Searching for intermediate-mass black holes in the Central Molecular Zone of our
GalaxyShunya Takekawa,1 Tomoharu Oka, 2, 3 and Yuhei Iwata 3
1

Faculty of Engineering, Kanagawa University, 3-27-1 Rokkakubashi, Kanagawa-ku,
Yokohama, Kanagawa 221-8686
2
Department of Physics, Institute of Science and Technology, Keio University
3-14-1 Hiyoshi, Kohoku-ku, Yokohama, Kanagawa 223-8522
3
School of Fundamental Science and Technology, Graduate School of Science and Technology, Keio University
3-14-1 Hiyoshi, Kohoku-ku, Yokohama, Kanagawa 223-8522
shunya.takekawa@jindai.jp

Abstract
Intermediate-mass black holes (IMBHs) with masses of ∼ 102 –105 M⊙ are key objects to reveal the origin of
supermassive black holes and to understand galactic evolution. However, no definitive evidence of the existenceof IMBHs has been obtained yet. Recently, we have discovered molecular gas streams suggesting the presence of
IMBHs in the central region of our Galaxy. The high sensitivity and spatial resolution observations of the ngVLA
will provide detailed kinematic information on such molecular clouds, enabling us to explore point-like radio sources
corresponding to IMBHs. The ngVLA is a powerful instrument to search for the missing link of black holes.
Key words: Galaxy: center — ISM: clouds — ISM: kinematics and dynamics — ISM: molecules
1. Introduction
Most large galaxies are thought to harbor supermassive
black holes (SMBHs) with masses of ∼ 106 –1010 M⊙ at theircenters. The observations of the S-stars around Sgr A∗ (e.g.,Gravity Collaboration 2018) and the BH shadow in M87 (e.g.,
Event Horizon Telescope Collaboration 2019) have provided
firm evidence for the existence of SMBHs. The discoveries ofhigh-redshift quasars at z > 7 have indicated that SMBHs canbe formed in less than a billion years after the Big Bang (e.g.,
Matsuoka et al. 2019).
The origin of SMBHs has been one of the key issues
in astronomy and cosmology. SMBHs are hypothesized to
grow through multiple merging and/or accretion of “seed”
intermediate-mass BHs (IMBHs) with masses of ∼ 102 –105M⊙ (e.g., see Figure 1 in Mezcua 2017). Since not all IMBHs
in the early universe should have grown into SMBHs, there arepresumed to be leftover IMBHs in the local universe.
Many efforts have been made to confirm the existence of
IMBHs so far (e.g., see the recent review by Greene et al.
2020). For example, several ultra-luminous X-ray sources
have been considered as promising IMBH candidates (e.g.,
Matsumoto et al. 2001). Several globular clusters and dwarf
galaxies have been suggested to harbor massive (∼ 104 –105
M⊙ ) IMBHs at their centers (e.g., Noyola et al. 2008).
Nevertheless, these results have been still controversial, i.e.,
the existence of IMBHs has not been definitively corroborated
yet. Finding IMBHs is essential for further understanding of
the formation of SMBHs and galactic evolution.
2. Indications of IMBHs in the Galactic center
Massive objects tend to sink toward central regions of galaxies by dynamical friction. The Galactic center is one of thebest regions to search for IMBHs because of its proximity.
Within ∼ 200 pc from the Galactic nucleus Sgr A∗ , there
are huge amounts of warm and dense molecular gas as wellas innumerable stars (e.g., Morris & Serabyn 1996). Thisregion is referred to as the Central Molecular Zone (CMZ).
Theoretical calculations have suggested that IMBHs may havebeen brought in the CMZ by infalling star clusters (Fujii etal. 2009; Arca-Sedda & Gualandris 2018). Such wandering
IMBHs have potential to gravitationally interact with the ambient molecular gas.
In the large-scale molecular gas surveys of the CMZ using single-dish telescopes, we have discovered a populationof compact (d <∼ 5 pc) molecular clouds with extremely broad−1
velocity widths (∆V >∼ 50 km s ), which are called highvelocity compact clouds (HVCCs; e.g., Oka et al. 1998;
Tokuyama et al. 2019). Most HVCCs are associated with nocounterparts in other wavelengths. The absences of apparent
energy sources render it difficult to understand them. The origins of HVCCs have been discussed in case studies of several
HVCCs so far. Interactions with supernova explosions (Oka etal. 1999; Yalinewich & Beniamini 2018) and cloud-cloud collisions (Tanaka et al. 2014; Tanaka 2018; Ravi et al. 2018) havebeen suggested as possible explanations for HVCCs. Besides
these interpretations, we have proposed that some HVCCs may
be the clouds that have been gravitationally accelerated by invisible massive objects such as IMBHs (Oka et al. 2016, 2017;
Takekawa et al. 2019a, 2019b, 2020).
The most promising HVCC which possibly hosts an IMBH
is HCN–0.009–0.044 (Takekawa et al. 2019a). The ALMA
molecular line observations with an angular resolution of ∼ 1′′
revealed the detailed structure and internal motion of HCN–
0.009–0.044. A ring-like structure (Balloon) appears at thecenter of the field of view and an elongated structure (Stream)
lies on the southeast of the Balloon (Figure 1(a)). Figure 1(b)2[Vol. ,

