2017MNRAS.464.4096L

Gravity plays a determining role in the evolution of the molecular ISM. In 2016, we proposed a measure called gravitational energy spectrum to quantify the importance of gravity on multiple physical scales. In this paper, using a wavelet-based decomposition technique, we derive the gravitational energy spectra of the Orion A and the Orion B molecular cloud from observational data. The gravitational energy spectra exhibit power-law-like behaviours. From a few parsec down to ∼0.1 pc scale, the Orion A and Orion B molecular cloud have E_p(k) ∼ k^–1.88 and E_p(k) ∼ k^–2.09, respectively. These scaling exponents are close to the scaling exponents of the kinetic energy power spectrum of compressible turbulence (where E ∼ k^–2), with a near-equipartition of turbulent versus gravitational energy on multiple scales. This provides a clear evidence that gravity is able to counteract effectively against turbulent motion for these length-scales. The results confirm our earlier analytical estimates. For the Orion A molecular cloud, gravity inevitably dominates turbulence inside the cloud. Our results provide a clear observational proof that gravity is playing a determining role in the evolution these molecular clouds from the cloud scale down to ∼ 0.1 pc. However, turbulence is likely to dominate in clouds such as California. The method is general and should be applicable to all the astrophysical problems where gravity plays a role.