Slow stellar halo rotation as a signature of disc flips and GES-like mergers
Abstract
The stellar halo of the Milky Way (MW) exhibits a weak net prograde rotational signal (vϕ ≲ 25 km~s−1), yet its origin remains unexplained. To investigate this, we use the Auriga cosmological simulation suite of MW-mass haloes to probe the rotation of stellar haloes over the redshift range zred=0−2.5. The rotational signal is found to persist over this redshift range. We show that satellite progenitors of the stellar halo exhibit a non-uniform distribution of infall directions, with a tendency to align with the host disc, albeit with significant scatter. Thus, we attribute the net rotation to the anisotropic accretion of satellites. Haloes that host \textit{Gaia}-Enceladus-Sausage (GES)-like substructures exhibit consistently slower rotational velocity, likely due to the head-on, more radial trajectory of the dominant progenitor in such systems. Haloes whose stellar discs have reorientated by ≥ 90∘ also exhibit slower rotation, likely because the longer dynamical timescales of haloes prevent them from quickly adjusting to disc reorientations. We also report a correlation between the rotation of stellar and dark matter haloes, suggesting a possible common origin driven by anisotropic accretion. We therefore suggest that the MW experienced a disc flip and has a slowly rotating dark matter halo.
Versions
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BibTeX
@misc{batrakov2026slow,
title = {{Slow stellar halo rotation as a signature of disc flips and GES-like mergers}},
author = {Kirill Batrakov and Alis J. Deason and Francesca Fragkoudi and Thomas Tomlinson and Azadeh Fattahi and Vasily Belokurov},
howpublished = {arXiv},
year = {2026},
month = sep,
doi = {10.48550/arxiv.2609.01208},
eprint = {2609.01208},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2609.01208},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - Slow stellar halo rotation as a signature of disc flips and GES-like mergers
AU - Kirill Batrakov
AU - Alis J. Deason
AU - Francesca Fragkoudi
AU - Thomas Tomlinson
AU - Azadeh Fattahi
AU - Vasily Belokurov
PY - 2026
DA - 2026/09/01
PB - arXiv
DO - 10.48550/arxiv.2609.01208
UR - https://arxiv.org/abs/2609.01208
AB - The stellar halo of the Milky Way (MW) exhibits a weak net prograde rotational signal (v_ϕ ≲ 25 km~s^−1), yet its origin remains unexplained. To investigate this, we use the Auriga cosmological simulation suite of MW-mass haloes to probe the rotation of stellar haloes over the redshift range z_red=0−2.5. The rotational signal is found to persist over this redshift range. We show that satellite progenitors of the stellar halo exhibit a non-uniform distribution of infall directions, with a tendency to align with the host disc, albeit with significant scatter. Thus, we attribute the net rotation to the anisotropic accretion of satellites. Haloes that host \textit{Gaia}-Enceladus-Sausage (GES)-like substructures exhibit consistently slower rotational velocity, likely due to the head-on, more radial trajectory of the dominant progenitor in such systems. Haloes whose stellar discs have reorientated by ≥ 90^∘ also exhibit slower rotation, likely because the longer dynamical timescales of haloes prevent them from quickly adjusting to disc reorientations. We also report a correlation between the rotation of stellar and dark matter haloes, suggesting a possible common origin driven by anisotropic accretion. We therefore suggest that the MW experienced a disc flip and has a slowly rotating dark matter halo.
ER -