2018ApJ...856...91T

The l=-1○ / . 2 region in the Galactic center has a high CO J = 3-2/J = 1-0 intensity ratio and extremely broad velocity width. This paper reports the detection of five expanding shells in the l=-1○ / . 2 region based on the CO J = 1-0, ¹³CO J = 1-0, CO J = 3-2, and SiO J = 8-7 line data sets obtained with the Nobeyama Radio Observatory 45 m telescope and James Clerk Maxwell Telescope. The kinetic energy and expansion time of the expanding shells are estimated to be 10^48.3–50.8 erg and 10^4.7–5.0 yr, respectively. The origin of these expanding shells is discussed. The total kinetic energy of 10⁵¹ erg and the typical expansion time of ∼10⁵ yr correspond to multiple supernova explosions at a rate of 10^–5-10^–4 yr^–1. This indicates that the l=-1○ / . 2 region may be a molecular bubble associated with an embedded massive star cluster, although the absence of an infrared counterpart makes this interpretation somewhat controversial. The expansion time of the shells increases as the Galactic longitude decreases, suggesting that the massive star cluster is moving from Galactic west to east with respect to the interacting molecular gas. We propose a model wherein the cluster is moving along the innermost x₁ orbit and the interacting gas collides with it from the Galactic eastern side.