ALMA Observations of the Galactic Center: SiO Outflows and High-mass Star Formation near Sgr A*
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Author/Creator ORCID
Date
2013-04-05
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Citation of Original Publication
Yusef-Zadeh, F., “ALMA Observations of the Galactic Center: SiO Outflows and High-mass Star Formation near Sgr A*”, The Astrophysical Journal Letters, vol. 767, no. 2, 2013. doi:10.1088/2041-8205/767/2/L32.
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Abstract
ALMA observations of the Galactic center with a spatial resolution of 2farcs61 × 0farcs97 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) ∼1014 cm−2, and the H2 gas density $n_{\rm H_2}=(3\hbox{--}9)\times 10^5$ 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 104–105 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.