The tangential velocity of M31: CLUES from constrained simulations

Abstract

Determining the precise value of the tangential component of the velocity of M31 is a nontrivial astrophysical issue that relies on complicated modelling. This has recently lead to conflicting estimates, obtained by several groups that used different methodologies and assumptions. This Letter addresses the issue by computing a Bayesian posterior distribution function of this quantity, in order to measure the compatibility of those estimates with Λ cold dark matter (Λ CDM). This is achieved using an ensemble of Local Group (LG) look-alikes collected from a set of constrained simulations (CSs) of the local Universe, and a standard unconstrained Λ CDM. The latter allows us to build a control sample of LG-like pairs and to single out the influence of the environment in our results. We find that neither estimate is at odds with Λ CDM; however, whereas CSs favour higher values of vtan, the reverse is true for estimates based on LG samples gathered from unconstrained simulations, overlooking the environmental element.