  Aims. We use Fermi and Planck data to calculate the cross correlation between
gamma ray signal and gas distribution in different scales in giant molecular
clouds (GMC). Then we investigate the cosmic rays (CRs) penetration in GMCs
with these informations.
  Methods.We use the wavelet technique to decompose both the gamma ray and dust
opacity maps in different scales, then we calculate the wavelet cross
correlation functions in these scales. We also define wavelet response as an
analog to the impulsive response in Fourier transform and calculate that in
different scales down to Fermi angular resolution.
  Results. The gamma ray maps above 2 GeV show strong correlation with the dust
opacity maps, the correlation coefficient is larger than 0.9 above a scale of
0.4 degree.The derived wavelet response is uniform in different scales.
  Conclusions. We argue that the CR above 10 GeV can penetrate the giant
molecular cloud freely and the CRs distributions in the same energy range are
uniform down to parsec scale.

High Energy Astrophysical Phenomena