Constraining the mass of the Local Group

The mass of the Local Group (LG) is a crucial parameter for galaxy formation theories. However, its observational determination is challenging - its mass budget is dominated by dark matter which cannot be directly observed. To meet this end the posterior distributions of the LG and its massive constituents have been constructed by means of constrained and random cosmological simulations. Two priors are assumed - the LCDM model that is used to set up the simulations and a LG model,which encodes the observational knowledge of the LG and is used to select LG-like objects from the simulations. The constrained simulations are designed to reproduce the local cosmography as it is imprinted onto the Cosmicflows-2 database of velocities. Several prescriptions are used to define the LG model, focusing in particular on different recent estimates of the tangential velocity of M31. It is found that (a) different $v_{tan}$ choices affect the peak mass values up to a factor of 2, and change mass ratios of $M_{M31}$ to $M_{MW}$ by up to 20%; (b) constrained simulations yield more sharply peaked posterior distributions compared with the random ones; (c) LG mass estimates are found to be smaller than those found using the timing argument; (d) preferred MW masses lie in the range of $(0.6 - 0.8)\times10^{12} M_{\odot}$ whereas (e) $M_{M31}$ is found to vary between $(1.0 - 2.0)\times10^{12} M_{\odot}$, with a strong dependence on the $v_{tan}$ values used.