Details of a Researcher - Oka, Tomoharu

Formation Process of Supermassive Black Holes in Galactic Nuclei,

2006

-

Present
Observational Study of Interstellar Matter in the Galactic Center Region,

1996

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Present

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Books 【 Display / hide 】

銀河の中心に潜むもの　ブラックホールと重力波の謎にいどむ

OKA Tomoharu, 慶應義塾大学出版会, 2018.01
人物でよむ物理法則の事典

Tomato Yonezawa, Kazuhiko Tsuji, etc., Asakura Publishing Co., Ltd., 2015.11

Scope: R. W. ウィルソン、エディントン、オテルマ、B. P. シュミット、チャンドラセカール、ディッケ、ホイヘンス

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Papers 【 Display / hide 】

Time Variations in the Flux Density of Sgr A∗ at 230 GHz Detected with ALMA

Iwata Y., Oka T., Tsuboi M., Miyoshi M., Takekawa S.

Astrophysical Journal Letters （Astrophysical Journal Letters) 892 （ 2 ） 2020.04

ISSN 20418205

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© 2020. The Author(s). Published by the American Astronomical Society.. A radio source at the Galactic center Sgr A∗ is a prime supermassive black hole candidate and therefore key to developing our understanding of them. Time variations in the 230 GHz band flux of Sgr A∗ have been found with the Atacama Large Millimeter/submillimeter Array Cycle 5 observations. Measuring the flux density of Sgr A∗ in 1 minute snapshots at 217.5, 219.5, and 234.0 GHz, we obtained light curves for ten 70 minute periods. The light curves show variations at a few tens of minutes and hourly scales. The shorter timescale is similar to the orbital period of the innermost stable circular orbit around a 4 × 106 M black hole, suggesting that the variation originates from the immediate vicinity of Sgr A∗. We also detected no time lag between 217.5 and 234.0 GHz and a dependence of the spectral index on the flux density.
- Access to Document (DOI)
The Fifth Candidate for an Intermediate-mass Black Hole in the Galactic Center

Takekawa S., Oka T., Iwata Y., Tsujimoto S., Nomura M.

Astrophysical Journal （Astrophysical Journal) 890 （ 2 ） 2020.02

ISSN 0004637X

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© 2020 The Author(s). Published by the American Astronomical Society. We report the results of high-resolution molecular line observations of the high-velocity compact cloud HCN-0.085-0.094 with the Atacama Large Millimeter/submillimeter Array. The HCN J = 4-3, HCO+ J = 4-3, and CS J = 7-6 line images reveal that HCN-0.085-0.094 consists mainly of three small clumps with extremely broad velocity widths. Each of the three clumps has a 5.5 GHz radio continuum counterpart in its periphery toward Sgr A∗. The positional relationship indicates that their surfaces have been ionized by ultraviolet photons from young stars in the central cluster, suggesting the clumps are in close proximity to the Galactic nucleus. One of the three clumps has a ring-like structure with a very steep velocity gradient. This kinematical structure suggests an orbit around a point-like object with a mass of ∼104 M o&dot;. The absence of stellar counterparts indicates that the point-like object may be a quiescent black hole. This discovery adds another intermediate-mass black hole candidate in the central region of our Galaxy.
- Access to Document (DOI)
High-resolution CO images of the Galactic central molecular zone

Tokuyama S., Oka T., Takekawa S., Iwata Y., Tsujimoto S., Yamada M., Furusawa M., Nomura M.