Fig. 1. (a) Integrated intensity map of HCN–0.009–0.044 in the HCN J=4–3 line obtained with the ALMA. (b) Moment-1 map of HCN–0.009–0.044
made by the HCN J=4–3 image smoothed with a 3′′ FWHM Gaussian kernel. (c) Color map of the line-of-sight velocities of the modeled orbits
with the best-fit values. The ellipses are the modeled orbits for the Balloon and Stream, respectively. The red star indicates the dynamical center.

shows the moment-1 map of HCN–0.009–0.044. The Balloon
and Stream display clear velocity gradients: clockwise fromthe northern side to the south for the Balloon, and for the
Stream, as it curves slightly from the southern side to the north.
These velocity gradients can be naturally explained by orbital
motions caused by gravity.
We found that the Balloon and Stream are moving probably along two different Keplerian orbits around a common single gravitational source through orbital fittings to the kinematic
data. The mass (Mdyn ) and line-of-sight velocity (Vdyn ) of the
dynamical center were derived as Mdyn = (3.2 ± 0.6) × 104−1
M⊙ and Vdyn = −49.5+1.0. Figure 1(c) shows the−0.7 km s
reproduced velocity field with the best-fit parameters. These
results strongly suggest that an enormous mass of 3 × 104 M⊙
is packed within an area much smaller than 0.07 pc. Since
no bright counterparts have been detected in this direction, it isquite likely that a massive IMBH with a very low accretion rateis lurking in HCN–0.009–0.044. This putative IMBH may be
identified as a radio-continuum point source with the ngVLA.
3. Potential of the ngVLA to search for IMBHs
The ngVLA is expected to discover many Galactic stellarmass BHs (Maccarone et al. 2018), IMBHs in globular clusters (Wrobel et al. 2018; Wrobel & Nyland 2020), and massive
5
BHs with masses of >∼ 10 M⊙ wandering around the outskirtsof galaxies (Guo et al. 2020). We propose that HVCCs suchas HCN–0.009–0.044 (Takekawa et al. 2019a), HCN–0.085–
0.094 (Takekawa et al. 2020), CO–0.40–0.22 (Oka et al. 2016,2017), and CO–0.31+0.11 (Takekawa et al. 2019b), are goodcandidates for ngVLA observations to search for point-like radio sources corresponding to IMBHs. The high sensitivity andspatial resolution of the ngVLA will enable us to detect a number of point sources toward HVCCs. The wide-band spectra
are essential since non-thermal radio emission is expected tocome from BHs. Radio-loud point sources could be effectively
detected as IMBH candidates.
Radio variability is also an important clue to investigate theidentity of the detected point sources. IMBHs could exhibit

