Recent ammonia ( 1,1 ) inversion line data on the Galactic star forming region Sgr B2 show that the column density is consistent with a radial Gaussian density profile with a standard deviation of 2.75 pc .
Deriving a formula for the virial mass of spherical Gaussian clouds , we obtain M _ { vir } = 1.9 \times 10 ^ { 6 } M _ { \odot } for Sgr B2 .
For this matter distribution , a reasonable magnetic field and an impinging flux of cosmic rays of solar neighbourhood intensity , we predict the expected synchrotron emission from the Sgr B2 giant molecular cloud due to secondary electrons and positrons resulting from cosmic ray interactions , including effects of losses due to pion production collisions during diffusive propagation into the cloud complex .

We assemble radio continuum data at frequencies between 330 MHz and 230 GHz .
From the spectral energy distribution the emission appears to be thermal at all frequencies .
Before using these data to constrain the predicted synchrotron flux , we first model the spectrum as free-free emission from the known ultra compact H ii regions plus emission from an envelope or wind with a radial density gradient , and obtain an excellent fit .
We thus find the spectrum at all frequencies to be dominated by thermal emission , and this severely constrains the possible synchrotron emission by secondary electrons to quite low flux levels .
The absence of a significant contribution by secondary electrons is almost certainly due to multi-GeV energy cosmic rays being unable to penetrate far into giant molecular clouds .
This would also explain why 100 MeV–GeV gamma-rays ( from neutral pion decay or bremsstrahlung by secondary electrons ) were not observed from Sgr B2 by EGRET , while TeV energy gamma-rays were observed , being produced by higher energy cosmic rays which more readily penetrate giant molecular clouds .