2017MNRAS.467.2019R

We fit the rotation curves of isolated dwarf galaxies to directly measure the stellar mass-halo mass relation (M_*-M₂₀₀) over the mass range 5 x 10⁵ <= M_*/M_☉ <= 10⁸. By accounting for cusp-core transformations due to stellar feedback, we find a monotonic relation with little scatter. Such monotonicity implies that abundance matching should yield a similar M_*-M₂₀₀ if the cosmological model is correct. Using the 'field galaxy' stellar mass function from the Sloan Digital Sky Survey (SDSS) and the halo mass function from the Λ cold dark matter Bolshoi simulation, we find remarkable agreement between the two. This holds down to M₂₀₀ ∼ 5 x 10⁹ M_☉, and to M₂₀₀ ∼ 5 x 10⁸ M_☉ if we assume a power-law extrapolation of the SDSS stellar mass function below M_* ∼ 10⁷ M_☉. However, if instead of SDSS we use the stellar mass function of nearby galaxy groups, then the agreement is poor. This occurs because the group stellar mass function is shallower than that of the field below M_* ∼ 10⁹ M_☉, recovering the familiar 'missing satellites' and 'too big to fail' problems. Our result demonstrates that both problems are confined to group environments and must, therefore, owe to 'galaxy formation physics' rather than exotic cosmology. Finally, we repeat our analysis for a Λ Warm Dark Matter cosmology, finding that it fails at 68 per cent confidence for a thermal relic mass of m_WDM < 1.25 keV, and m_WDM < 2 keV if we use the power-law extrapolation of SDSS. We conclude by making a number of predictions for future surveys based on these results.