NIHAO V: too big does not fail - reconciling the conflict between ΛCDM predictions and the circular velocities of nearby field galaxies

We compare the half-light circular velocities, V_1/2, of dwarf galaxies in the Local Group to the predicted circular velocity curves of galaxies in the Numerical Investigations of Hundred Astrophysical Objects (NIHAO) suite of Λ cold dark matter (ΛCDM) simulations. We use a subset of 34 simulations in which the central galaxy has a stellar luminosity in the range 0.5 × 10⁵ < L_V/ L_⊙ < 2 × 10⁸. The NIHAO galaxy simulations reproduce the relation between stellar mass and halo mass from abundance matching, as well as the observed half-light size versus luminosity relation. The corresponding dissipationless simulations overpredict the V_1/2, recovering the problem known as too big to fail (TBTF). By contrast, the NIHAO simulations have expanded dark matter haloes, and provide an excellent match to the distribution of V_1/2 for galaxies with L_V ≳ 2 × 10⁶ L_⊙. For lower luminosities, our simulations predict very little halo response, and tend to overpredict the observed circular velocities. In the context of ΛCDM, this could signal the increased stochasticity of star formation in haloes below M_halo ∼ 10¹⁰ M_⊙, or the role of environmental effects. Thus, haloes that are `TBTF', do not fail ΛCDM, but haloes that are `too small to pass' (the galaxy formation threshold) provide a future test of ΛCDM.