The Formation of Planes of Satellite Galaxies: New Horizons Simulation
Written on August 20th, 2025 by Janvi Madhani
Cover Image: High Resolution Gas Cube of New Horizon, Dubois et al. 2020, 20 Mpc/h on a side
Numerous observations in recent years have shown that the satellite galaxies orbiting our local galaxies tend to align their orbits in one or two thin planes around the host galaxy. This has been observed in local galaxies, Andromeda and Centaurus A, and our own Milky Way. Numerical simulations in a cosmological context find these planes to be rare or short-lived leading to tension between observation and theory. This leads to considerable debate on whether observations are compatible with the standard, Lambda Cold Dark Matter, model of cosmology. This is known as the “Planes of Satellites Problem”.
I work with Dr. Charlotte Welker and the Gotham Web Lab on this project. We argue that on large scales, these simulations did not sufficiently resolve the nearby large-scale structure, cosmic filaments, which we believe to be responsible for the anisotropic infall of satellites forming planar alignments, and on smaller scales, they did not sufficiently resolve dwarf satellite galaxies. We use the high precision, hydrodynamic, cosmological zoom simulation, New Horizon, which has both the large volume, (16 Mpc)3 and the small-scale resolution, ~ 35 pc, required to study the interplay between cosmic web dynamics and the formation, funneling, and eventually the anisotropic distribution of satellites around local galaxies.
Broadly, there are three scenarios astronomers consider as possible solutions to this problem.
- Filamentary accretion
Massive galaxies form at the nodes of gas and dark matter rich filaments. Smaller galaxies form at the spines of cold pristine filaments and are funneled towards the nodes. Their anisotropic infall towards the central galaxy gives rise to alignments along the axis of infall and could provide a natural explanation for the planar systems we observe in our local universe due to our proximity to a large cosmic filament, the Virgo cluster. - Group Infall
Dwarf galaxies formed in a group or infalling with a more massive galaxy in the process of merging with another massive galaxy or group, fall into physical alignment, kinematically cohering and co-orbiting around the central galaxy. - Tidal Dwarf Galaxies
As two massive galaxies undergo a close encounter, smaller dwarf galaxies are formed by stripping gas from the central galaxy. These tidally stripped dwarfs fall into kinematic alignment around the central galaxy after the encounter. However, in this scenario, the tidally stripped dwarfs should have no dark matter content, which puts this scenario at odds with our observations of dark matter rich dwarves.
We investigate the first scenario as the most likely one as we believe
We’ve recently submitted our publication reporting our findings and investigation into the hydrodynamic origins of these planes, find the pre-print here!.
A movie of the formation of the large scale structure, the cosmic web, as gas cools and collapses into filaments over cosmic time in New Horizon (Dubois et al. 2020).
Recently, Charlotte and I were given the wonderful opportunity to present our results at the AAS meeting in Pasadena in a press conference! You can listen to our talk here, starting at 18:30!
Pictured left to right: Katya Gozman, Eric Bell, Janvi Madhani, Charlotte Welker, and Hollis Akins. (Photo Credit: AAS Press)
