The Stellar-to-Halo Mass Relation for Local Group Galaxies

We contend that a single power-law halo mass distribution is appropriate for direct matching to the stellar masses of observed Local Group dwarf galaxies, allowing the determination of the slope of the stellar mass–halo mass relation for low-mass galaxies. Errors in halo masses are well defined as the Poisson noise of simulated Local Group realizations, which we determine using local volume simulations. For the stellar mass range 10 7 M ☉ < M * < 10 8 M ☉ , for which we likely have a complete census of observed galaxies, we find that the stellar mass–halo mass relation follows a power law with slope of 3.1, significantly steeper than most values in the literature. This steep relation between stellar and halo masses would indicate that Local Group dwarf galaxies are hosted by dark matter halos with a small range of mass. Our methodology is robust down to the stellar mass to which the census of observed Local Group galaxies is complete, but the significant uncertainty in the currently measured slope of the stellar-to-halo mass relation will decrease dramatically if the Local Group completeness limit was 10 6.5 M ☉ or below, highlighting the importance of pushing such limit to lower masses and larger volumes.