Published papers

The spectral line of the guide map (Figure 1) was mischosen in the published article. The revised Figure 1 with the correct line maps is provided here. (Figure Presented).

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, H13CN J = 4-3, H13CO+ J = 4-3, SiO J = 8-7, CH3OH J K a, K c = 71,7-61,6 A+ lines, and 900 μm continuum show several new features, which have not been identified in previous observations. The dense gas probe (H13CN, SiO, CH3OH) images are dominated by a pair of northeast-southwest elongated filaments, which may be the main body of CO 0.02-0.02. Two striped patterns perpendicular to each other (F1 and F2) and a high-velocity feature (HV), which appear in different velocity ranges, were prominent in the CO image. An emission hole that may represent an expanding feature was found in the F1 velocity range. F2 appeared to align along the western edge of a 20 pc × 13 pc ellipse (the Large Shell) identified in the single-dish CO map. The HV contains eight compact clumps at the positive high-velocity end of the CO emissions. Based on these results, we propose a formation scenario for CO 0.02-0.02; internal explosions of supernovae, external perturbations by the Large Shell, and gravitational acceleration by a less-luminous star cluster have formed CO 0.02-0.02 in its current state.

We performed a systematic search for broad-velocity-width molecular features (BVFs) in the disk part of our Galaxy by using the CO J = 1-0 survey data obtained with the Nobeyama Radio Observatory 45 m telescope. From this search, 58 BVFs were identified. In comparisons with the infrared and radio continuum images, 36 BVFs appeared to have both infrared and radio continuum counterparts, and 15 of them are described as molecular outflows from young stellar objects in the literature. In addition, 21 BVFs have infrared counterparts only, and eight of them are described as molecular outflows in the literature. One BVF (CO 16.134-0.553) does not have any luminous counterpart in the other wavelengths, which suggests that it may be an analog of high-velocity compact clouds in the Galactic center.

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 James Clerk Maxwell Telescope, to identify 184 high-velocity dispersion compact clouds (HVDCCs), which are a category of peculiar molecular clouds found in the central molecular zone of our Galaxy. A list of HVDCCs in the area -1.°4 ≤ l ≤ +2.°0, -0.°25 ≤ b ≤ +0.°25 was presented with their physical parameters, CO J = 3 - 2/J = 1 - 0 intensity ratios, and morphological classifications. Consequently, the list provides several intriguing sources that may have been driven by encounters with pointlike massive objects, local energetic events, or cloud-to-cloud collisions.

The youngest known Galactic supernova remnant (SNR) G1.9+0.3 has high-velocity supernova shock beyond 10000 km s−1, and it is considered to be one of the major candidates of a PeVatron. Despite these outstanding properties, the surrounding interstellar matter of this object is poorly understood. We investigated the interstellar gas toward G1.9+0.3 using the 12CO(J = 3−2) data with the angular resolution of 15◦ obtained by the CHIMPS2 survey by the James Clerk Maxwell Telescope, and discovered three individual clouds at −1, 7, and 45 km s−1. From its morphological and velocity structures, the −1 km s−1 cloud, having the largest velocity width >20 km s−1 and located at the distance of the Galactic Center, is possibly associated with the SNR. The associated cloud shows a cavity structure both in space and velocity and coincides well with the SNR. We found that the associated cloud has higher column densities toward three bright, radio synchrotron-emitted rims where the radial expansion velocity of the supernova shock is decelerated, and the cloud is faint in the other parts of the SNR. This is the first direct evidence indicating that the highly anisotropic expansion of G1.9+0.3 observed by previous studies results from the deceleration by the interaction between the supernova shock and surrounding dense interstellar medium.

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 position-velocity space. By tracing the CO J = 3-2 intensity peak in each velocity channel, we noticed that the kinematics of the Tadpole can be well reproduced by a Keplerian motion around a point-like object with a mass of 1 × 105 M ⊙. Changes in line intensity ratios along the orbit are consistent with the Keplerian orbit model. The spatial compactness of the Tadpole and absence of bright counterparts in other wavelengths indicate that the object could be an intermediate-mass black hole.

The l = +1.3 region in the Galactic center is characterized by multiple shell-like structures and their extremely broad velocity widths. We revisit the molecular superbubble hypothesis for this region, based on high-resolution maps of CO J = 1-0, 13CO J = 1-0, H13CN J = 1-0, H13CO+ J = 1-0, SiO J = 2-1, and CS J = 2-1 lines obtained from the Nobeyama Radio Observatory 45 m telescope, as well as CO J = 3-2 maps obtained from the James Clerk Maxwell telescope. We identified 11 expanding shells with total kinetic energy and typical expansion time E kin ∼ 1051.9 erg and t exp ∼ 104.9 yr, respectively. In addition, the l = +1.3 region exhibited high SiO J = 2-1/H13CN J = 1-0 and SiO J = 2-1/H13CO+ J = 1-0 intensity ratios, indicating that the region has experienced dissociative shocks in the past. These new findings confirm the molecular superbubble hypothesis for the l = +1.3 region. The nature of the embedded star cluster, which may have supplied 20-70 supernova explosions within 105 yr, is discussed. This work also shows the importance of compact broad-velocity-width features in searching for localized energy sources hidden behind severe interstellar extinction and stellar contamination.

