The magnetic field strength in molecular clouds is a fundamental quantity for theories of star formation. It is estimated by Zeeman splitting measurements in a few dense molecular cores, but its volume--averaged value within large molecular clouds (over several parsecs) is still uncertain. In this work we provide a new method to constrain the average magnetic field strength in molecular clouds. We compare the power spectrum of gas density of molecular clouds with that of two $350^3$ numerical simulations of supersonic MHD turbulence. The numerical simulation with approximate equipartition of kinetic and magnetic energies (model A) yields the column density power spectrum $P(k)\propto k^{ -2.25\pm 0.01}$, the super--Alfv\'{ e}nic simulation (model B) $P(k)\propto k^{ -2.71\pm 0.01}$. The column density power spectrum of the Perseus, Taurus and Rosetta molecular cloud complexes is found to be well approximated by a power law, $P_{ \rm o}(k)\propto k^{ -a}$, with $a=2.74\pm 0.07$, $2.74\pm 0.08$ and $2.76\pm 0.08$ respectively. We conclude that the observations are consistent with the presence of super--Alfv\'{ e}nic turbulence in molecular clouds (model B) while model A is inconsistent (more than 99% confidence) with the observations.
Categories: astro-ph