  It is widely accepted that black holes (BHs) with masses greater than a
million solar masses (Msun) lurk at the centres of massive galaxies. The
origins of such `supermassive' black holes (SMBHs) remain unknown (Djorgovski
et al. 1999), 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 (Portagies Zwart et
al. 1999), merge at the centre of a galaxy to form an SMBH (Ebisuzaki et al.
2001). 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 ~1E5 Msun is lurking in this
cloud (Oka et al. 2016). 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.