The l=-1buildrel{circ}over{.} 2 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=-1buildrel{circ}over{.} 2 region based on the CO J = 1–0, 13CO J = 1–0, CO J = 3–2, and SiO J = 8–7 line data sets obtained with the Nobeyama Radio Observatory 45 m telescope and James Clerk Maxwell Telescope. The kinetic energy and expansion time of the expanding shells are estimated to be {10}48.3{--50.8} erg and {10}4.7{--5.0} yr, respectively. The origin of these expanding shells is discussed. The total kinetic energy of 1051 erg and the typical expansion time of ∼105 yr correspond to multiple supernova explosions at a rate of 10‑5–10‑4 yr‑1. This indicates that the l=-1buildrel{circ}over{.} 2 region may be a molecular bubble associated with an embedded massive star cluster, although the absence of an infrared counterpart makes this interpretation somewhat controversial. The expansion time of the shells increases as the Galactic longitude decreases, suggesting that the massive star cluster is moving from Galactic west to east with respect to the interacting molecular gas. We propose a model wherein the cluster is moving along the innermost x 1 orbit and the interacting gas collides with it from the Galactic eastern side.

© 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.

© 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.

© 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˙. 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.

© 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.

© 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.

© 2018. The American Astronomical Society. All rights reserved.. Using two-dimensional magnetohydrodynamic simulations, we investigated the gas dynamics around a black hole (BH) plunging into a molecular cloud. In these calculations, we assumed a parallel-magnetic-field layer in the cloud. The size of the accelerated region is far larger than the Bondi-Hoyle-Lyttleton radius, being approximately inversely proportional to the Alfvén Mach number for the plunging BH. Our results successfully reproduce the "Y" shape in position-velocity maps of the "Bullet" in the W44 molecular cloud. The size of the Bullet is also reproduced within an order of magnitude using a reasonable parameter set. This consistency supports the shooting model of the Bullet, according to which an isolated BH plunged into a molecular cloud to form a compact broad-velocity-width feature.

The high-velocity compact cloud CO-0.40-0.22 was mapped in 22 molecular lines with the NRO 45 m radio telescope and the ASTE 10 m telescope. The map of each detected line shows that this cloud has a compact appearance (d~=3 pc) and extremely broad velocity width (Δ V~=100 km s-1). The representative position-velocity map along the major axis shows that CO-0.40-0.22 consists of an intense region with a shallow velocity gradient and a less intense high-velocity wing. This kinematical structure can be attributed to a gravitational kick to the molecular cloud caused by an invisible compact object with a mass of ~105 M ⊙. Its compactness and the absence of a counterpart at other wavelengths suggest that this massive object is an intermediate-mass black hole.

We report the discovery of physical contact between the Galactic circumnuclear disk (CND) and an adjacent giant molecular cloud. The central 10 pc of our Galaxy has been imaged in molecular lines at millimeter wavelength using the Nobeyama Radio Observatory 45 m radio telescope. In the position-velocity maps of several high-density probe lines, we have found an emission ``bridge'' connecting the +20 km s-1 cloud (M-0.13-0.08) and the negative longitude extension of the CND. The collision between the +20 km s-1 cloud and the CND may be responsible for the formation of the bridge. This event can promote mass accretion onto the CND and/or into the inner cavity.

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 velocity widths (Δ V > 50 km s-1). Some of them show high CO J=3-2/J=1-0 intensity ratios (>= 1.5), indicating that they consist of dense and warm molecular gas. Dispite a number of efforts, we have not reached a comprehensive interpretation of HVCCs. Recently, we detected an extraordinaly broad velocity width feature, the `Bullet', in the molecular cloud interacting with the W44 supernova remnant. The Bullet shares essential properties with HVCCs. Because of its proximity, a close inspection of the Bullet must contribute to the understanding of HVCCs.

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 Experiment 10 m telescope. The Bullet clearly appears in the CO J = 1-0, CO J = 3-2, CO J = 4-3, and HCO+ J = 1-0 maps with a compact appearance (0.5 × 0.8 pc2) and an extremely broad-velocity width (ΔV ≃ 100 km s-1). The line intensities indicate that the Bullet has a higher density and temperature than those in the expanding molecular shell. The kinetic energy of the Bullet amounts to 1048.0 erg, which is approximately 1.5 orders of magnitude greater than the kinetic energy shared to the small solid angle of it. Two possible formation scenarios with an inactive isolated black hole are presented.

This paper reports the discovery of evidence for physical contact between the Galactic circumnuclear disk (CND) and an exterior giant molecular cloud. The central 10 pc of our Galaxy has been imaged in the HCN J = 1-0, HCO+ J = 1-0, CS J = 2-1, H13CN J = 1-0, SiO J = 2-1, SO NJ = 23-12, and HC3N J = 11-10 lines using the Nobeyama Radio Observatory 45 m radio telescope. Based on our examination of the position-velocity maps of several high-density probe lines, we have found that an emission “bridge” may be connecting the +20 km s-1 cloud (M-0.13-0.08) and the negative-longitude extension of the CND. Analyses of line intensity ratios imply that the chemical property of the bridge is located between the +20 km s-1 cloud and the CND. We introduce a new interpretation that a part of the CND may be colliding with the 20 km s-1 cloud and the collision may be responsible for the formation of the bridge. Such collisional events could promote mass accretion onto the CND or into the inner ionized cavity, which may be further tested by proper motion studies.