The Hubble flow around the Local Group

Abstract

We use updated data on distances and velocities of galaxies in the proximity of the Local Group (LG) in order to establish properties of the local Hubble flow. For 30 neighbouring galaxies with distances 0.7 < D_LG < 3.0 Mpc, the local flow is characterized by the Hubble parameter H_loc = (78 +/- 2)kms^-1 Mpc^-1, the mean-square peculiar velocity σ_v = 25kms^-1, corrected for errors of radial velocity measurements (~4kms^-1) and distance measurements (~10kms^-1), as well as the radius of the zero-velocity surface R₀ = (0.96 +/- 0.03) Mpc. The minimum value for σ_v is achieved when the barycentre of the LG is located at the distance D_c = (0.55 +/- 0.05) D_M31 towards Andromeda galaxy (M31) corresponding to the Milky Way (MW)-to-M31 mass ratio M_MW/M_M31 ~= 4/5. In the reference frame of the 30 galaxies at 0.7-3.0 Mpc, the LG barycentre has a small peculiar velocity ~(24 +/- 4)kms^-1 towards the Sculptor constellation. The derived value of R₀ corresponds to the total mass M_T(LG) = (1.9 +/- 0.2) 10¹² M_solar with Ω_m = 0.24 and a topologically flat universe, a value in good agreement with the sum of virial mass estimates for the MW and M31.