arXiv subjects

Shiho Tsujimoto

Publications and source records attributed to Shiho Tsujimoto.

14 recordsLinked to original sources

arXiv2026

The Nobeyama 45 m Survey of Shocked Molecular Gas in the Central Molecular Zone. I. Survey Data, Cloud Catalog, and SiO Line-Ratio Trends

We present large-scale molecular line maps of the Central Molecular Zone (CMZ) in our Galaxy obtained with the Nobeyama Radio Observatory 45 m telescope. The observations cover a 3.5 deg x 0.5 deg region with 20 arcsec resolution in eight molecular lines including SiO J=2-1, CS J=2-1, H13CN J=1-0, and HCN J=1-0. We release the calibrated data cubes and a catalog of SiO-emitting clouds identified by the SCIMES algorithm. For each cloud, we provide cloud-integrated intensities of the observed lines and derive an H13CN-based molecular gas mass and a cloud-averaged H2 number density. We compare SiO intensity ratios with the density, dynamical time, and their product n_H2 t_dyn. We find that SiO intensity ratios relative to six other molecular lines all decrease systematically with increasing n_H2 t_dyn. The tightest correlations are found for SiO/H13CN (r_s = -0.82) and SiO/CS (r_s = -0.70). The released data cubes and cloud catalog enable systematic studies of SiO enhancement, shock chemistry, and gas dynamical evolution across the CMZ.

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arXiv2026

Discovery of Multiple Ultra-Broad-Velocity Molecular Features Associated with the W44 Molecular Cloud

We report the discovery of multiple compact molecular features exhibiting extremely broad velocity widths toward the W44 molecular cloud. ALMA CO $J$=3--2 data reveal eight ``Petit--Bullets'' surrounding the previously known ``Bullet.'' Each Petit--Bullet shows a distinct V-shaped structure in position--velocity space, reminiscent of the Y-shaped morphology of the Bullet, suggesting a common origin. These features are interpreted as the result of high-velocity plunges of compact gravitational objects into dense molecular gas. The spatial and kinematic properties of the Petit--Bullets suggest that the plunging material was not a single object but rather a small cluster of compact bodies. A virial mass of $1.0\!\times\! 10^{5}\, M_\odot$ inferred from their velocity dispersion is comparable to that of typical globular clusters. Momentum analysis further implies that the main Bullet likely formed by an isolated black hole. These findings provide new evidence for dynamical interactions between halo clusters and disk molecular gas.

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arXiv2024

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

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-sensitivity imaging observations in molecular lines including SiO $J$=2$-$1 and CS $J$=2$-$1. By identifying SiO-emitting clouds and examining their intensity ratios relative to the other molecular lines, we unveiled a parabolic-like trend showing lower ratios near the Galactic nucleus, Sgr A$^*$, with gradual increases toward the edges of the CMZ. This pattern suggests a possible outburst of the nucleus within the last $\sim 10^5$ yr, which may have propagated through the entire CMZ with strong shocks. Alternatively, the observed trend may also be attributed to the destruction of small dust grains by high-energy photons. Our results can potentially lead to a new perspective on the history of nuclear activity and its impact on the surrounding molecular environment.

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arXiv2024

Millimeter-wave CO and SiO Observations toward the Broad-velocity-width Molecular Feature CO 16.134-0.553: a Smith cloud scenario?

We report the results of the CO $\textit J$=1-0 and SiO $\textit J$=2-1 mapping observations towards the broad-velocity-width molecular feature CO 16.134-0.553 with the Nobeyama Radio Observatory 45 m telescope. The high quality CO map shows that the 5-pc size broad-velocity-width feature bridges two separate velocity components at $\textit V_{\rm{LSR}}$$\quad$$\simeq$ 40 km s$^{-1}$ and 65 km s$^{-1}$ in the position-velocity space. The kinetic power of CO 16.134-0.553 amounts to $7.8\times10^2$ $\textit L$$_\odot$ whereas no apparent driving sources were identified. Prominent SiO emission was detected from the broad-velocity-width feature and its root in the $\textit V_{\rm{LSR}}$$\quad$$\simeq$ 40 km s$^{-1}$ component. In the CO Galactic plane survey data, CO 16.134-0.553 appears to correspond to the Galactic eastern rim of a 15-pc diameter expanding CO shell. An $1°$-diameter H I emission void and $4°$-long vertical H I filament were also found above and below the CO shell, respectively. We propose that the high-velocity plunge of a dark matter subhalo with a clump of baryonic matter was responsible for the formation of the H I void, CO 16.134-0.553/CO shell, and the H I filament.

