ALMA Observations of the Galactic Center: SiO Outflows and High-mass Star Formation near Sgr A*

Abstract

ALMA observations of the Galactic center with a spatial resolution of 2."61 × 0."97 resulted in the detection of 11 SiO (5-4) clumps of molecular gas within 0.6 pc (15") of Sgr A^*, interior to the 2 pc circumnuclear molecular ring. The three SiO (5-4) clumps closest to Sgr A^* show the largest central velocities, ~150 km s^(–1), and the broadest asymmetric line widths with full width zero intensity (FWZI) ~110-147 km s^(–1). The remaining clumps, distributed mainly to the NE of the ionized mini-spiral, have narrow FWZI (~18-56 km s^(–1)). Using CARMA SiO (2-1) data, Large Velocity Gradient modeling of the SiO line ratios for the broad velocity clumps constrains the column density N(SiO) ~10^(14) cm^(–2), and the H_2 gas density n_H_2 = (3-9) x 10^5 cm^(–3) for an assumed kinetic temperature 100-200 K. The SiO clumps are interpreted as highly embedded protostellar outflows, signifying an early stage of massive star formation near Sgr A^* in the last 10^4-10^5 yr. Support for this interpretation is provided by the SiO (5-4) line luminosities and velocity widths which lie in the range measured for protostellar outflows in star-forming regions in the Galaxy. Furthermore, spectral energy distribution modeling of stellar sources shows two young stellar object candidates near SiO clumps, supporting in situ star formation near Sgr A^*. We discuss the nature of star formation where the gravitational potential of the black hole dominates. In particular, we suggest that external radiative pressure exerted on self-shielded molecular clouds enhances the gas density, before the gas cloud becomes gravitationally unstable near Sgr A^*. Alternatively, collisions between clumps in the ring may trigger gravitational collapse.

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