arXiv subjects

Rei Enokiya

Publications and source records attributed to Rei Enokiya.

At least 19 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.

astro-ph.GA↗

arXiv2026

Development of TIFUUN: Terahertz Integral Field Units with Universal Nanotechnology

TIFUUN (THz Integral Field Units with Universal Nanotechnology) is an ultra-wideband mm-submm wave imaging spectrometer that capitalizes on the highly scalable integrated superconducting spectrometer technology. TIFUUN has two slots for integral field units (IFUs), which can jointly be optimized as open-hardware for each astronomical observation in terms of spatial and spectral coverage. These IFUs can have observation frequencies in the range of 90--360 GHz, with spectral resolution up to $R\equiv F/\Delta F \le 1,000$, with up to $\sim$18,000 kinetic inductance detectors (shared by the two IFUs with a flexible ratio). The ultra-wide 4:1 (2 octave) bandwidth optics fits in a remarkably compact volume, by means of thin silicon lenses and a high chief ray angle design. The first pair of IFUs are being developed for the SUBLIME (Study of the Universe By Line Intensity Mapping Experiments) experiment that aims to map CII emission at redshift $\sim$6 to trace the cosmic large-scale structure and the buildup of galaxies during reionization, using TIFUUN on the ASTE 10-m telescope. The scalability, flexibility and compactness makes TIFUUN a highly compatible and portable system suited also for upcoming telescope facilities in the vicinity, such as FYST and AtLAST/LST.

astro-ph.IM↗

arXiv2026

First Determination of the Cosmic Microwave Background Radiation Temperature at $z\!=\!0.68$ Using Molecular Absorption Lines

We analyzed millimeter-wave data toward the quasar B0218+357 observed with the Atacama Large Millimeter/submillimeter Array and obtained absorption spectra of the $J$=2-1 and $J$=3-2 rotational transitions of HCN, HCO$^{+}$, HNC, H$^{13}$CN, and H$^{13}$CO$^{+}$ at the cosmological redshift of $z\!=\!0.68$. For HCN, HCO$^{+}$, and HNC, we identified two distinct absorption components that are common to both transitions, whereas a single component was detected in the isotopologue spectra. In this paper, we accurately evaluate the excitation temperatures and their uncertainties from the absorption strengths of these components, and use them to determine the CMB temperature. Uncertainties in the continuum covering factor were propagated into the excitation temperature via Monte Carlo sampling. We further corrected the observed optical depths for biases due to column-density nonuniformity by assuming a lognormal column-density distribution. Under the assumption that the rotational levels are in radiative equilibrium with the cosmic microwave background (CMB), we derived excitation temperature profiles in the optically thin regime. Because the excitation of HCO$^+$ is biased by an additional velocity component and partial collisional excitation, this species was excluded from the final determination of the CMB temperature. From a weighted mean of the excitation temperatures obtained from HCN and HNC, we determined the CMB temperature at $z\!=\!0.68$ to be ${4.50\pm0.17\,\mathrm{K}}$. This constitutes the first measurement of the CMB temperature at $z\!=\!0.68$ based on a quasar absorption line system and represents the most precise determination at this redshift, highly consistent with the standard Big Bang cosmological model.

astro-ph.CO↗

arXiv2026

The Supernova Remnant G284.3$-$1.8 and Its Relation to the Gamma-ray Binary 1FGL J1018.6$-$5856

G284.3$-$1.8 is a supernova remnant with a radio shell and thermal X-ray emission. Located near its center is the gamma-ray binary 1FGL J1018.6$-$5856, although the physical association between the two systems is not clear yet. Our X-ray spectroscopy with Suzaku reveals that G284.3$-$1.8 and 1FGL J1018.6$-$5856 have compatible absorption column densities of $N_\mathrm{H} = 6\textrm{--}7 \times 10^{21}~\mathrm{cm}^{-2}$, indicating that the two systems have similar distances. The actual distance is determined as $3~\mathrm{kpc}$ using $\mathrm{^{12}CO}$ ($J=1\textrm{--}0$) data obtained with NANTEN. The X-ray spectrum of G284.3$-$1.8 shows a strong K-shell emission line of Mg, confirming that the earlier claim that the SNR is one of the few Mg-rich SNRs. Comparing recent stellar models taking into account the "shell merger" processes, we find that the obtained Mg-to-Ne mass ratio of $M_\mathrm{Mg}/M_\mathrm{Ne} = 0.73^{+0.07}_{-0.03}$ and Si-to-Mg mass ratio of $M_\mathrm{Si}/M_\mathrm{Mg} = 0.44\pm0.03$ suggest a supernova explosion that would have left a neutron star. The characteristics of 1FGL J1018.6$-$5856, on the other hand, are better explained with a model in which its compact object is neutron star. The present results, therefore, would suggest a possible scenario where G284.3$-$1.8 and 1FGL J1018.6$-$5856 are both remnants of a common supernova explosion although further observational tests are necessary.

