Expanded haloes, abundance matching and too-big-to-fail in the Local Group

TL;DR

This paper investigates the dark matter distribution in Local Group galaxies using kinematic data, comparing standard cuspy and baryon-influenced profiles to address the too-big-to-fail problem and match abundance predictions.

Contribution

It introduces a mass-dependent density profile accounting for baryonic effects, which alleviates the too-big-to-fail problem and aligns galaxy-halo relations with abundance matching.

Findings

Cuspy profiles fit low-mass haloes, leaving many massive haloes dark.

Mass-dependent profiles assign dwarf galaxies to more massive, cored haloes.

Using baryon-influenced profiles resolves the too-big-to-fail issue.

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$\gtrsim$10$^{10}$\msun: this reflects the "too big to fail" problem and results in a \mstar-\mhalo\ relation that differs from abundance matching predictions. Moreover, the star formation efficiency of isolated LG galaxies increases with decreasing halo mass when adopting a cuspy model. By contrast, using the mass dependent density profile, dwarf galaxies with \mstar$\gtrsim$10$^{6}$\msun 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.