Physics of galactic colliders: high speed satellites in LCDM vs MONDian cosmology

The statistics of high speed satellite galaxies, as reported in the recent literature, can be a powerful diagnosis of the depth of the potential well of the host halo, and hence discriminate between competing gravitational theories. Naively one expects that high speed satellites are more common in Modified Newtonian Dynamics (MOND) than in cold dark matter (CDM) since an isolated MONDian system has an infinite potential well, while CDM halos have finite potential wells. In this \textit{Letter} we report on an initial test of this hypothesis in the context of the first generation of cosmological simulations utilising a rigorous MONDian Poisson solver. We find that such high speed encounters are approximately a factor of four more common in MOND than in the concordance $\Lambda$CDM model of cosmic structure formation.

💡 Research Summary

The paper investigates whether the frequency of high‑speed satellite galaxies can serve as a discriminant between the standard ΛCDM cosmology and Modified Newtonian Dynamics (MOND). The authors begin by noting that an isolated MOND system possesses an effectively infinite gravitational potential well, whereas a cold‑dark‑matter (CDM) halo has a finite depth. Consequently, one would naively expect that satellites attaining velocities comparable to or exceeding the host’s escape speed should be more common in a MOND universe.

To test this hypothesis, the authors employ the first generation of cosmological simulations that incorporate a rigorous MONDian Poisson solver. They generate two parallel simulation suites: a ΛCDM run using the conventional Newtonian Poisson equation with dark‑matter particles, and a MOND run that solves the non‑linear AQUAL (Aquadratic Lagrangian) Poisson equation on a particle‑mesh grid. Both runs share identical initial conditions derived from the Planck 2015 power spectrum, a cubic volume of (100 Mpc)^3, and a particle count of 512^3, ensuring comparable mass resolution. Haloes are identified with a Friends‑of‑Friends algorithm, and sub‑haloes (satellites) are extracted using a Subfind‑type procedure.

The key observable is the three‑dimensional relative speed of each satellite with respect to its host’s centre. The authors define a “high‑speed” satellite as one whose relative velocity exceeds 1.5 times the host’s maximum circular velocity (v_max), a threshold commonly used in observational studies of fast‑moving dwarf galaxies and colliding clusters. By measuring this quantity across the satellite population, they construct a statistical distribution of satellite speeds for each cosmology.

The results are striking. In the MOND simulation, roughly 12 % of satellites satisfy the high‑speed criterion, whereas in the ΛCDM counterpart only about 3 % do. This translates into an approximately four‑fold increase in the incidence of high‑speed encounters under MOND. The authors attribute this enhancement directly to the deeper (effectively unbounded) MOND potential: satellites falling into a MOND halo experience stronger acceleration as they approach the centre, allowing them to reach higher velocities without requiring an excessively massive host. In contrast, CDM haloes have a finite mass budget, limiting the maximum kinetic energy that infalling satellites can acquire.

To assess robustness, the authors perform several convergence tests. They repeat the simulations at lower (256^3) and higher (1024^3) resolutions, finding that the high‑speed fraction remains within the 3.5–4.5 × range. They also vary the random seed of the initial density field across five realizations, confirming that the four‑fold excess persists on average. An analysis of satellite‑host distance and mass‑ratio distributions reveals that MOND high‑speed satellites typically have mass ratios between 0.01 and 0.1 and reside at 50–150 kpc from the host centre—parameters that are broadly consistent with observed fast dwarfs such as the Magellanic Clouds or the ultra‑fast satellites reported in recent surveys.

The paper concludes that the prevalence of high‑speed satellite galaxies is a sensitive probe of the underlying gravitational law. The current simulations, albeit limited to dark‑matter‑only dynamics (no gas, star formation, or feedback), already demonstrate a clear, order‑of‑magnitude difference between MOND and ΛCDM predictions. The authors acknowledge that baryonic processes could modify satellite orbits, and they advocate for future work that incorporates hydrodynamics, radiative cooling, and feedback mechanisms within MOND frameworks. Moreover, they suggest expanding the simulation volume to gigaparsec scales to improve statistical power and to enable direct comparison with large‑scale surveys such as SDSS, DESI, and forthcoming JWST observations.

In summary, this study provides an initial but compelling quantitative test of the “high‑speed satellite” diagnostic, showing that MOND predicts roughly four times more fast‑moving satellites than the concordance ΛCDM model. If future, more sophisticated simulations and observational analyses confirm this trend, the high‑speed satellite population could become a decisive arena for testing competing theories of gravity and dark matter.