It is widely accepted that black holes (BHs) with masses greater than a million solar masses (M ⊙ ) lurk at the centres of massive galaxies. The origins of such 'supermassive' black holes (SMBHs) remain unknown 1 , while those of stellar-mass BHs are well-understood. One possible scenario is that intermediate-mass black holes (IMBHs), which are formed by the runaway coalescence of stars in young compact star clusters 2 , merge at the centre of a galaxy to form an SMBH 3 . Although many candidates for IMBHs have been proposed to date, none of them are accepted as definitive. Recently we discovered a peculiar molecular cloud, CO-0.40-0.22, with an extremely broad velocity width near the centre of our Milky Way galaxy.Based on the careful analysis of gas kinematics, we concluded that a compact object with a mass of ∼ 10 5 M ⊙ is lurking in this cloud 4 . Here we report the detection of a point-like continuum source as well as a compact gas clump near the center of CO-0.40-0.22. This point-like continuum source (CO-0.40-0.22 * ) has a wide-band spectrum consistent with 1/500 of the Galactic SMBH (Sgr A * ) in luminosity. Numerical simulations around a point-like massive object reproduce the kinematics of dense molecular gas well, which suggests that CO-0.40-0.22 * is the most promising candidate for an intermediate-mass black hole.CO-0.40-0.22 is a compact cloud (∼ 5 pc) with an extremely broad velocity width (∼ 100 km s −1 ) and very high CO J=3-2/J=1-0 intensity ratio (≥ 1.5) detected at a projected distance of ∼ 60 pc away from the galactic nucleus 5 . It belongs to a peculiar category of molecular clouds called high-velocity compact clouds (HVCCs) that were originally identified in the CO J=1-0 survey data 6-8 . CO-0.40-0.22 is only a dense cloud with a negative velocity detected in HCN J=4-3 line within the 0.3 • ×0.3 • area including it 4 . It has a continuous and roughly straight entity in the position-velocity maps, seeming not to be an aggregate of unrelated clouds with less broad velocity widths. The kinematical structure of CO-0.40-0.22 can be explained as being due to a gravitational kick experienced by the molecular cloud caused by an invisible compact object with a mass of ∼ 10 5 M ⊙ . The compactness and absence of a counterpart at other wavelengths suggest that this massive object is an inactive IMBH, which is not currently accreting matter. This is the second-largest black hole candidate in the Milky Way galaxy affter Sgr A * , as well as the second IMBH candidate in the Galaxy after that in the nuclear subcluster IRS13E (M BH ≃ 1300 M ⊙ ) 9,10 .ALMA band 6 observations towards CO-0.40-0.22 have provided high-resolution HCN J=3-2 (265.9 GHz) and CO J=2-1 (230.5 GHz) images. Dense molecular gas traced by HCN J=3-2 emission seems to concentrate near the centre of CO-0.40-0.22 as previously determined by the coarse-resolution ASTE HCN J=4-3 map (Fig. 1a). The displacement of 0.2 pc from the centre is within the ASTE beamwidth (22 ′′ = 0.9 pc). This dense gas clump is very compact (∼ 0.3

We report the discovery of molecular gas streams orbiting around an invisible massive object in the central region of our Galaxy, based on the high-resolution molecular line observations with the Atacama Large Millimeter/submillimeter Array (ALMA). The morphology and kinematics of these streams can be reproduced well through two Keplerian orbits around a single point mass of (3.2±0.6)×10 4 M . We also found ionized gas toward the inner part of the orbiting gas, indicating dissociative shock and/or photoionization. Our results provide new circumstantial evidences for a wandering intermediate-mass black hole in the Galactic center, suggesting also that high-velocity compact clouds can be probes of quiescent black holes abound in our Galaxy.

We report the results of high-resolution molecular line observations of the high-velocity compact cloud HCN-0.085-0.094 with the Atacama Large Millimeter/submillimeter Array. The HCN J=4-3, HCO + J=4-3, and CS J=7-6 line images reveal that HCN-0.085-0.094 consists mainly of three small clumps with extremely broad velocity widths. Each of the three clumps has a 5.5 GHz radio continuum counterpart in its periphery toward Sgr A * . The positional relationship indicates that their surfaces have been ionized by ultraviolet photons from young stars in the central cluster, suggesting the clumps are in close proximity to the Galactic nucleus. One of the three clumps has a ring-like structure with a very steep velocity gradient. This kinematical structure suggests an orbit around a point-like object with a mass of ∼ 10 4 M . The absence of stellar counterparts indicates that the point-like object may be a quiescent black hole. This discovery adds another intermediate-mass black hole candidate in the central region of our Galaxy.

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.