Infall and outflow of molecular gas in Sgr B2

Observations of two H₂CO (3₀₃-2₀₂ and 3₂₁-2₂₀) lines and continuum emission at 1.3 mm toward Sgr B2(N) and Sgr B2(M) have been carried out with the SMA. The mosaic maps of Sgr B2(N) and Sgr B2(M) in both continuum and lines show a complex distribution of dust and molecular gas in both clumps and filaments surrounding the compact star formation cores. We have observed a decelerating outflow originated from the Sgr B2( M ) core, showing that both the redshifted and blueshifted outflow components have a common terminal velocity. This terminal velocity is 58 ±2 km s^-1. It provides an excellent method for determination of the systematic velocity of the molecular cloud. The SMA observations have also shown that a large fraction of absorption against the two continuum cores is redshifted with respect to the systematic velocities of Sgr B2(N ) and Sgr B2(M ), respectively, suggesting that the majority of the dense molecular gas is flowing into the two major cores where massive stars have been formed. We have solved the radiative transfer in a multilevel system with LVG approximation. The observed H₂CO line intensities and their ratios can be adequately fitted with this model for the most of the gas components. However, the line intensities between the higher energy level transition H₂CO(3 ₂₁-2₂₀) and the lower energy level transition H ₂CO(3₂₁-2₂₀) is reversed in the redshifted outflow region of Sgr B2(M), suggesting the presence of inversion in population between the ground levels in the two K ladders (K _-1= 0 and 2). The possibility of weak maser processes for the H₂CO emission in Sgr B2(M) is discussed.