Tango of Titans: Centaurus A and M83 as a Local Group Analog

TL;DR

The study investigates whether the nearby Cen A–M83 pair behaves as a bound, Local Group–like system. It extends the Timing Argument to a two-body system in a $\Lambda$CDM background, calibrated with AbacusSummit analogs, and employs two projection schemes to infer the radial motion from line-of-sight data. The PCM approach yields a timing-mass of $M_{\rm TA,PCM}=(6.36\pm1.31)\times10^{12}\,M_\odot$, consistent with virial and $K$-band mass estimates when combined with a $K$-band prior to give $M_{\rm total}=(8.26\pm1.18)\times10^{12}\,M_\odot$, indicating a bound or marginally bound configuration. This work establishes Cen A–M83 as a robust nearby analog to the Local Group and demonstrates the TA’s applicability to proximate galaxy pairs, with NGC 4945 discussed as a perturber within Cen A’s complex.

Abstract

Centaurus A (CenA) and M83 form one of the most massive galaxy pairs in the nearby Universe. Although their observed heliocentric velocities suggest motion that is not obviously indicative of mutual attraction, this work presents evidence that CenA and M83 are in fact infalling toward each other, exhibiting a dynamical interaction analogous to the binary-like motion of the Milky Way and Andromeda in the Local Group (LG). Using the Timing Argument (TA), calibrated with analog galaxy pairs from the AbacusSummit simulation, we estimate the total mass of the CenA/M83 system under the assumption that the line-of-sight (LoS) velocity is dominated by motion toward the system's barycenter. This yields a total mass of $(6.36 \pm 1.30) \cdot 10^{12}\, M_\odot$. The inferred mass agrees well with independent estimates based on virial mass measurements and $K$-band luminosity--to-mass ratios. Together, the consistent bound signature and robust mass determination highlight the CenA/M83 system as a compelling nearby analog to the LG. Further discussion of NGC 4945 as a main perturber (as the Large Magellanic Could) for the CenA is also discussed.

Figures (4)

• Figure 1: Relative motion of two galaxies, labeled galaxy 1 and galaxy 2. The vector $v_1$ ($v_2$) represents the velocity of galaxy 1 (galaxy 2). The angle $\theta$ is measured between $r_1$ and the line connecting the two galaxies. $v_c$ is the LoS velocity of the CoM of the pair. The assumptions for different infall models are shown in the top left.
• Figure 2: Inferred quantities for the Cen A--M83 radial motion.
• Figure 3: Kinematic properties of the simulated pair sample. The panels show the radial velocity cuts used to define the PCM (green) and minor (blue) candidates (left); the velocity anisotropy distribution (middle); and the resulting $M_{TA}$ vs. $M_{200}$ relation for each group (right).
• Figure 4: Total mass estimates for the Cen A--M 83 galaxy system from the virial, projected mass, Hubble-flow, and TA methods. The PCM solution agrees well with dynamical and stellar-based estimates, while the minor solution aligns more closely with Hubble-flow prediction.