The young massive star cluster Westerlund 2 observed with MUSE. I. First
  results on the cluster internal motion from stellar radial velocities
  Westerlund 2 (Wd2) is the central ionizing star cluster of the \ion{H}{2}
region RCW~49 and the second most massive young star cluster (${\rm M} = (3.6
\pm 0.3)\times 10^4\,{\rm M}_\odot$) in the Milky Way. Its young age
($\sim2\,$Myr) and close proximity to the Sun ($\sim 4\,$kpc) makes it a
perfect target to study stars emerging from their parental gas cloud, the large
number of OB-stars and their feedback onto the gas, and the gas dynamics. We
combine high-resolution multi-band photometry obtained in the optical and
near-infrared with the \textit{Hubble} Space Telescope (HST), and VLT/MUSE
integral field spectroscopy to study the gas, the stars, and their
interactions, simultaneously. In this paper we focus on a small,
$64\times64\,{\rm arcsec}^2$ region North of the main cluster center, which we
call the Northern Bubble (NB), a circular cavity carved into the gas of the
cluster region. Using MUSE data, we determined the spectral types of 17 stars
in the NB from G9III to O7.5. With the estimation of these spectral types we
add 2 O and 5 B-type stars to the previously published census of 37 OB-stars in
Wd2. To measure radial velocities we extracted 72 stellar spectra throughout
Wd2, including the 17 of the NB, and show that the cluster member stars follow
a bimodal velocity distribution centered around $(8.10 \pm 1.53)\,{\rm
km}\,{\rm s}^{-1}$ and $(25.41 \pm 1.57)\,{\rm km}\,{\rm s}^{-1}$ with a
dispersion of $(4.52 \pm 1.78)\,{\rm km}\,{\rm s}^{-1}$ and $(3.46 \pm
1.29)\,{\rm km}\,{\rm s}^{-1}$, respectively. These are in agreement with
CO($J=1$-2) studies of RCW~49 leaving cloud-cloud collision as a viable option
for the formation scenario of Wd2. The bimodal distribution is also detected in
the Gaia DR2 proper motions.
