The binary ballet: Mapping local expansion around M 81 and M 82

¹ Helsinki Institute of Physics P.O. Box 64 FI-00014 University of Helsinki, Finland
² Academia Sinica Institute of Astronomy and Astrophysics, 11F of AS/NTU Astronomy-Mathematics Building Roosevelt Rd Taipei 106216, Taiwan, R.O.C.
³ Bahamas Advanced Study Institute and Conferences, 4A Ocean Heights, Hill View Circle Stella Maris Long Island, The Bahamas
⁴ Leibniz-Institut für Astrophysik Potsdam (AIP) An der Sternwarte 16 D-14482 Potsdam, Germany
⁵ Special Astrophysical Observatory, Russian Academy of Sciences, Nizhny Arkhyz Karachai-Cherkessian Republic 369167, Russia

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Received: 28 October 2025
Accepted: 13 December 2025

Abstract

This study of the M 81 complex and its Hubble flow delivers new and improved tip of the red giant branch (TRGB)-based distances for nine member galaxies, yielding a total of 58 galaxies with high-precision TRGB distances. With those, we perform a systematic analysis of the group’s dynamics in the core and its embedding in the local cosmic environment. Our analysis confirms that the satellite galaxies of the M 81 complex exhibit a flattened, planar distribution almost perpendicular to the supergalactic pole, and thus aligned with a larger-scale filamentary structure in the local Universe. We demonstrate that the properties of the group’s barycentre are robustly constrained by the two brightest members, M 81 and M 82, and that correcting heliocentric velocities for the solar motion in the Local Group decreases the velocity dispersion of the group. Then applying minor and major infall models, we fit the local Hubble flow to constrain the Hubble constant and the total mass of the M 81 complex. The joint best-fit parameters from both models yield H₀ = (63±6) km/s/Mpc and a total mass of (2.28 ± 0.49)×10¹² M_⊙. We thus arrive at an increased mass estimate compared to prior work but reach a higher consistency with virial ((2.74 ± 0.36)×10¹² M_⊙) and projected ((3.11 ± 0.69)×10¹² M_⊙) mass estimates. Moreover, our H₀ estimate shows an agreement with Planck, consistent with other TRGB-based local-Universe inferences of H₀ and still within a 2σ agreement with Cepheid-based local-Universe probes.