THE ROTATION OF THE HOT GAS AROUND THE MILKY WAY

Abstract

ABSTRACT 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 × 10 6 -->?> K halo that is detected in emission and by absorption in the O vii resonance line against bright background active galactic nuclei (AGNs), and for which the best current model is an extended spherical distribution. Using XMM-Newton Reflection Grating Spectrometer 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 &sgr; -->?> and a co-rotating halo at 2 &sgr; -->?> , and leading to a best-fit rotational velocity of v φ = 183 ± 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.