We discovered two small high-velocity compact clouds (HVCCs) in the HCN J = 4–3 and J = 3–2 maps of the central 20 pc of our Galaxy. Both HVCCs have broad velocity widths ( km s−1) and compact sizes ( ), and originate from the dense molecular clouds in the position–velocity space. One of them has a faint counterpart in a Paschen-α image. Their spatial structure, kinematics, and absence of luminous stellar object are compatible with the notion that each of the small HVCCs is driven by the plunge of an invisible compact object into a molecular cloud. Such objects are most likely inactive, isolated black holes.

We performed Nyquist-sampled mapping observations of the central molecular zone of our Galaxy in the J = 1–0 lines of CO, 13CO, and C18O using the 45 m telescope at the Nobeyama Radio Observatory. The newly obtained data sets were an improvement by a factor of four in spatial resolution of the CO data previously obtained with the same telescope 22 years ago, providing the highest angular resolution CO atlas of this special area of the Galaxy. The data cover the area: −0${^{\circ}_{.}}$8 ≤ l ≤ +1${^{\circ}_{.}}$4 and −0${^{\circ}_{.}}$35 ≤ b ≤ +0${^{\circ}_{.}}$35 with a 15″ beamwidth. Total intensity ratios for CO J = 3–2/J = 1–0, 13CO/CO J = 1–0 and C18O/13CO J = 1–0, are 0.70 ± 0.06, 0.12 ± 0.01, and 0.14 ± 0.01, respectively. The high-resolution CO images show the fine structure of the molecular gas and enable us to identify a number of compact clouds with broad velocity widths, i.e., high-velocity compact clouds. We conducted a detailed comparison of our CO J = 1–0 data with the CO J = 3–2 data obtained with the James Clerk Maxwell Telescope to derive the distribution and kinematics of the highly excited gas. Three, out of four, of the previously identified high CO J = 3–2/J = 1–0 ratio areas at l = +1${^{\circ}_{.}}$3, 0${^{\circ}_{.}}$0, and −0${^{\circ}_{.}}$4 were confirmed with a higher spatial resolution. In addition to these, we identified several very compact, high CO J = 3–2/J = 1–0 spots with broad velocity widths for the first time. These are candidates for accelerated gas in the vicinity of invisible, point-like massive objects.

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, 13 CO 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 10 51 erg and the typical expansion time of ∼ 10 5 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 x 1 orbit and the interacting gas collides with it from the Galactic eastern side.

We have discovered an energetic high-velocity compact cloud CO$\, -0.31+$0.11 in the central molecular zone of our Galaxy. CO$\, -0.31+$0.11 is located at a projected distance of ∼45 pc from the Galactic nucleus Sgr A*. It is characterized by its compact spatial appearance (d ≃ 4 pc), extremely broad velocity width (ΔV > 100 km s−1), and high CO J = 3–2/J = 1–0 intensity ratio. The total gas mass and kinetic energy are estimated as approximately $10^{4}\, M_{\odot }$ and 1051 erg, respectively. Two expanding bubble-like structures are found in our HCN J = 1–0 map obtained with the Nobeyama Radio Observatory 45 m telescope. In the longitude–velocity maps, CO$\, -0.31+$0.11 exhibits an asymmetric V shape. This kinematical structure can be well fitted by Keplerian motion on an eccentric orbit around a point mass of $2\times 10^{5}\, M_\odot$. The enhanced CO J = 3–2/J = 1–0 ratio is possibly attributed to the tidal compression during the pericenter passage. The model suggests that a huge mass is packed within a radius of r < 0.1 pc. The huge mass, compactness, and absence of luminous stellar counterparts may correspond to a signature of an intermediate-mass black hole (IMBH) inside. We propose a formation scenario of CO$\, -0.31+$0.11 in which a compact cloud has gravitationally interacted with an IMBH and a bipolar molecular outflow was driven by the past activity of the putative IMBH.

We mapped the ultra-high-velocity feature (the "Bullet") detected in the expanding molecular shell associated with the W44 supernova remnant using the Nobeyama Radio Observatory 45-m telescope and the ASTE 10-m telescope.The Bullet clearly appears in the CO J=1-0, CO J=3-2, CO J=4-3, and HCO + J=1-0 maps with a compact appearance (0.5 × 0.8 pc 2 ) and an extremely broad velocity width (∆V ≃ 100 km s −1 ). The line intensities indicate that the Bullet has a higher density and temperature than those in the expanding molecular shell.The kinetic energy of the Bullet amounts to 10 48.0 erg which is approximately 1.5 orders of magnitude greater than the kinetic energy shared to the small solid angle of it. Two possible formation scenarios with an inactive isolated black hole (BH) are presented.