Publications of the Astronomical Society of Japan （Publications of the Astronomical Society of Japan) 71 2019.12

ISSN 00046264

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© 2019 The Author(s) 2019. Published by Oxford University Press on behalf of the Astronomical Society of Japan. We performed Nyquist-sampled mapping observations of the central molecular zone of our Galaxy in the J = 1-0 lines of CO, 13CO, and C18O using the 45 m telescope at the Nobeyama Radio Observatory. The newly obtained data sets were an improvement by a factor of four in spatial resolution of the CO data previously obtained with the same telescope 22 years ago, providing the highest angular resolution CO atlas of this special area of the Galaxy. The data cover the area: -0${^{\circ}_{.}}$8 ≤ l ≤ +1${^{\circ}_{.}}$4 and -0${^{\circ}_{.}}$35 ≤ b ≤ +0${^{\circ}_{.}}$35 with a 15″ beamwidth. Total intensity ratios for CO J = 3-2/J = 1-0, 13CO/CO J = 1-0 and C18O/13CO J = 1-0, are 0.70 ± 0.06, 0.12 ± 0.01, and 0.14 ± 0.01, respectively. The high-resolution CO images show the fine structure of the molecular gas and enable us to identify a number of compact clouds with broad velocity widths, i.e., high-velocity compact clouds. We conducted a detailed comparison of our CO J = 1-0 data with the CO J = 3-2 data obtained with the James Clerk Maxwell Telescope to derive the distribution and kinematics of the highly excited gas. Three, out of four, of the previously identified high CO J = 3-2/J = 1-0 ratio areas at l = +1${^{\circ}_{.}}$3, 0${^{\circ}_{.}}$0, and -0${^{\circ}_{.}}$4 were confirmed with a higher spatial resolution. In addition to these, we identified several very compact, high CO J = 3-2/J = 1-0 spots with broad velocity widths for the first time. These are candidates for accelerated gas in the vicinity of invisible, point-like massive objects.
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An energetic high-velocity compact cloud: CO-0.31+0.11

Takekawa S., Oka T., Tokuyama S., Tanabe K., Iwata Y., Tsujimoto S., Nomura M., Shibuya Y.

Publications of the Astronomical Society of Japan （Publications of the Astronomical Society of Japan) 71 2019.12

ISSN 00046264

　View Summary

© 2019 The Author(s) 2019. Published by Oxford University Press on behalf of the Astronomical Society of Japan. We have discovered an energetic high-velocity compact cloud CO-0.31+$0.11 in the central molecular zone of our Galaxy. CO-0.31+$0.11 is located at a projected distance of ∼45 pc from the Galactic nucleus Sgr A∗. It is characterized by its compact spatial appearance (d 4 pc), extremely broad velocity width (ΔV > 100 km s-1), and high CO J = 3-2/J = 1-0 intensity ratio. The total gas mass and kinetic energy are estimated as approximately 104, M⊙ and 1051 erg, respectively. Two expanding bubble-like structures are found in our HCN J = 1-0 map obtained with the Nobeyama Radio Observatory 45 m telescope. In the longitude-velocity maps, CO-0.31+$0.11 exhibits an asymmetric V shape. This kinematical structure can be well fitted by Keplerian motion on an eccentric orbit around a point mass of 2× 105 M⊙. The enhanced CO J = 3-2/J = 1-0 ratio is possibly attributed to the tidal compression during the pericenter passage. The model suggests that a huge mass is packed within a radius of r < 0.1 pc. The huge mass, compactness, and absence of luminous stellar counterparts may correspond to a signature of an intermediate-mass black hole (IMBH) inside. We propose a formation scenario of CO-0.31+0.11 in which a compact cloud has gravitationally interacted with an IMBH and a bipolar molecular outflow was driven by the past activity of the putative IMBH.
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Indication of Another Intermediate-mass Black Hole in the Galactic Center

Takekawa S., Oka T., Iwata Y., Tsujimoto S., Nomura M.

Astrophysical Journal Letters （Astrophysical Journal Letters) 871 （ 1 ） 2019.01

ISSN 20418205

　View Summary

© 2019. The American Astronomical Society. We report the discovery of molecular gas streams orbiting around an invisible massive object in the central region of our Galaxy, based on the high-resolution molecular line observations with the Atacama Large Millimeter/submillimeter Array. The morphology and kinematics of these streams can be reproduced well through two Keplerian orbits around a single point mass of (3.2 ± 0.6) ×10 4 M o . We also found ionized gas toward the inner part of the orbiting gas, indicating dissociative shock and/or photoionization. Our results provide new circumstantial evidences for a wandering intermediate-mass black hole in the Galactic center, suggesting also that high-velocity compact clouds can be probes of quiescent black holes that abound in our Galaxy.
- Access to Document (DOI)