astro-ph.HE↗

arXiv2025

Head-tail molecular clouds falling onto the Milky Way disk

We report discovery of two CO clouds which are likely falling down to the Galactic plane at more than $35$ km s$^{-1}$. The clouds show head-tail distributions elongated perpendicular to the Galactic plane at $l=331.6^{\circ}$ and $b=0^{\circ}$ as revealed by an analysis of the Mopra CO $J=$1-0 survey data. We derived the distance of the clouds to be $2.46 \pm 0.18$ kpc based on the Gaia Data Release 3. The CO clouds have molecular masses of $4.8\times 10^3\ M_{\odot}$ and $3.5\times 10^3\ M_{\odot}$, respectively, and show kinetic temperature of 30-50 K as derived from the line intensities of the $^{13}$CO $J$=2-1, $^{12}$CO $J$=1-0, and $^{13}$CO $J$=1-0 emission. The temperature in the heads of the clouds is significantly higher than 10 K of the typical molecular clouds, although no radiative heat source is found inside or close to the clouds. Based on the results, we interpret that the present clouds are falling onto the Milky Way disk and are significantly heated up by the strong shock interaction with the disk HI gas. We suggest that the clouds represent part of the HI intermediate velocity clouds falling to the Galactic plane which were converted into molecular clouds by shock compression. This is the first case of falling CO clouds having direct observed signatures of the falling motion including clear directivity and shock heating. Possible implications of the CO clouds in the evolution of the Galactic interstellar medium are discussed.

astro-ph.GA↗

arXiv2025

A New Precise Measurement of the Cosmic Microwave Background Radiation Temperature at $z=0.89$ Toward PKS1830-211

In this study, we analyzed millimeter-wave data toward the quasar PKS1830-211 observed with the Atacama Large Millimeter/submillimeter Array to obtain absorption spectra of HCN $J$=2-1, $J$=3-2, $J$=4-3, and $J$=5-4 lines at the cosmological redshift of $z=0.89$. We confirmed multiple absorption components in each spectrum, and the two lower-$J$ lines are highly saturated at velocity centers of the two most prominent components. The effect of uncertainty in the continuum covering factor was carefully assessed using a Monte Carlo approach. We also accounted for systematic uncertainties in the HCN $J$=4-3 and $J$=5-4 absorption due to time variability during an intense flaring event of the quasar. Assuming local thermodynamic equilibrium and neglecting molecular collisions, we derived the excitation temperature profiles and their uncertainties in the optically thin regime. We determined the cosmic microwave background radiation temperature at $z = 0.89$ to be ${5.13\pm0.06\,\mathrm{K}}$ by taking a weighted average of calculated excitation temperatures; this is highly consistent with that expected from the standard model of the universe.

astro-ph.CO↗

arXiv2025

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

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 contains the high-velocity compact cloud (HVCC) at $(l,b,v_{\rm lsr})\sim(+0^\circ.02,-0^\circ.02, 100 {\rm km~s}^{-1})$ (hereafter G0.02). The longitude-velocity diagram (LVD) of the cloud draws an elliptical structure, which is interpreted as an orbital trajectory in the $(l,V_{\rm lsr})$ space of a noncircular (eccentric) motion of the molecular gas in the gravitational potential of an extended mass distribution in the central 10 pc of the Galaxy. We argue that G0.02 is a kinematic tracer of the inner potential, a rare case of a dense gas following an eccentric orbit in the nuclear gravitational field.

astro-ph.GA↗

arXiv2025

Giant Molecular Clouds in RCW 106 (G333): Galactic Mini-starbursts and Massive Star Formation Induced by Supersonic Cloud-Cloud Collisions

