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 (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 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 at least 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.