  We report the observation of the melting of a vortex lattice in a fast
rotating quasi-two dimensional Bose gas, under the influence of thermal
fluctuations. We image the vortex lattice after a time-of-flight expansion, for
increasing rotation frequency at constant atom number and temperature. We
detect the vortex positions and study the order of the lattice using the pair
correlation function and the orientational correlation function. We evidence
the melting transition by an abrupt change in the decay of orientational
correlations, associated to a proliferation of dislocations. Our findings are
consistent with the hexatic to liquid transition in the KTHNY scenario for
two-dimensional melting.
