Interpretation of radio continuum and molecular line observations of Sgr B2: free-free and synchrotron emission, and implications for cosmic rays

Abstract

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 a virial mass of 1.9 million solar masses 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 HII regions plus emission from an envelope or wind with a radial density gradient. 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.

11 pages, 10 figures. New section on diffusion of primary and secondary cosmic ray electrons into and within the Sgr B2 Giant Molecular Cloud added. Main corrections to proofs made in this version

References in corpus (5)

• Gamma rays from molecular clouds
• The Origin of Diffuse X-ray and $\gamma$-ray Emission from the Galactic Center Region: Cosmic Ray Particles
• The sky distribution of positronium annihilation continuum emission measured with SPI/INTEGRAL
• APEX mapping of H3O+ in the Sgr B2 region
• Diffuse Gamma-ray Emission from the Galactic Center - A Multiple Energy Injection Model

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• Radio Emission from Accreting Isolated Black Holes in Our Galaxy
• Probes of turbulent driving mechanisms in molecular clouds from fluctuations in synchrotron intensity
• Australia Telescope Compact Array Radio Continuum 1384 and 2368 Mhz Observations of Sagittarius B
• Sgr B2 hard X-ray emission with INTEGRAL after 2009: still detectable?
• A 3D Diffusive and Advective Model of Electron Transport Applied to the Pulsar Wind Nebula HESS J1825-137
• Self-modulation of cosmic rays in molecular clouds: Imprints in the radio observations
• Prospects for Detection of Synchrotron Emission from Secondary Electrons and Positrons in Starless Cores: Application to G0.216+0.016
• Clustered Star Formation in the center of NGC 253 Contributes to Driving the Ionized Nuclear Wind