The Binary Ballet: Mapping Local Expansion Around M81 & M82

TL;DR

The study leverages new TRGB-based distances for the M81 complex to map its local Hubble flow and constrain the system's total mass. It combines velocity corrections to the Local Group frame with minor and major infall models around a barycentre, and uses a 58-galaxy TRGB sample to jointly estimate $H_0$ and $M$. The resulting Hubble-flow mass is $M_{HFF} = (2.28 \pm 0.49) \times 10^{12} M_\odot$ with $H_0 = (62.6 \pm 5.4)$ km s$^{-1}$ Mpc$^{-1}$, while projected and virial masses are $(2.74 \pm 0.36) \times 10^{12} M_\odot$ and $(3.11 \pm 0.69) \times 10^{12} M_\odot$, respectively. The findings support a filamentary environment for the M81 complex, demonstrate consistency with Planck within uncertainties, and provide a refined local-universe benchmark for testing cosmology on small scales.

Abstract

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.

Figures (9)

• Figure 1: Histogram of distances for all 71 galaxies with respect to M 81 as the centre (black bars) and only those 57 galaxies having TRGB-based distances (red bars). The bin size is 0.25 Mpc.
• Figure 2: Locations of the 72 galaxies having velocity measurements: physical positions in supergalactic coordinates in the SGX-SGY-plane (left), SGX-SGZ-plane (centre), and SGY-SGZ-plane (right). Galaxies in the environment are marked as "field" (grey pluses), galaxies having M 81, M 82, NGC 3077 or NGC 2976 as their major disturber are summarised under "M81 complex" (black dots), the galaxy attributed to Holm II as its major disturber and Holm II itself are called the "HolmII system" (red xs), NGC 2403 and two satellites galaxies are called the "N2403 system" (yellow squares). M 81, Holm II, and NGC 2403 are additionally highlighted by green stars. Larger symbols indicate galaxies having $\Theta_1 < 0$ and thus being unbound.
• Figure 3: (Movie online, left plot) Filamentary structure of the local M 81-group including all galaxies of Table \ref{['tab:M81-environment']}: Determining the directions of smallest extent for all 103 galaxies anchored at their mean location, the normal vector to the grey plane is obtained. An analogous calculation for the volume in a 3 Mpc radius around M 81 yields the red plane and its normal direction. For the volume in the 0.25 Mpc radius around M 81, the blue plane and its normal direction are obtained. The former two planes are becoming more aligned to the SGX-SGY-plane for increasing volume size. For further orientation, the most luminous galaxies in our data set are marked in colour and their names are attached. The right plot shows a projection into the plane of SGL and the distance from us, also showing the locations of the mean, median and the centre of mass of the six most luminous galaxies.
• Figure 4: Relative location and motion of a galaxy $j$ at distance $r_j$ from an observer with respect to the centre of mass at distance $r_\mathrm{c}$. The vector $\boldsymbol{v}_j$ represents the line-of-sight velocity of galaxy $j$, while $\boldsymbol{v}_\mathrm{c}$ is the line-of-sight velocity of the barycentre. The angle $\theta$ is measured between $\boldsymbol{r}_j$ and $\boldsymbol{r}_\mathrm{c}$. The prerequisites for the two infall models to accurately describe the radial velocity $\boldsymbol{v}_\mathrm{rad}$ are listed in the table.
• Figure 5: Dependence of the Hubble flow fit on the minimum distance from the barycentre for the minor (left column) and major infall models (right column). The first and third rows have M 81 as the centre of mass, the second and fourth row use the barycentre located on the connection line between M 81 and M 82, the first and second row use $v_\mathrm{hel}$, the third and fourth row use $v_\mathrm{LG}$. For the major infall model, velocities up to a maximum value of 700 km/s were included in the fit.