The frozen outskirts: a cold Hubble flow and the mass of the Local Group

Abstract

We analyze the velocity field of peripheral members of the Local Group. The Hubble flow at distances from 400 to 1400~kpc, formed by 7 of 11 nearby galaxies, is characterized by an extremely small line-of-sight velocity dispersion of 15 km/s, which differs significantly from the predictions of cosmological simulations of about 70 km/s. This fact allows us to determine the total mass of the Local Group as $M_{LG} = (2.47 \pm 0.15) \times 10^{12}$ $M_\odot$ using an analytical model of the Hubble flow around a spherical overdensity in the standard flat \LCDM{} universe. The practical equality of this mass to the sum of the masses of our Galaxy and the Andromeda Galaxy, as well as the absence of mass growth in the range of distances under consideration, gives grounds to conclude that the entire mass of the Local Group is confined within the virial radii around its two main galaxies. The barycenter, found from the minimal scatter of mass estimates, corresponds to the mass ratio of the Milky Way and the Andromeda Galaxy equal to $M_{MW}/M_{M31} = 0.74\pm0.10$. The velocity of our Galaxy to the barycenter turns out to be $62.6\pm2.6$ km/s. This allows us to determine the apex of the Sun relative to the barycenter of the Local Group to be $(l,b,V) = ( +94.0^\circ \pm 0.7^\circ, -2.7^\circ \pm 0.3^\circ, 301 \pm 3$ km/s in the Galactic coordinates.