The combination of precise radial velocities from multi-object spectroscopy and highly accurate proper motions from Gaia DR2 opens up the possibility for detailed 3D kinematic studies of young star forming regions and clusters .
Here , we perform such an analysis by combining Gaia -ESO Survey spectroscopy with Gaia astrometry for \sim 900 members of the Lagoon Nebula cluster , NGC 6530 .
We measure the 3D velocity dispersion of the region to be 5.35 ^ { +0.39 } _ { -0.34 } km s ^ { -1 } , which is large enough to suggest the region is gravitationally unbound .
The velocity ellipsoid is anisotropic , implying that the region is not sufficiently dynamically evolved to achieve isotropy , though the central part of NGC 6530 does exhibit velocity isotropy that suggests sufficient mixing has occurred in this denser part .
We find strong evidence that the stellar population is expanding , though this is preferentially occurring in the declination direction and there is very little evidence for expansion in the right ascension direction .
This argues against a simple radial expansion pattern , as predicted by models of residual gas expulsion .
We discuss these findings in the context of cluster formation , evolution and disruption theories .