The hot gaseous halos of galaxies likely contain a large amount of mass and are an integral part of galaxy formation and evolution .
The Milky Way has a 2 \times 10 ^ { 6 } K halo that is detected in emission and by absorption in the O vii resonance line against bright background AGNs , and for which the best current model is an extended spherical distribution .
Using XMM-Newton RGS data , we measure the Doppler shifts of the O vii absorption-line centroids toward an ensemble of AGNs .
These Doppler shifts constrain the dynamics of the hot halo , ruling out a stationary halo at about 3 \sigma and a co-rotating halo at 2 \sigma , and leading to a best-fit rotational velocity v _ { \phi } = 183 \pm 41 km s ^ { -1 } for an extended halo model .
These results suggest that the hot gas rotates and that it contains an amount of angular momentum comparable to that in the stellar disk .
We examined the possibility of a model with a kinematically distinct disk and spherical halo .
To be consistent with the emission-line X-ray data the disk must contribute less than 10 % of the column density , implying that the Doppler shifts probe motion in the extended hot halo .