Testing MOND using the dynamics of nearby stellar streams

Abstract: The stellar halo of the Milky Way is built up, at least in part, from debris from past mergers. Stars from such merger events define substructures in phase-space, for example in the form of streams, which are groups of stars moving on similar trajectories. The nearby Helmi streams discovered more than two decades ago are a well-known example. Using 6D phase-space information from the Gaia space mission, Dodd et al. (2022) have recently reported that the Helmi streams are split into two clumps in angular momentum space. Such substructure can be explained and sustained in time if the dark matter halo of the Milky Way takes a prolate shape in the region probed by the orbits of the stars in the streams. Here, we explore the behaviour of the two clumps identified in the Helmi streams in a Modified Newtonian Dynamics (MOND) framework to test this alternative model of gravity. We perform orbit integrations of Helmi streams member stars in a simplified MOND model of the Milky Way and using the more sophisticated Phantom of RAMSES simulation framework. We find with both approaches that the two Helmi streams clumps do not retain their identity and dissolve after merely 100 Myr. This extremely short timescale would render the detection of two separate clumps as very unlikely in MONDian gravity. The observational constraints provided by the streams, which MOND fails to reproduce in its current formulation, could potentially also be used to test other alternative gravity models.