The Binary Ballet: Mapping Local Expansion Around M81 & M82

This study of the M81 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 M81 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, M81 and M82, 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 M81 complex. The joint best-fit parameters from both models yield $H_0 = \left(63 \pm 6 \right)$ km/s/Mpc and total mass of $(2.28\pm 0.49) \times 10^{12} M_{\odot}$. We thus arrive at an increased mass estimate compared to prior work but reach a higher consistency with virial, $(2.74 \pm 0.36)\times 10^{12},M_\odot$, and projected-mass estimates, $(3.11 \pm 0.69)\times 10^{12} M_\odot$. Moreover, our $H_0$ estimate shows an agreement with Planck, consistent with other TRGB-based Local-Universe inferences of $H_0$ and still within a 2-$σ$ agreement with Cepheid-based Local-Universe probes.

💡 Research Summary

This paper presents a comprehensive study of the M81 galaxy complex, a prominent group in the nearby universe, leveraging an expanded and refined dataset of precise distances to map its dynamics, environmental context, and the local Hubble flow. The core of the analysis is built upon Tip of the Red Giant Branch (TRGB)-based distances for 58 member and nearby galaxies, including new measurements for nine key objects like M82. This homogeneous high-precision dataset enables a robust dynamical investigation.

The study begins by characterizing the sample. From an initial 98 galaxies, 72 with velocity measurements are analyzed, with the 58 TRGB-based distances forming the primary set. Using the concept of a “Major Disturber” and tidal index (Θ1), galaxies are grouped into the core M81 complex (31 galaxies), the Holm II system, and the NGC 2403 system. Spatial analysis in supergalactic coordinates reveals a striking alignment: the satellite galaxies of the M81 complex exhibit a flattened, planar distribution that is oriented almost perpendicular to the supergalactic pole (SGZ-axis). This alignment persists when analyzing a larger region up to 3 Mpc around M81, indicating that the group’s structure is co-aligned with a larger-scale filament in the Local Universe, connecting to structures mapped in other surveys.

Prior to dynamical modeling, crucial preprocessing steps are applied. Heliocentric velocities are corrected for the Sun’s motion within the Local Group to obtain velocities in the Local Group rest frame (v_LG), which reduces the group’s internal velocity dispersion. The barycenter (center of mass) of the M81 complex is calculated, and it is demonstrated that its position and velocity are robustly constrained by just the two brightest members, M81 and M82.

The central dynamical analysis involves fitting the local Hubble flow around the group using two independent “infall models”: a minor infall model and a major infall model. These models describe how galaxies fall into the group’s gravitational potential, imprinting a characteristic pattern on their recession velocities as a function of distance from the group’s center. By fitting this pattern, both the total mass of the M81 complex and the Hubble Constant (H0) can be constrained simultaneously from the local kinematics.

The joint best-fit parameters from both models yield: H0 = (63 ± 6) km/s/Mpc and a total mass for the M81 complex of M = (2.28 ± 0.49) × 10^12 M⊙. This mass estimate is higher than some previous works but shows good consistency with independent dynamical mass estimators applied to the same data: the virial mass estimator gives (2.74 ± 0.36) × 10^12 M⊙ and the projected-mass estimator gives (3.11 ± 0.69) × 10^12 M⊙.

The derived value of H0 = 63 ± 6 km/s/Mpc is consistent with the Planck satellite’s measurement from the Cosmic Microwave Background and with other TRGB-based measurements in the Local Universe. It remains within 2-sigma agreement with higher values derived from Cepheid variable stars in the Local Universe, thus contributing a precise data point from a nearby galaxy group to the ongoing investigation of the Hubble tension.

In summary, this work provides a detailed kinematic portrait of the M81 complex. It confirms the planar alignment of its satellites within a larger cosmic filament, refines the group’s barycenter and mass estimate, and delivers a constraint on the Hubble Constant from local expansion dynamics. The study underscores the value of precise TRGB distances and sophisticated dynamical modeling of nearby groups as critical tools for testing cosmology and understanding galaxy assembly on small scales.