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A stray black hole that swirls gas clouds
-More reliable evidence of an intermediate-mass black hole
lurking around the center of the Milky Way-

A joint team consisting of Shunya Takekawa, a project researcher at the Nobeyama Radio Observatory of the National Astronomical Observatory of Japan, Professor Tomoharu Oka of the Keio University Faculty of Science and Technology, Department of Physics, and others, elucidated the entity of the peculiar molecular cloud located near the center of the Milky Way using the Atacama Large Millimeter/submillimeter Array (ALMA). Analysis of its internal motion revealed the existence of an intermediate-mass black hole with a mass 30,000 times that of the Sun. The findings of this research are extremely important not only because they lead to an explanation of the origin of supermassive black holes and an understanding of galactic evolution, but also because they lead to the possibility of opening new doors for the investigation of black holes.
The findings of this research were published in the January 20 issue of the American astrophysics journal, The Astrophysical Journal Letters.

[Background of Research]
Many galaxies, including the Milky Way, are known to harbor supermassive black holes at their nuclei, but their formation process is regarded as one of the big mysteries of the universe. In theory, it is thought that intermediate-mass black holes, which have masses of about 100 to 100,000 times that of the Sun, act as “seeds,” which then merge with each other and grow by swallowing surrounding material to form a supermassive black hole. However, even though there have been several reported cases of intermediate-mass black holes, no definitive evidence proving their existence has yet been obtained. In the black hole community, it can be said that intermediate-mass black holes are the missing link and key to explaining the origin of supermassive black holes, so their discovery holds great meaning in astronomy.

[Research Results]
The research team discovered a small peculiar molecular cloud, HCN–0.009–0.044, located about 20 light-years away from “Sagittarius A*,” the nucleus of the Milky Way, from observations carried out in 2016 using the East Asian Observatory’s James Clerk Maxwell Telescope (JCMT). In general, molecular clouds move around the nucleus broadly in accordance with the galactic rotation, but this molecular cloud, HCN–0.009–0.044, showed odd behavior as it seemingly moved in the opposite direction to the gravitational rotation. In order to uncover the identity of this peculiar cloud, the team conducted a high-resolution submillimeter spectral line observations using ALMA. From this, it seemed that this molecular cloud was made up of multiple gas streams having orbital motions influenced by a strong pull of an “invisible gravitational source.” Detailed kinematic analyses revealed that an enormous mass equivalent to 30,000 times that of the Sun was concentrated in a region much smaller than our Solar System. This strongly suggests that a heavy intermediate-mass black hole is drifting near the nucleus of the Milky Way.

[Significance of Research Results ]
The outcomes of this research have great significance for the following 2 reasons. The first is that an intermediate-mass black hole was discovered in the vicinity (at a distance of about 20 light-years away) of Sagittarius A*, the nucleus of the Milky Way. In the future, this intermediate-mass black hole will probably be swallowed by the supermassive black hole at the nucleus and contribute to its growth. Intermediate-mass black holes are key to explaining the origin of supermassive black holes and understanding galactic evolution, so the discovery itself has great meaning.
The other significance is that the black hole discovered in the course of this study is “faint.” Dozens of black holes have been detected in the Milky Way to date, but most of these are black hole binary star systems with a secondary star and a brightly shining accretion disk. According to theoretical predictions, there are at least 100 million black holes in the Milky Way, including large and small ones. In other words, most of the black holes in the Milky Way are faint because not enough material is supplied to them by secondary stars, and for this reason, discovering them through the conventional method of detecting bright emissions from the accretion disk is difficult. However, as was done in this study, attempting to detect gas clouds being swung around by strong gravitational forces is an effective method to search for faint black holes. By focusing on molecular gas clouds with unusual velocities, it is anticipated that many black hole candidates will start to be discovered one after the other. This pioneering series of studies could serve as a cornerstone for breaking new ground in the search for black holes.

<Details of Original Paper>
The findings of this research were published in the January 20 issue of the American astrophysics journal, The Astrophysical Journal Letters. The title of the paper, the authors, and their affiliation at the time the research was carried out are as follows:
“Indication of Another Intermediate-mass Black Hole in the Galactic Center”
Shunya Takekawa (National Astronomical Observatory of Japan, Nobeyama Radio Observatory, Project Researcher)
Tomoharu Oka (Keio University Faculty of Science and Technology, Department of Physics, Professor)
Yuhei Iwata (Keio University, Graduate School of Science and Technology, 1st year doctoral student)
Shiho Tsujimoto (Keio University, Graduate School of Science and Technology, 1st year doctoral student)
Mariko Nomura (Tohoku University, Astronomical Institute, Postdoctoral researcher)
The Astrophysical Journal Letters, January 17, 2019, vol. 871, Number 1, L1 (6pp)
doi: 10.3847/2041-8213/aafb07

fig-512
Figure:Artist’s impression of a gas cloud swirling around a black hole (enlarged view)

<Related links>
National Astronomical Observatory of Japan, Nobeyama Radio Observatory
https://www.nro.nao.ac.jp
Atacama Large Millimeter/submillimeter Array (ALMA)
https://alma-telescope.jp
Keio University Faculty of Science and Technology, Tomoharu Oka Laboratory
http://aysheaia.phys.keio.ac.jp/index.html

<Announcements of Related Research>
・ National Astronomical Observatory of Japan and Keio University Press Release (September , 2017)
First Detection of an Intermediate-mass Black Hole Candidate in the Milky Way
https://www.nao.ac.jp/en/news/science/2017/20170928-alma.html
https://www.keio.ac.jp/en/press-releases/2017/Sep/27/49-24314/

The Atacama Large Millimeter/submillimeter Array (ALMA), an international astronomy facility, is a partnership of the European Organisation for Astronomical Research in the Southern Hemisphere (ESO), the U.S. National Science Foundation (NSF) and the National Institutes of Natural Sciences (NINS) of Japan in cooperation with the Republic of Chile. ALMA is funded by ESO on behalf of its Member States, by NSF in cooperation with the National Research Council of Canada (NRC) and the Ministry of Science and Technology (MOST) and by NINS in cooperation with the Academia Sinica (AS) in Taiwan and the Korea Astronomy and Space Science Institute (KASI). ALMA construction and operations are led by ESO on behalf of its Member States; by the National Radio Astronomy Observatory (NRAO), managed by Associated Universities, Inc. (AUI), on behalf of North America; and by the National Astronomical Observatory of Japan (NAOJ) on behalf of East Asia. The Joint ALMA Observatory (JAO) provides the unified leadership and management of the construction, commissioning and operation of ALMA.

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