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arXiv2023

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

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, H$^{13}$CO$^{+}$ $J$=4-3, SiO $J$=8-7, CH$_3$OH $J_{K_a, K_c}$ = 7$_{1, 7}$-6$_{1, 6}$ A$^{+}$ lines, and 900 $μ$m continuum show several new features, which have not been identified in previous observations. The dense gas probe (H$^{13}$CN, SiO, CH$_{3}$OH) 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 $\times$ 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.

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arXiv2023

Discovery of the Tadpole Molecular Cloud near the Galactic Nucleus

In this paper, we report the discovery of an isolated, peculiar compact cloud with a steep velocity gradient at $2\farcm 6$ 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 {\it 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 $1\!\times\! 10^{5}\,M_{\odot}$. 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.

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arXiv2021

Broad-velocity-width Molecular Features in the Galactic Plane

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.

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arXiv2021

New Look at the Molecular Superbubble Candidate in the Galactic Center

The $l\!=\!+1.\!\!^\circ3$ 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 {\it J}=1--0, $^{13}$CO {\it J}=1--0, H$^{13}$CN {\it J}=1--0, H$^{13}$CO$^{+}$ {\it J}=1--0, SiO {\it J}=2--1, and CS {\it J}=2--1 lines obtained from the Nobeyama radio observatory 45-m telescope, as well as CO {\it J}=3--2 maps obtained from the James Clerk Maxwell telescope. We identified eleven expanding shells with total kinetic energy and typical expansion time $E_{\rm kin}\!\sim\! 10^{51.9}$ erg and $t_{\rm exp}\!\sim\! 10^{4.9}$ yr, respectively. In addition, the $l\!=\!+1.\!\!^\circ3$ region exhibited high SiO {\it J}=2--1/H$^{13}$CN {\it J}=1--0 and SiO {\it J}=2--1/H$^{13}$CO$^{+}$ {\it 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 $l\!=\!+1.\!\!^\circ3$ region. The nature of the embedded star cluster, which may have supplied 20--70 supernova explosions within 10$^5$ yr, is discussed. This work also show the importance of compact broad-velocity-width features in searching for localized energy sources hidden behind severe interstellar extinction and stellar contamination.

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arXiv2020

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

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 $\sim 10^4$ $M_\odot$. 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.

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arXiv2019

An energetic high-velocity compact cloud: CO$-$0.31+0.11

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 $\sim 45$ pc from the Galactic nucleus Sgr A$^*$. It is characterized by its compact spatial appearance ($d\simeq4$ 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 $10^4$ $M_\odot$ and $10^{51}$ 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 $2\times 10^5$ $M_\odot$. 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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arXiv2018

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

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 (ALMA). The morphology and kinematics of these streams can be reproduced well through two Keplerian orbits around a single point mass of $(3.2 \pm 0.6)\times 10^4 \ M_\odot$. 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 abound in our Galaxy.

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arXiv2018

Detection of Another Molecular Bubble in the Galactic Center

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 $J$=1-0, $^{13}$CO $J$=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 $10^{51}$ erg and the typical expansion time of ${\sim}10^5$ yr correspond to multiple supernova explosions at a rate of $10^{-5}$-$10^{-4}$ yr$^{-1}$. This indicates that the $l=-1.2^{\circ}$ 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 x1 orbit and the interacting gas collides with it from the Galactic eastern side.

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arXiv2017

Millimetre-wave Emission from an Intermediate-Mass Black Hole Candidate in the Milky Way

It is widely accepted that black holes (BHs) with masses greater than a million solar masses (Msun) lurk at the centres of massive galaxies. The origins of such `supermassive' black holes (SMBHs) remain unknown (Djorgovski et al. 1999), while those of stellar-mass BHs 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 clusters (Portagies Zwart et al. 1999), merge at the centre of a galaxy to form an SMBH (Ebisuzaki et al. 2001). Although many candidates for IMBHs have been proposed to date, none of them are 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 ~1E5 Msun is lurking in this cloud (Oka et al. 2016). Here we report the detection of a point-like continuum source as well as a compact gas clump near the center 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 the most promising candidate for an intermediate-mass black hole.

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arXiv2016

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

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 ASTE 10-m telescope. The Bullet clearly appears in the CO ${\it J}$=1-0, CO ${\it J}$=3-2, CO ${\it J}$=4-3, and HCO$^+$ ${\it J}$=1-0 maps with a compact appearance ($0.5\times 0.8$ pc$^2$) and an extremely broad velocity width ($ΔV\!\simeq\!100 \ \rm km \, s ^{-1}$). 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 $10^{48.0} \ {\rm erg}$ 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 (BH) are presented.

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