Explanipedia

A Face-on Accretion Disk Geometry Revealed by Millimeter-wave Periodicity in Sgr A <sup>∗</sup> Open

Tomoharu Oka, Ryo Ariyama, Yuhei Iwata, Mikiya M. Takahashi · 2025

We analyzed 77 epochs of Atacama Large Millimeter/submillimeter Array archival data to investigate flux variability in Sagittarius A ∗ , the supermassive black hole at the Galactic center. Among these, we identified a rare but unusually cl…

Circum-nuclear eccentric gas flow in the Galactic Center revealed by ALMA CMZ Exploration Survey (ACES) Open

Yoshiaki Sofue, Tomoharu Oka, S. N. Longmore, Daniel L. Walker, Adam Ginsburg , et al. · 2025

We analyze the CS (J=2-1) line cube from the internal data release obtained by the large-scale program "ALMA CMZ Exploration Survey (ACES)" to investigate the kinematic structure of the innermost $\sim 10$ pc region of the Galaxy, which co…

RAMBO. I. Project Introduction and First Results with uGMRT Open

Z. Keszthelyi, Kohei Kurahara, Yuhei Iwata, Yuka Fujii, Haruka Sakemi , et al. · 2025

Magnetic hot stars can emit both coherent and incoherent nonthermal radio emission. Understanding the nature of these emissions and their connection to stellar rotation and magnetic field characteristics remains incomplete. The RAdio Magne…

Radio Follow-up Observations of SN 2023ixf by Japanese and Korean Very Long Baseline Interferometers Open

Yuhei Iwata, Masanori Akimoto, Tomoki Matsuoka, Keiichi Maeda, Yoshinori Yonekura , et al. · 2025

We report on radio follow-up observations of the nearby Type II supernova SN 2023ixf, spanning from 1.7 to 269.9 days after the explosion, conducted using three very long baseline interferometers (VLBIs), which are the Japanese VLBI Networ…

Radio Follow-up Observations of SN 2023ixf by Japanese and Korean VLBIs Open

Yuhei Iwata, Masanori Akimoto, Tomoki Matsuoka, Keiichi Maeda, Yoshinori Yonekura , et al. · 2024

We report on radio follow-up observations of the nearby Type II supernova, SN 2023ixf, spanning from 1.7 to 269.9 days after the explosion, conducted using three very long baseline interferometers (VLBIs), which are the Japanese VLBI Netwo…

The Japanese vision for the Black Hole Explorer mission Open

Kazunori Akiyama, Kotaro Niinuma, Kazuhiro Hada, Akihiro Doi, Yoshiaki Hagiwara , et al. · 2024

Akiyama, Kazunori et al.-- Full list of authors: Akiyama, Kazunori; Niinuma, Kotaro; Hada, Kazuhiro; Doi, Akihiro; Hagiwara, Yoshiaki; Higuchi, Aya E.; Honma, Mareki; Kawashima, Tomohisa; Kolev, Dimitar; Koyama, Shoko; Masui, Sho; Ohsuga, …

Parabolic-like Trend in SiO Ratios throughout the Central Molecular Zone: Possible Signature of a Past Nuclear Activity in the Galactic Center Open

Shunya Takekawa, Tomoharu Oka, Shiho Tsujimoto, Hiroki Yokozuka, Nanase Harada , et al. · 2024

We report the discovery of a characteristic trend in the intensity ratios of SiO emissions across the Central Molecular Zone (CMZ) of our Galaxy. Using the Nobeyama Radio Observatory 45 m telescope, we conducted large-scale, high-sensitivi…

Follow-up surveys for gravitational wave events using Subaru/Hyper Suprime-Cam Open

Nozomu Tominaga, Takayuki Ohgami, Yousuke Utsumi, M. Yoshida, Yuhei Iwata · 2023

Hyper Suprime-Cam (HSC) is a wide-field imager on the Subaru telescope. Its field of view of 1.77 deg$^2$ is the largest among current 8m-class telescopes, which makes the Subaru/HSC the most efficient instrument for the optical survey. We…

ALMA View of the High-velocity-dispersion Compact Cloud CO 0.02–0.02 at the Galactic Center Open

Yuhei Iwata, Tomoharu Oka, Shunya Takekawa, Shiho Tsujimoto, Rei Enokiya · 2023

We report the results of observations toward the center of the molecular cloud CO 0.02–0.02 made using the Atacama Large Millimeter/Submillimeter Array. The successfully obtained 1″ resolution images of CO J = 3–2, H 13 CN J = 4–3, H 13 CO…

