The Formation of Planes of Satellite Galaxies: NewHorizon Simulation
Written on August 20th, 2025 by Janvi Madhani
Cover Image: High Resolution Gas Cube of NewHorizon, 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, NewHorizon, 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.
Click each panel below for more information about suggested solutions!
Filamentary Accretion
Satellites fall in along cosmic filaments.
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
Dwarfs arrive together as a coherent group.
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
Dwarfs form from stripped tidal material.
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 and find that planes of satellites do exist in simulations when the surrounding cosmic environment is properly resolved in our paper, Madhani et al 2026. In this paper, we report our findings as well as investigate the hydrodynamic origins of these planes and find that satellite alignments are closely related to the local vorticity of gas streams as well as the large-scale direction of filamentary accretion, as predicted. Together, these findings offer a robust solution to a decades-long question of discrepancy with our $\Lambda$CDM cosmological model. Find the paper here!.
And take a look at my page on filamentary evolution to visualize the gas-rich cosmic web around Milky Way-like systems in NewHorizon, where we investigate the environmental drivers of satellite distributions.
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).
At the AAS 240 Conference in Pasadena that took place during the summer of 2022, Charlotte and I were given the wonderful opportunity to present our results at a press conference! You can listen to our talk here, starting at 18:30! Here we discuss our discovery of planes of satellites in NewHorizon.
Pictured left to right: Katya Gozman, Eric Bell, Janvi Madhani, Charlotte Welker, and Hollis Akins. (Photo Credit: AAS Press)
