Press Release

March 6, 2023
Keio University
National Astronomical Observatory of Japan
Kanagawa University“Tadpole” Molecular Cloud Discovered near the Center of the Milky Way－Interacting with a Black Hole?－
A team of researchers has identified an isolated “Tadpole” molecular cloud near Sagittarius
A* at the galactic nucleus of the Milky Way. The discovery was made by Miyuki Kaneko, a
2nd Year Master’s student in the Graduate School of Science and Technology at Keio
University, and Professor Tomoharu Oka who works in the Department of Physics at theFaculty of Science and Technology at the same school. Researchers from the NationalAstronomical Observatory of Japan and Kanagawa University also contributed to the study.
The molecular cloud they discovered is shaped like an arc in space and its radial velocity clearlyvaries monotonically along this arc. The researchers were able to replicate this spatial-velocity
behavior by using a Keplerian orbit modeled around a point-like massive object (point mass)estimated to have a mass 100,000 times that of the Sun. Examinations of other wavelengths
did not reveal any luminous counterpart, meaning that this point mass is not a high-density
stellar cluster. At this point in time, the most likely explanation is that the point mass is anintermediate-mass black hole1. This object may be the most convincing candidate for anintermediate-mass black hole among any that have been discovered from molecular gas
kinematics.
This paper was published in the January 10 issue of The Astrophysical Journal, an
American academic journal that specializes in astrophysics research.
1. Main Points of Research・Researchers have discovered a molecular cloud with a tadpole-like structure in position–velocity
space near the galactic nucleus of the Milky Way.
・This position-velocity structure can be reproduced by Keplerian motion around a point mass of
100,000 times that of the Sun.
・The discovered point-like massive “object” may, in fact, be an intermediate-mass black hole due
to its assumed mass and the absence of such corresponding objects in that vicinity.
2. Background of ResearchBlack holes with millions of times more mass than the Sun lurk at the center of most galaxies
in our universe. These “supermassive” black holes are believed to form and grow throughsmaller black holes merging. However, scientists have yet to confirm the existence ofintermediate-mass black holes (those with 102–105 solar masses)2 which should appear during
the formation of a supermassive black hole, although several potential candidates have been
proposed.
The nucleus of our Milky Way Galaxy, Sagittarius A*, also harbors a supermassive black holewith a mass of 4 million times that of the Sun. There is a cluster of stars in this vicinity known
as “IRS 13E” which was suggested to have an intermediate-mass black hole (of several thousandsolar masses). However, this hypothesis has not been proved and there are other competing
explanations. The group conducting this current study, though, has pointed out the possibility
that multiple intermediate-mass black holes could exist in the same general area other than IRS
13E, based on the discovery of compact clouds with unusually broad velocity widths3 (ref. July

1/5

18, 2017, press release) in the Central Molecular Zone4. These phenomena, however, may not becaused by a black hole either. It is possible for other objects and factors to generate similarmolecular clouds, meaning that it is still open to interpretation. To confirm their hypothesis,
they must be able to detect and accurately reproduce the gas kinematics that are produced by agravitational mass point that resembles a black hole. This type of research is vital because it
sheds light on the formation and evolution of the supermassive black holes at galactic nuclei.

Figure 1: (a) Map of velocity-integrated CO 346 GHz emission near the galactic nucleus(Sagittarius A* is marked by a +). (b) Zoomed map of the area around the “Tadpole.” (c) Positionvelocity map along the light blue line in Figure (b). (d) 3D diagram of intensity peak positions ateach velocity (purple crosses) and Keplerian orbits (solid green lines).

3. Research Design and Findings
The research team used the James Clerk Maxwell Telescope5 to gather data on carbon
monoxide (CO) line spectra and searched specifically for molecular clouds that were bothcompact and had broad velocity widths, characteristics they believed would imply the workingsof a point mass as a source of gravity. One of their findings included a peculiar molecular cloudthat was to the northwest of Sagittarius A*, about three arcminutes away (equivalent to 20 lightyears). This molecular cloud was clearly isolated and had a distinctive tadpole-like shape inposition-velocity space (Figure 1: a-c). They were able to corroborate the existence of this object
by using CO and CS line data from the Nobeyama Radio Observatory’s 45m telescope.

2/5

Detailed analysis revealed that the discovered molecular cloud is shaped like an arc in space
and its radial velocity clearly varies monotonically along this arc. (Figure 1: d). This spatial velocity
structure suggests that the molecular gas is moving and being distributed along a single closedorbit. The team was also able to reproduce this when using a Keplerian orbit around a presumed
point mass of 100,000 times the mass of the Sun. This implies that the “Tadpole” is orbiting apoint-like gravitational source that has enormous mass. In addition, the behavior of physicalconditions suggested by the spectral line intensity ratios also demonstrates that the gas is being
trapped by a point-like gravitational source.
The team then tried to identify this point mass by examining various wavelength images of
the area surrounding the Tadpole. However, they were unable to find any luminous objects inthe predicted vicinity, meaning that it is very unlikely the point mass is a star cluster.
Furthermore, the lower limit to the mass density derived from the orbital parameters isenormous, suggesting the point mass is an intermediate-mass black hole. Taken together, thesefacts provide evidence that the “Tadpole” is a molecular cloud accelerated by the gravitational
interaction with an inactive black hole of approximately 100,000 solar masses.

