Hubble Constant and Mass Determination of Centaurus A & M83 from TRGB Distances

TL;DR

By combining TRGB-based distances with LG-frame kinematics for CenA and M83, the study derives a dynamical constraint on the local Hubble constant and the total group mass. Using two limiting infall scenarios (minor and major), a Bayesian fit to the turnaround velocity-distance relation is performed, yielding a convergent estimate of the Hubble constant $H_0 = 64.0 \,\pm\ 4.6$ km s$^{-1}$ Mpc$^{-1}$ and a group mass $M_{ m group} = (2.6 \pm 1.4) \times 10^{12}$ M$_\odot$, while a virial-mass calculation gives $M_{ m vir} = (7.3 \pm 2.0) \times 10^{12}$ M$_\odot$. A K-band luminosity-based mass estimate produces $M_{\rm K-band} = (6.91 \pm 1.21) \times 10^{12}$ M$_\odot$, highlighting a ~2σ tension between the virial and Hubble-flow masses. The results emphasize the role of peculiar velocities in the Local Volume and illustrate that M83’s proximity to the velocity surface biases simple Hubble-flow mass estimates, prompting a preference for the virial mass in this system. Overall, the work provides an independent, dynamical cross-check of $H_0$ from local galaxy dynamics and demonstrates the value of combining multiple dynamical mass estimators with TRGB distances.

Abstract

An independent determination of the Hubble constant is crucial given the persistent tension between early- and late-Universe measurements. In this study, we analyze the dynamics of the Centaurus~A (CenA) and M83 galaxies, along with their associated dwarf companions identified via Tip of the Red Giant Branch (TRGB) distance measurements, to constrain both the group mass and the local value of the Hubble constant ($H_0$). By examining the motions of these galaxies relative to the system's barycenter, we apply both the minor and major infall models, which provide bounds on the true radial velocity dispersion. From the overlap of these approaches, we obtain a virial mass estimate of $(7.3 \pm 2.0) \times 10^{12}\,M_{\odot}$ and a Hubble flow-based mass of $(2.6 \pm 1.4) \times 10^{12}\,M_{\odot}$. Modeling the cold Hubble flow around the group center of mass yields a corresponding Hubble constant of $(64.0 \pm 4.6)\,\mathrm{km\,s^{-1}\,Mpc^{-1}}$. These results offer an independent, dynamically motivated constraint on the local value of $H_0$, explicitly accounting for the impact of peculiar velocities in the nearby Universe. We also discuss the $\sim 2σ$ tension between the virial and Hubble flow-based mass estimates, which likely arises from the proximity of M83 to the velocity surface, breaking the assumptions of the Hubble flow model. While the Hubble flow fit emphasizes galaxies that follow smooth expansion on the lower branch of the velocity-distance relation, the virial mass estimate is in good agreement with the group mass derived from the $K$-band luminosity of its brightest members and from projected mass methods.

Figures (7)

• Figure 1: Spatial distribution of galaxies in the CenA/M83 group shown in rotated Cartesian coordinates centered on the midpoint between Cen A and M83. Symbols are color–coded by heliocentric radial velocity (km s$^{-1}$). Dashed gray circles indicate the individual virial regions of Cen A and M83, and the black circle marks the 1.5 Mpc zero-velocity surface.
• Figure 2: Schematic illustration of barycenter ($c$) determination along the line connecting the CenA and M83 galaxies. For a given dwarf galaxy ($g$), the radial velocity is estimated using either the minor or major infall model. The barycenter is identified by minimizing the velocity dispersion of the radial velocities towards the barycenter. The angle $\theta$ is the angle between the angle between galaxy (g) and the center (c).
• Figure 3: Root mean square of the relative velocity dispersion, $\sqrt{\langle v^2 \rangle}$, with respect to the CenA/M83 system’s barycenter, shown as a function of the mass ratio $\bar{m}_{CenA} = m_{CenA} / m_{\mathrm{Tot}}$. Velocities are measured in the Local Group frame under the minor infall (blue) and major infall (red) models. Dotted lines: Raw results of $\sqrt{\langle v^2 \rangle}$ as a function of $\bar{m}{\mathrm{CenA}}$, mostly lying slightly below the smoothed (solid) curves, except for a local spike near $\bar{m}_{\mathrm{CenA}} \approx 0.85$ in the minor infall case. This spike is interpreted as a numerical bias arising from the clustering of galaxies near the CenA/M83 barycenter position (Muller:2021Muller:2025). Vertical dashed lines: Positions of the minima in $\sqrt{\langle v^2 \rangle}$ for each model, indicating the best-fitting mass ratio. To avoid confusion, these vertical lines are plotted with a distinct dashed style, different from the dotted raw data lines.
• Figure 4: The radial velocity vs. the 3D distance from the group barycenter for the minor (upper) and the major (lower) infall models. The Muse galaxies are shown in green and the galaxies that use for fit are shown in black. The Hubble Flow fit model is presented with $3\sigma$ error.
• Figure 5: Posterior distributions for the Hubble Constant $H_0$, the group mass $M\, [10^{12}\,\odot]$, and intrinsic velocity dispersion $\delta_I\,[km/s]$. The blue contour shows the minor infall model while the red shows major infall posterior.