ar X iv : 0 80 1 . 03 48 v 1 [ as tr o - ph ] 2 J an 2 00 8 Infall and Outflow of Molecular Gas in Sgr B 2

Observations of two H2CO (303−202 and 321−220) lines and continuum emission at 1.3 mm towards 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 red-shifted and blue-shifted outflow components have a common terminal velocity. This terminal velocity is 58±2 km s−1. It provides an excellent method in 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 red-shifted 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 multi-level 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 (321 − 220) and the lower energy level transition H2CO (303 − 202) is reversed in the red-shifted 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. Subject headings: Galaxy:center — ISM:individual (Sgr B2) — ISM:kinematics and dynamics — ISM:molecules — radio lines: ISM — star:formation Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, MS 42, Cambridge, MA 02138; [email protected]. National Astronomical Observatories, Chinese Academy of Sciences, Beijing, 100012. Academia Sinica Institute of Astronomy and Astrophysics, P.O.Box 23-141, Taipei 106, Taiwan. Department of Earth Sciences, National Taiwan Normal University, 88 Section 4, Ting-Chou Road, Taipei 116, Taiwan.