The Local Group of galaxies holds a unique place in our exploration of the Universe. Modern deep surveys provide a continuous stream of high-precision measurements, including distances, the motions of stars and galaxies, the discovery of new satellites, and the detection of faint structural features. Data of comparable quality remain inaccessible beyond the Local Group, making it a unique laboratory for probing the evolution of stellar systems, the distribution of dark matter, and its role in shaping galactic dynamics.

    Using the most comprehensive collection of data to date on nearby galaxies, we carried out a detailed study of the velocity field in the neighbourhood of the Milky Way and the Andromeda galaxy. This analysis provides a much clearer picture of the dynamics within the Local Group. We obtained new, robust estimates of the total masses of both the Milky Way and Andromeda within their virial boundaries. For the first time, we show that galaxies on the outskirts of the Local Group form an exceptionally “cold” flow, with a remarkably small spread in radial velocities of just 15 km/s—far lower than predicted by standard cosmological models. This discrepancy opens new directions for both theoretical and observational research. This cold Hubble flow extends down to the boundaries of the virial zones of the Milky Way and Andromeda Galaxy. This allowed us to measure the total mass of the Local Group, (M_LG = 2.47×10¹² M⊙), with a precision of about 6% across a wide range of distances. Remarkably, this total mass is fully consistent with the sum of masses of the Milky Way and Andromeda Galaxy derived from the motions of their satellites. Therefore, our results suggest that nearly all mass of the Local Group is concentrated within the virial regions of these two giant spiral galaxies, placing strong constraints on the distribution of the dark matter in the system.

Fig. 1. The Hubble Flow in the Local Group. View from the barycenter of the system.

Fig. 2. The Hubble Flow in the Local Group. View from the barycenter of the system.

The work was carried out within the framework of the Russian Science Foundation grant 24-12-00277.

Published:
1. Makarov Danila, Makarov Dmitry, Makarova Lidia, Libeskind Noam, “The frozen outskirts: A cold Hubble flow and the mass of the Local Group”, 2025, Astronomy & Astrophysics, 698, id.A178, pp.8
2. Makarov Danila, Makarov Dmitry, Kozyrev Kirill, Libeskind Noam, “Line-of-Sight Mass Estimator and the Masses of the Milky Way and Andromeda Galaxy”, 2025, Universe, 11, id.144