To reveal the origin of the mini-starbursts in the Milky Way, we carried out large-scale CO observations toward the RCW 106 giant molecular cloud (GMC) complex using the NANTEN2 4-m radio telescope operated by Nagoya University. We also analyzed the Mopra Southern Galactic plane CO survey and Herschel infrared continuum archival data. The RCW 106 GMC complex contains the radial velocity components of $-68$ km s$^{-1}$ and $-50$ km s$^{-1}$ reported by Nguyen et al. (2015). Focusing on the RCW 106 East and West region with the massive star formation having the bright infrared dust emission, we found that these regions have three different velocity components with $\sim 10$ km s$^{-1}$ differences. The two out of three velocity components show morphological correspondence with the infrared cold dust emission and connect with the bridge feature on a position-velocity diagram. Therefore, two molecular clouds (MCs) with $\sim 10$ km s$^{-1}$ differences are likely to be physically associated with massive star-forming regions in the GMC complex. Based on these observational results, we argue that mini-starbursts and massive star/cluster formation in the RCW 106 GMC complex are induced by supersonic cloud-cloud collisions in an agglomerate of molecular gas on the Scutum-Centaurus arm.

astro-ph.GA↗

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.

astro-ph.GA↗

arXiv2024

Progenitor Constraint Incorporating Shell Merger: The Case of Supernova Remnant G359.0-0.9

It is generally hard to put robust constraints on progenitor masses of supernovae (SNe) and remnants (SNRs) observationally, while they offer tantalizing clues to understanding explosion mechanisms and mass distribution. Our recent study suggests that ``shell merger'', which is theoretically expected for stellar evolution, can appreciably affect final yields of inter-mediate mass elements (IMEs; such as Ne, Mg, and Si). In light of this, here we report results of X-ray spectral analysis of a Galactic SNR G359.0-0.9, whose abundance pattern may possibly be anomalous according to a previous study. Our spectroscopy using all the available data taken with XMM-Newton reveals that this remnant is classified as Mg-rich SNRs because of its high Mg-to-Ne ratio (Z_Mg/Z_Ne=1.90+0.27-0.19; mass ratio 0.66+0.09-0.07) and conclude that the result cannot be explained without the shell merger. By comparing the observation with theoretical calculations, we prefer the so-called Ne-burning shell intrusion and in this case the progenitor mass M_ZAMS is likely <15M_sun. We confirm the result also by our new molecular line observations with the NRO-45 m telescope: G359.0-0.9 is located in the Scutum-Centaurus arm (2.66--2.94 kpc) and in this case the resultant total ejecta mass ~6.8M_sun is indeed consistent with the above estimate. Our method using mass ratios of IMEs presented in this paper will become useful to distinguish the type of the shell merger, the Ne-burning shell intrusion and the O-burning shell merger, for future SNR studies.

astro-ph.HE↗

arXiv2023

The JWST Galactic Center Survey -- A White Paper

The inner hundred parsecs of the Milky Way hosts the nearest supermassive black hole, largest reservoir of dense gas, greatest stellar density, hundreds of massive main and post main sequence stars, and the highest volume density of supernovae in the Galaxy. As the nearest environment in which it is possible to simultaneously observe many of the extreme processes shaping the Universe, it is one of the most well-studied regions in astrophysics. Due to its proximity, we can study the center of our Galaxy on scales down to a few hundred AU, a hundred times better than in similar Local Group galaxies and thousands of times better than in the nearest active galaxies. The Galactic Center (GC) is therefore of outstanding astrophysical interest. However, in spite of intense observational work over the past decades, there are still fundamental things unknown about the GC. JWST has the unique capability to provide us with the necessary, game-changing data. In this White Paper, we advocate for a JWST NIRCam survey that aims at solving central questions, that we have identified as a community: i) the 3D structure and kinematics of gas and stars; ii) ancient star formation and its relation with the overall history of the Milky Way, as well as recent star formation and its implications for the overall energetics of our galaxy's nucleus; and iii) the (non-)universality of star formation and the stellar initial mass function. We advocate for a large-area, multi-epoch, multi-wavelength NIRCam survey of the inner 100\,pc of the Galaxy in the form of a Treasury GO JWST Large Program that is open to the community. We describe how this survey will derive the physical and kinematic properties of ~10,000,000 stars, how this will solve the key unknowns and provide a valuable resource for the community with long-lasting legacy value.

astro-ph.GA↗

arXiv2023

Discovery of a molecular cloud possibly associated with the youngest Galactic SNR G1.9+0.3

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.