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Papers, etc., Registered in KOARA 【 Display / hide 】

Search for isolated black holes in the Milky Way galaxy based on molecular spectral line observations

Oka, Tomoharu

学事振興資金研究成果実績報告書（慶應義塾大学) 2019
Intensive observations of the intermediate-mass black hole candidate CO-0.40-0.22*

Oka, Tomoharu

学事振興資金研究成果実績報告書（慶應義塾大学) 2018
Search for intermediate-mass black holes in the galactic center based on molecular line observations

Oka, Tomoharu

科学研究費補助金研究成果報告書 2017
Search for super star clusters and intermediate-mass black holes in the depths of our galaxy

Oka, Tomoharu

科学研究費補助金研究成果報告書 2014
Observational Study of the Galactic Nuclues Sgr A* : Mass Accretion and Evolution

OKA, TOMOHARU

科学研究費補助金研究成果報告書 2008

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Reviews, Commentaries, etc. 【 Display / hide 】

銀河系中心のもう一つのブラックホール

OKA Tomoharu

パリティ (Maureen Publishing Co., Ltd.) 32 ( 1 ) 58 - 60 2016.12

Article, review, commentary, editorial, etc. (trade magazine, newspaper, online media), Single Work
Atomic Carbon in the Galaxy

OKA Tomoharu

The Astronomical Herald 99 ( 4 ) 190 - 196 2006.03

Article, review, commentary, editorial, etc. (scientific journal), Single Work
超新星残骸と相互作用する星間分子雲

岡朋治、瀬田益道

原子核研究「超高エネルギー天体 (I)」 42 ( 5 ) 141 - 148 1998

Article, review, commentary, editorial, etc. (scientific journal)
Galactic Center Me¥molecular Clouds -- Micro-scale Structure and Physical Conditions --

OKA Tomoharu

天文月報 88 ( 12 ) 506 - 511 1995.11

Article, review, commentary, editorial, etc. (scientific journal)

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Presentations 【 Display / hide 】

ブラックホールとは何か

岡朋治

10MTV オピニオン「大人のための教養講座」,

2019.03
,
Media coverage, イマジニア株式会社
銀河系中心で発見された高速度雲の起源

岡朋治

ブラックホール地平面勉強会＜銀河系中心とJASMINE＞ (慶應義塾大学日吉キャンパス) ,

2018.12
,
Oral presentation (invited, special)
ALMAによる小型高速度コンパクト雲の詳細観測

竹川俊也, 岡朋治, 岩田悠平, 辻本志保, 野村真理子

日本天文学会2018年秋季年会 (千葉大学) ,

2018.09
,
Oral presentation (general)
高速度コンパクト雲CO 0.02-0.02の3 mm帯ラインサーベイ観測

岩田悠平, 岡朋治, Justin A. Roll, 辻本志保, 竹川俊也

日本天文学会2018年秋季年会 (千葉大学) ,

2018.09
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Oral presentation (general)
高速度コンパクト雲 CO–0.40–0.22 中に発見された点状電波源の観測的研究

岩田悠平, 岡朋治, 杉本知穂, 竹川俊也, 田邊恭介, 三好真

日本天文学会2018年春季年会 (千葉大学) ,

2018.03
,
Oral presentation (general)

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Research Projects of Competitive Funds, etc. 【 Display / hide 】

Search for Missing Black Holes in the Galaxy based on Submillimeter-wave Observations

2020.04

-

2025.03

MEXT,JSPS, Grant-in-Aid for Scientific Research, Grant-in-Aid for Scientific Research (A) , Principal investigator
Search for Intermediate-mass Black Holes in the Galactic Center Based on Molecular Line Observations

2015.04

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2018.03

MEXT,JSPS, Grant-in-Aid for Scientific Research, Grant-in-Aid for Scientific Research (B), Principal investigator