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Millimetre-wave emission from an intermediate-mass black hole candidate in the Milky Way

Nature Astronomy volume 1, pages 709–712 (2017)Cite this article

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Abstract

It is widely accepted that black holes with masses greater than a million solar masses (M ⊙) lurk at the centres of massive galaxies. The origins of such ‘supermassive’ black holes (SMBHs) remain unknown1, although those of stellar-mass black holes 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 clusters2, merge at the centre of a galaxy to form a SMBH3. Although many candidates for IMBHs have been proposed, none is 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 about 105 M ⊙ is lurking in this cloud4. Here we report the detection of a point-like continuum source as well as a compact gas clump near the centre 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 one of the most promising candidates for an intermediate-mass black hole.

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 105 M ⊙. 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 (M BH ≈ 1,300M ⊙)9,10.

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Fig. 1: ALMA views of CO–0.40–0.22.
Fig. 2: Gas kinematics around CO–0.40–0.22*.
Fig. 3: Wide-band spectrum of CO–0.40–0.22*.
Fig. 4: N-body simulations of the gravitational kick.

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Acknowledgements

This paper makes use of the following ALMA data: ADS/JAO.ALMA#2011.0.01234.S. ALMA is a partnership of ESO (representing its member states), NSF (USA) and NINS (Japan), together with NRC (Canada), MOST and ASIAA (Taiwan), and KASI (Republic of Korea), in cooperation with the Republic of Chile. The Joint ALMA Observatory is operated by ESO, AUI/NRAO and NAOJ. We thank the ALMA staff for the operation of the array and delivering the qualified data. We also thank S. Nakashima and M. Nobukawa for calculating the upper limit to the X-ray flux, and A. E. Higuchi for helping in ALMA data reduction with CASA. T.O. acknowledges support from JSPS Grant-in-Aid for Scientific Research (B) No. 15H03643.

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Authors and Affiliations

  1. Department of Physics, Institute of Science and Technology, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama, Kanagawa, 223-8522, Japan

    Tomoharu Oka & Mariko Nomura

  2. School of Fundamental Science and Technology, Graduate School of Science and Technology, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama, Kanagawa, 223-8522, Japan

    Tomoharu Oka, Shiho Tsujimoto, Yuhei Iwata & Shunya Takekawa

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Contributions

T.O. directed the research, analysed the data and wrote the manuscript. S.Ts. and M.N. performed the model calculation. Y.I. and S.Ta. contributed to the analyses and discussion.

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Correspondence to Tomoharu Oka.

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Oka, T., Tsujimoto, S., Iwata, Y. et al. Millimetre-wave emission from an intermediate-mass black hole candidate in the Milky Way. Nat Astron 1, 709–712 (2017). https://doi.org/10.1038/s41550-017-0224-z

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