Expanded haloes, abundance matching and too-big-to-fail in the local group

Abstract

Observed kinematical data of 40 Local Group (LG) members are used to derive the dark matter halo mass of such galaxies. Haloes are selected from the theoretically expected LG mass function and two different density profiles are assumed, a standard universal cuspy model and a mass-dependent profile which accounts for the effects of baryons in modifying the dark matter distribution within galaxies. The resulting relations between stellar and halo mass are compared with expectations from abundance matching. Using a universal cuspy profile, the ensemble of LG galaxies is fit in relatively low-mass haloes, leaving 'dark' many massive haloes of Mhalo ≳ 1010 M⊙: this reflects the 'too-big-to-fail' problem and results in a Mstar- Mhalo relation that differs from abundance matching predictions. Moreover, the star formation efficiency of isolated LG galaxies increases with decreasing halo mass when adopting a cuspy model. By contrast, using the mass-dependent density profile, dwarf galaxies with Mstar ≳ 106 M⊙ are assigned to more massive haloes, which have a central cored distribution of dark matter: the 'too-big-to-fail' problem is alleviated, the resultant Mstar-Mhalo relation follows abundance matching predictions down to the completeness limit of current surveys, and the star formation efficiency of isolated members decreases with decreasing halo mass, in agreement with theoretical expectations. Finally, the cusp/core space of LG galaxies is presented, providing a framework to understand the non-universality of their density profiles.