astro-ph.GA↗

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 $\mu$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.

astro-ph.GA↗

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.

astro-ph.GA↗

arXiv2022

The Effect of Shock Wave Duration on Star Formation and the Initial Condition of Massive Cluster Formation

Stars are born in dense molecular filaments irrespective of their mass. Compression of the ISM by shocks cause filament formation in molecular clouds. Observations show that a massive star cluster formation occurs where the peak of gas column density in a cloud exceeds 10^23 cm^-2. In this study, we investigate the effect of the shock-compressed layer duration on filament/star formation and how the initial conditions of massive star formation are realized by performing three-dimensional (3D) isothermal magnetohydrodynamics (MHD) simulations with {gas inflow duration from the boundaries (i.e., shock wave duration)} as a controlling parameter. Filaments formed behind the shock expand after the duration time for short shock duration models, whereas long duration models lead to star formation by forming massive supercritical filaments. Moreover, when the shock duration is longer than two postshock free-fall times, the peak column density of the compressed layer exceeds 10^23 cm^-2, and {the gravitational collapse of the layer causes that} the number of OB stars expected to be formed in the shock-compressed layer reaches the order of ten (i.e., massive cluster formation).

astro-ph.GA↗

arXiv2022

Catalog of High Velocity Dispersion Compact Clouds in the Central Molecular Zone of Our Galaxy

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 (HVCCs), which is a category of peculiar molecular clouds found in the central molecular zone of our galaxy. A list of HVCCs 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 point-like massive objects, local energetic events, or cloud-to-cloud collisions.

astro-ph.GA↗

arXiv2022

A Multiwavelength Study of the Sgr B Region: Contiguous Cloud-Cloud Collisions Triggering Widespread Star Formation Events?

The Sgr\,B region, including Sgr\,B1 and Sgr\,B2, is one of the most active star-forming regions in the Galaxy. Hasegawa et al. (1994) originally proposed that Sgr\,B2 was formed by a cloud-cloud collision (CCC) between two clouds with velocities of $\sim$45 km~s$^{-1}$ and $\sim$75 km~s$^{-1}$. However, some recent observational studies conflict with this scenario. We have re-analyzed this region, by using recent, fully sampled, dense-gas data and by employing a recently developed CCC identification methodology, with which we have successfully identified more than 50 CCCs and compared them at various wavelengths. We found two velocity components that are widely spread across this region and that show clear signatures of a CCC, each with a mass of $\sim$10$^6$ $M_\odot$. Based on these observational results, we suggest an alternative scenario, in which contiguous collisions between two velocity features with a relative velocity of $\sim$20 km~s$^{-1}$ created both Sgr\,B1 and Sgr\,B2. The physical parameters, such as the column density and the relative velocity of the colliding clouds, satisfy a relation that has been found to apply to the most massive Galactic CCCs, meaning that the triggering of high-mass star formation in the Galaxy and starbursts in external galaxies can be understood as being due to the same physical CCC process.

astro-ph.GA↗

arXiv2022

Massive core/star formation triggered by cloud-cloud collision: II High-speed collisions of magnetized clouds

We study the effects of the magnetic fields on the formation of massive, self-gravitationally bound cores (MBCs) in high-speed cloud-cloud collisions (CCCs). Extending our previous work (Sakre et al. 2021), we perform magnetohydrodynamic simulations following the high-speed (20 - 40 km s$^{-1}$) collisions between two magnetized (4 $\mu$G initially), turbulent clouds of different sizes in the range of 7 - 20 pc. We show that a magnetic field effect hinders the core growth, particularly after a short-duration collision during which cores cannot get highly bound. In such a case, a shocked region created by the collision rapidly expands to the ambient medium owing to the enhanced magnetic pressure, resulting in the destruction of the highly unbound cores and suppression of gas accretion to massive cores. This negative effect on the MBC formation is a phenomenon not seen in the past hydrodynamic simulations of similar CCC models. Together with our previous work, we conclude that the magnetic fields provide the two competing effects on the MBC formation in CCC; while they promote the mass accumulation into cores during a collision, they operate to destroy cores or hinder the core growth after the collision. The duration of collision determines which effect prevails, providing the maximum collision speed for the MBC formation with given colliding clouds. Our results agree with the observed trend among CCC samples in the corresponding column density range; clouds with higher relative velocity require higher column density for the formation of massive stars (Enokiya et al. 2021).

astro-ph.GA↗