ALMA View of the High-velocity-dispersion Compact Cloud CO 0.02-0.02 at the Galactic Center Open

Yuhei Iwata, Tomoharu Oka, Shunya Takekawa, Shiho Tsujimoto, Rei Enokiya · 2023

We report the results of observations toward the center of the molecular cloud CO 0.02-0.02 made using the Atacama Large Millimeter/Submillimeter Array. The successfully obtained 1 arcsec resolution images of CO $J$=3-2, H$^{13}$CN $J$=4-3…

Discovery of the Tadpole Molecular Cloud near the Galactic Nucleus Open

Miyuki Kaneko, Tomoharu Oka, Hiroki Yokozuka, Rei Enokiya, Shunya Takekawa , et al. · 2023

In this paper, we report the discovery of an isolated, peculiar compact cloud with a steep velocity gradient at 2.′6 northwest of Sgr A*. This “Tadpole” molecular cloud is unique owing to its characteristic head-tail structure in the posit…

Catalog of High-velocity Dispersion Compact Clouds in the Central Molecular Zone of Our Galaxy Open

Tomoharu Oka, Asaka Uruno, Rei Enokiya, Taichi Nakamura, Yuto Yamasaki , et al. · 2022

This study developed an automated identification procedure for compact clouds with broad velocity widths in the spectral-line data cubes of highly crowded regions. The procedure was applied to the CO J = 3 − 2 line data, obtained using the…

The Fifth Candidate for an Intermediate-mass Black Hole in the Galactic Center Open

Shunya Takekawa, Tomoharu Oka, Yuhei Iwata, Shiho Tsujimoto, Mariko Nomura · 2020

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 revea…

Indication of Another Intermediate-mass Black Hole in the Galactic Center Open

Shunya Takekawa, Tomoharu Oka, Yuhei Iwata, Shiho Tsujimoto, Mariko Nomura · 2019

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 Arra…

Detection of Another Molecular Bubble in the Galactic Center Open

Shiho Tsujimoto, Tomoharu Oka, Shunya Takekawa, Masaya Yamada, Sekito Tokuyama , et al. · 2018

The $l=-1.2^{\circ}$ region in the Galactic center has a high CO $J$=3-2/$J$=1-0 intensity ratio and extremely broad velocity width. This paper reports the detection of five expanding shells in the $l=-1.2^{\circ}$ region based on the CO $…

KINEMATICS OF ULTRA-HIGH-VELOCITY GAS IN THE EXPANDING MOLECULAR SHELL ADJACENT TO THE W44 SUPERNOVA REMNANT Open

Masaya Yamada, Tomoharu Oka, Shunya Takekawa, Yuhei Iwata, Shiho Tsujimoto , et al. · 2016

We mapped the ultra-high-velocity feature (the “Bullet”) detected in the expanding molecular shell associated with the W44 supernova remnant using the Nobeyama Radio Observatory 45 m telescope and the Atacama Submillimeter Telescope Experi…

Kinematics of the Ultra-High-Velocity Gas in the Expanding Molecular Shell Adjacent to the W44 Supernova Remnant Open

Masaya Yamada, Tomoharu Oka, Kunihiko Tanaka, Mariko Nomura, Shunya Takekawa , et al. · 2016

High-velocity compact cloud (HVCC) is a peculiar category of molecular clouds detected in the central molecular zone of our Galaxy (Oka et al. 1998, 2007, and 2012). They are characterized by compact appearances ( d < 5 pc) and very large …

Possible Detection of Quasi-Periodic Oscillations from Sgr A* at 43 GHz Open

Yuhei Iwata, Tomoharu Oka, Makoto Miyoshi · 2016

Quasi-periodic oscillations (QPOs) are believed to be indirect evidence for black holes. Several authors have reported detections of QPOs from Sgr A*, the nucleus of our Galaxy, in infrared and X-ray wavelength during flare-ups. Miyoshi et…

Statistical Study of High-Velocity Compact Clouds Based on the Complete CO Imagings of the Central Molecular Zone Open

Sekito Tokuyama, Tomoharu Oka, Shunya Takekawa, Masaya Yamada, Yuhei Iwata , et al. · 2016

High-velocity compact clouds (HVCCs) is one of the populations of peculiar clouds detected in the Central Molecular Zone (CMZ) of our Galaxy. They have compact appearances (< 5 pc) and large velocity widths (> 50 km s −1 ). Several explana…

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