short-timescale radio variability like that of Sgr A∗ . The variability of Sgr A∗ at 43 and 22 GHz has been detected with aduration of ∼ 2 hr (Yusef-Zadeh et al. 2006). Assuming avariability timescale of a BH is proportional to its mass, thevariability timescale of an IMBH can be expected to be ∼ 20
sec for MBH = 104 M⊙ (or ∼ 2 sec for MBH = 103 M⊙ ) on theanalogy of the flaring behavior of Sgr A∗ . Combining the observed spectra and variability with the kinematic informationof the ambient gas, we may obtain more convincing evidenceof the presence of IMBHs in HVCCs.
Molecular line observations at millimeter wavelengths (SiO
J=2–1, CS J=2–1, etc.) are useful to trace kinematic structures of HVCCs in more detail. In our previous ALMA observations (Takekawa et al. 2019a, 2020), we have discovered
several ultra-compact clumps with very broad velocity widths(∆V ∼ 50 km s−1 ). These clumps were not spatially resolvedby the angular resolution of ∼ 1′′ (∼ 0.04 pc at the Galactic
center) and there are no counterparts in other wavelengths.
Figure 2 shows one example of the clumps, which is also indicated as “UCC” in Figure 1(a). Such compact clumps are also
good targets for the ngVLA observations. As is the case with
HCN–0.009–0.044, the high-resolution molecular line obser′′vations (∆θ <∼ 0.1 ) will potentially reveal the kinematics ofthe compact clumps. If the broad velocity width of each of
the clumps is attributed to a Keplerian motion around a massive object in the CMZ, its orbital radius and rotational velocity′′
is R <∼ 0.5 (∼ 0.02 pc at the Galactic center) and Vrot ≃ 20
km s−1 , respectively. The enclosed mass can be estimated to
be Mr <
103 M⊙ from these values. If the orbital radius is∼′′
R = 0.1 , the enclosed mass is Mr ∼ 200 M⊙ . The inner structure of a molecular clump with a diameter of 0.1′′ can be tracedby the high angular resolution of the ngVLA. Therefore, notonly massive IMBHs (MBH ∼ 104 –105 M⊙ ) but also lighter
3
ones (MBH <∼ 10 M⊙ ) would possibly be found in the CMZ
based on the observations toward HVCCs.
The ngVLA with long baselines may enable us to measure
the proper motions of point sources even in the CMZ (at 8 kpc
from the Sun). A transverse velocity of 100 km s−1 at a distance of 8 kpc corresponds to a proper motion of 2.7 mas yr−1 ,100Galactic Latitude [deg]3–0.045 –0.0445 –0.044 –0.0435

No. ]
(a)

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[ Jy beam–1 km s–1 ]References
HPBW
0.1 pc(b)0

0.1 0.2 0.3 0.4 0.5[ Jy beam–1 ]

–100

–50

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50

l-V diagram

LSR Velocity [km s–1]

be an order of 108 (e.g., Caputo et al. 2017). Some of them
could be dynamically identified by the detailed spectral line
observations. The ngVLA has much potential to find Galactic
isolated BHs including IMBHs, contributing to understanding
the origin of SMBHs and galactic evolution.

–0.006 –0.0065 –0.007 –0.0075

Galactic Longitude [deg]Fig. 2. (a) Integrated intensity map of the ultra-compact clump(UCC) toward HCN–0.009–0.044 in the HCN J=4–3 line obtained with the ALMA. The white ellipse in the lower left corner indicates the synthesized beam size (0.87′′ × 0.71′′ ). (b)Longitude–velocity (l-V) diagram along the yellow line in the panel (a).

which is detectable with the ngVLA. If we can identify an
IMBH candidate source associated with molecular gas streams
constituting a HVCC, we will infer a line-of-sight velocity ofthe source by orbital fits to the gas streams. Thus, we may
be able to constrain the three-dimensional velocities of IMBH
candidates, which should be useful to understand their origins.
Hydrogen recombination lines may also be used in the
search of IMBHs. Tsuboi et al. (2017) have suggested the possibility that IRS13E, which is an infrared source in the vicinityof Sgr A∗ , can contain a ∼ 104 M⊙ IMBH based on the radio
recombination line observations of the high-velocity ionizedgas. We found the faint Pα line emission at 1.87 µm toward
the dynamical center of HCN–0.009–0.044 (see Figures 2 and3 in Takekawa et al. 2019a). This faint emission blob may be
detectable by radio recombination lines with the high sensitivity of the ngVLA. The kinematic analysis of the ionized gas
will provide valuable information to diagnose the presence ofan IMBH.
As well as in the CMZ, broad-velocity-width features ofmolecular gas are possible probes of isolated BHs in the
Galactic disk. Indeed, we have found several HVCC-like features in the disk, which are potentially driven by high-velocity
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