any environmental processes. According to models that reproduce the LG stellar mass function, all of these missing massive systems should have M_★ > 10^6 M_⊙. We find, unexpectedly, that there is no obvious trend in the M_★−V_(max) relation for LG field galaxies with stellar masses in the range of ∼ 10^5 − 10^8 M_⊙. Solutions to the too big to fail problem that rely on ram pressure stripping, tidal effects, or statistical flukes appear less likely in the face of these results.

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