Probing the boundary between star clusters and dwarf galaxies: A MUSE
  view on the dynamics of Crater/Laevens I
  We present MUSE observations of the debated ultra faint stellar system
Crater. We spectroscopically confirm 26 member stars of this system via radial
velocity measurements. We derive the systematic instrumental velocity
uncertainty of MUSE spectra to be 2.27$\rm \,km\,s^{-1}$. This new dataset
increases the confirmed member stars of Crater by a factor of 3. One out of
three bright blue stars and a fainter blue star just above the
main-sequence-turn-off are also found to be likely members of the system. The
observations reveal that Crater has a systemic radial velocity of $v_{\rm
sys}=148.18^{\rm +1.08}_{\rm -1.15}\rm \,km\,s^{-1}$, whereas the most likely
velocity dispersion of this system is $\sigma_{\rm v}=2.04^{\rm +2.19}_{\rm
-1.06} \rm \,km\,s^{-1}$. The total dynamical mass of the system, assuming
dynamical equilibrium is then $M_{\rm tot}=1.50^{+4.9}_{-1.2}\cdot 10^{\rm
5}M_{\odot}$ implying a mass-to-light ratio of M/L$_{\rm
V}$=8.52$^{+28.0}_{-6.5}\, M_{\odot}/L_{\odot}$, which is consistent with a
purely baryonic stellar population within its errors and no significant
evidence for the presence dark matter was found. We also find evidence for a
velocity gradient in the radial velocity distribution. We conclude that our
findings strongly support that Crater is a faint intermediate-age outer halo
globular cluster and not a dwarf galaxy.
