Probing the multi-scale interplay between gravity and turbulence -
  Power-law like gravitational energy spectra of the Orion Complex
  Gravity plays a determining role in the evolution of the molecular ISM. In
\citet{2016arXiv160304342L}, we proposed a measure called gravitational energy
spectrum to quantify the importance of gravity on multiple physical scales. In
this work, 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 pc down to $\sim 0.1 $ pc scale, the
Orion A and Orion B molecular cloud have $E_{\rm p}(k)\sim k^{-1.88}$ and
$E_{\rm p}(k)\sim k^{-2.09}$, respectively. These scaling exponents are close
to the scaling exponents of the kinetic energy power spectrum of compressible
turbulence (where $E\sim k^{-2}$), 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 over 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 $\sim 0.1\;\rm pc$. { However, turbulence is likely to
dominate in clouds like California. } The method is general and should be
applicable to all the astrophysical problems where gravity plays a role.
