The object CO–0.40–0.22 is a compact cloud (~5 pc) with an extremely broad velocity width (~100 km s−1) and very high intensity ratio (≥1.5) for CO J = 3–2/J = 1–0 detected at a projected distance of ~60 pc away from the Galactic nucleus5. 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 data6,7,8. The CO–0.40–0.22 cloud is the only dense cloud with a negative velocity detected in the HCN J = 4–3 line within the 0.3° × 0.3° area including it4. It has a continuous and roughly straight entity in the position–velocity maps, seeming not to be an aggregate of unrelated clouds with narrower 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 about 105M⊙. 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 after Sgr A*, as well as the second IMBH candidate in the Galaxy after that in the nuclear subcluster IRS13E (MBH ≈ 1,300M⊙)9,10.

Observations with the Atacama Large Millimeter/submillimeter Array (ALMA) band 6 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 HCN J = 4–3 map from the Atacama Submillimeter Telescope Experiment (ASTE) (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 pc) and has a broad velocity width (~100 km s−1). We also see about 20 rather faint clumps. These faint clumps and the central dense clump occupy around 10% of the field of view. The faint clumps have velocity widths less than 20 km s−1. Thus, the chance probability of their alignment over a width of about 100 km s−1 is less than (0.1/(100/20))100/20 = 10−8.5. The main body of the central clump appears at line-of-sight velocities from −80 to −40 km s−1 with respect to the local standard of rest (LSR), being associated with high-velocity components that reach VLSR = −105 and −5 km s−1 (Fig. 2). It is slightly elongated in roughly the same direction as the major axis of CO–0.40–0.22, with a steep velocity gradient from southeast to northwest. The mass of the clump was estimated from the HCN J = 3–2 integrated intensity to be 40M⊙, according to an excitation model at the kinetic temperature Tk = 60 K and number density n(H2) = 106.5 cm−3. On the other hand, the size parameter S = 0.15 pc and velocity dispersion σV = 22 km s−1 give a virial theorem mass of Mvir ≈ 4.1 × 103M⊙, which indicates that the clump must not be bound by its self-gravity.