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

¹ Special Astrophysical Observatory, Russian Academy of Sciences, Nizhnij Arkhyz 369167, Russia
² Leibniz Institut für Astrophysik Potsdam (AIP), An der Sternwarte 16, D-14482 Potsdam, Germany

^⋆ Corresponding author: dim@sao.ru

Received: 26 March 2025
Accepted: 12 May 2025

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⁻¹, which differs significantly from the predictions of cosmological simulations of about 70 km s⁻¹. This fact allows us to determine the total mass of the Local Group as M_LG = (2.47 ± 0.15)×10¹² M_⊙ using an analytical model of the Hubble flow around a spherical overdensity in the standard flat ΛCDM 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 us 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 ± 0.10. The velocity of our Galaxy to the barycenter turns out to be 62.6 ± 2.6 km s⁻¹. 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° ±0.7° , − 2.7° ±0.3° ,301 ± 3 km s⁻¹) in Galactic coordinates.