4. Future Developments
This research paper reveals the discovery of a new candidate intermediate-mass black hole inthe vicinity of the galactic nucleus, Sagittarius A*. Their proof is the existence of a “Tadpole”
molecular cloud which is clearly isolated from other molecular clouds, and which has amonotonous spatial velocity structure. This rules out the possibility that the Tadpole’s unusual
velocity width could be caused by anything other than an interaction with a point-likegravitational source. Therefore, the Tadpole’s is the most convincing candidate for anintermediate-mass black hole that has been discovered from molecular gas kinematics thus
far. Due to its extremely close proximity to Sagittarius A*, it is likely that the intermediate-mass
black hole which drives the Tadpole will eventually be swallowed by the supermassive black holeat the galactic nucleus.
The research team from this paper plans to make high-resolution observations using theALMA6. This may allow them to delineate the spatial velocity structure of molecular gas in a
Keplerian orbit more clearly. Once this detailed structure is obtained, the scientists hope todetermine its precise orbital parameters. In a similar previous study, high-resolution
observations have captured emission from the immediate vicinity of a candidate intermediatemass black hole (Press Release, September 27, 2017). Following these examples, the team’s
ALMA observations and search for counterparts may unveil the nature and entity of the point
mass that has formed the Tadpole.

Fig. 2: An artistic depiction of the “Tadpole” orbiting a black hole

3/5

References・Keio University Press Release (July 18, 2017)
Two New Possible “Rogue Black Holes” found in the Center of the Milky Way
https://www.keio.ac.jp/ja/press-releases/2017/7/18/28-21984/ (Japanese language only)
・Keio University Press Release (September 27, 2017)
First Detection of an Intermediate-Mass Black Hole Candidate in the Milky Way
https://www.keio.ac.jp/en/press-releases/2017/Sep/27/49-24314/
Details of Journal Article
This paper was published in the January 10 issue of The Astrophysical Journal, an American
academic journal that specializes in astrophysics research. Below is the official title of the paper
as well as the names and affiliations of its authors.
“Discovery of the Tadpole Molecular Cloud near the Galactic Nucleus”
Miyuki Kaneko
Tomoharu Oka
Hiroki Yokozuka
Rei Enokiya
Shunya Takekawa
Yuhei Iwata
Shiho Tsujimoto

2nd year MA student, Graduate School of Science and Technology, KeioUniversity
Professor, Department of Physics, Faculty of Science and Technology, KeioUniversity
Completed MA program in March 2022, Graduate School of Science andTechnology, Keio University
Project Researcher, Department of Physics, Faculty of Science and Technology,
Keio University
Assistant Professor, Department of Physics, Faculty of Engineering, Kanagawa
University
Project Researcher, Division of Science, National Astronomical Observatory ofJapan
Completed Ph.D. program in March 2021, Graduate School of Science andTechnology, Keio University

The Astrophysical Journal, January 10, 2023, vol. 942, Number 1, 46（8pp）
https://iopscience.iop.org/article/10.3847/1538-4357/aca66a
doi: 10.3847/1538-4357/aca66a

Glossary and References*1 Black HoleA dark astronomical object that has such a strong gravitational pull that it distorts space-time
so that not even light can escape. Its existence was predicted by Einstein's general theory ofrelativity.
*2 Solar Mass
A unit of mass used in astronomy. 1 Solar Mass = 1.99×1030 kg.
*3 Velocity WidthObserved spectral lines are deviated in frequency according to the relative velocity (line-ofsight or radial velocity) between the object and observer due to the Doppler effect. An object’sradial velocity can be determined by measuring the amount of change in this frequency. A
spectral line width expressed in frequencies can also be converted into radial velocity, which isthen called “velocity width.”*4 Central Molecular Zone
A region of the Milky Way Galaxy extending around Sagittarius A* with a radius ofapproximately 1,000 light years.

4/5*5 James Clerk Maxwell Telescope
Submillimeter-wavelength radio telescope at the Mauna Kea Observatory in Hawaii. It iscurrently operated by the East Asia Observatory.
*6 The Atacama Large Millimeter/submillimeter Array (ALMA)
A large radio interferometer on a plateau 5,000 meters above sea level in the Atacama Desertof northern Chile. The project is an international collaboration promoted by countries aroundthe world with East Asia represented by the National Astronomical Observatory of Japan, theU.S represented by the North American Union, and Europe represented by the European
Southern Observatory.
*Please direct any requests or inquiries to the contacts listed below in advance of any press
coverage.
Inquiries about research
Department of Physics, Faculty of Science and Technology, Keio University
Professor Tomoharu Oka
Tel. +81 45-566-1833
Email: tomo@phys.keio.ac.jp
http://aysheaia.phys.keio.ac.jp/index.html
Source of this release
Keio University Office of Communications and Public Relations (Mochizuki)
Tel. +81 3-5427-1541 Fax +81 3-5441-7640
Email: m-pr@adst.keio.ac.jp
https://www.keio.ac.jp/en/
Public Relations Office, Public Relations Center, National Astronomical Observatory of JapanTel. +81 90-1257-7980 (Head of the Public Relations Office, Yamaoka)
Email: media_contact@ml.nao.ac.jp
Public Relations Group, Nobeyama Radio Observatory, National Astronomical Observatory ofJapan (Nishimura)
Tel. +81 90-9358-0365
Email: atsushi.nishimura@nao.ac.jp

Public Relations Department, Public Relations Division, Kanagawa University（Shiino）
TEL: +81-45-481-5661
Email: kohou-info@kanagawa-u.ac.jp

5/5