Molecular line mapping of the giant molecular cloud associated with RCW 106-II. Column density and dynamical state of the clumps

We present a fully sampled (CO)-O-18 (1-0) map towards the southern giant molecular cloud (GMC) associated with the H II region RCW 106, and use it in combination with previous (CO)-C-13 (1-0) mapping to estimate the gas column density as a function of position and velocity. We find localized regions of significant (CO)-C-13 optical depth in the northern part of the cloud, with several of the high-opacity clouds in this region likely associated with a limb-brightened shell around the H II region G333.6-0.2. Optical depth corrections broaden the distribution of column densities in the cloud, yielding a lognormal distribution as predicted by simulations of turbulence. Decomposing the (CO)-C-13 and (CO)-O-18 data cubes into clumps, we find relatively weak correlations between size and linewidth, and a more sensitive dependence of luminosity on size than would be predicted by a constant average column density. The clump mass spectrum has a slope near -1.7, consistent with previous studies. The most massive clumps appear to have gravitational binding energies well in excess of virial equilibrium; we discuss possible explanations, which include magnetic support and neglect of time-varying surface terms in the virial theorem. Unlike molecular clouds as a whole, the clumps within the RCW 106 GMC, while elongated, appear to show random orientations with respect to the Galactic plane.