Scientific achievements. Metagalaxy

2025

Extremely Cold Hubble Flow and the Mass of the Local Group

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 neighborhood 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.

The work was carried out within the framework of the Russian Science Foundation grant No. 24-12-00277.

Authors:
Makarov D.D., Makarov D.I., Makarova L.N. (SAO RAS), Libeskind N. (Leibniz Institut für Astrophysik Potsdam), Kozyrev K. (SAO RAS, KFU).

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.
Details

Detection of radio wave scattering screens towards the quasar 2005 + 403 with long-term RATAN-600 observations

Based on multifrequency measurements with the RATAN-600 radio telescope of the radio-bright quasar 2005 + 403, observed through thermal plasma in the gas-dust region of the Cygnus-X in the interstellar medium of the Milky Way, rare extreme scattering events (ESE) of the source's radio emission were detected for the first time. Three flux-density variations characteristic in shape were detected at frequencies of 4.8, 8, and 11.2 GHz in 2011, 2015, and 2020. These ESEs are approximated by a numerical model of a scattering screen-lens, which yields the following physical properties: the average angular and linear size of the lens 0.3 ± 0.1 mas, i.e., 0.6 ± 0.1 au, the lens proper motion 8.3 ± 0.7 mas per year, and its transverse velocity of 70 ± 6 km/s (the distance to the lens is 1.8 kpc). The electron density along the line of sight was 1200 ± 120 cm⁻³ and the mass of each lens is (0.8 ± 0.4) 10⁻¹⁵ M_⊙. Importantly, the intrinsic (pre-scattering) angular size of the quasar 2005 + 403 was reconstructed as 1.57 ± 0.14, 0.95 ± 0.08, and 0.68 ± 0.04 mas at frequencies 4.8, 8, and 11.2 GHz, respectively. Monitoring of the quasar 2005 + 403 in 2015–2016 first showed that the scattering lenses crossed the quasar's line of sight one after another. As a result, a clearer understanding of the spatial structure of the interstellar medium on the smallest scales was achieved, which in turn defines the undistorted radio properties of microquasars and quasars containing black holes in thermal plasma.

The work was carried out within the framework of the state assignment of SAO RAS.

Authors:
Koryukova T.A. (CrAO RAS, ASC LPI), Trushkin S.A. (SAO RAS), Pashchenko I.N. (ASC LPI), Pushkarev A.B. (CrAO RAS, ASC LPI).

Published:
Koryukova T.A., Trushkin S.A., Pashchenko I.N., Pushkarev A.B. Probing plasma scattering screens towards the quasar 2005 + 403 with long-term RATAN-600 observations, 2025, MNRAS, V. 542, Iss. 4, pp. 2733-2751.