Slow stellar halo rotation as a signature of disc flips and GES-like mergers

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 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.