arXiv subjects

Tsuyoshi Inoue

Publications and source records attributed to Tsuyoshi Inoue.

At least 19 recordsLinked to original sources

arXiv2026

Impact of Upstream Clumpiness on Supernova Remnant Forward Shock Evolution in Molecular Cloud Environments

Supernova remnants (SNRs) are widely considered to be the primary accelerators of Galactic cosmic rays. In recent years, detailed observations have significantly progressed for young SNRs interacting with molecular clouds, a prime example being RX J1713.7-3946. When molecular clouds are clumpy, their impact can affect not only radiation properties but also shock wave propagation. Therefore, a quantitative understanding linking observational quantities with the ambient medium structure is highly required. In this study, we perform three-dimensional hydrodynamic simulations to model a molecular cloud with an inhomogeneous density structure driven by supersonic turbulence and subsequent SNR formation. To investigate various pre-supernova environments, we systematically vary the medium clumpiness by replacing gas below a threshold number density with a low-density hot gas, quantifying the relationship between the forward shock velocity and the volume filling factor of the high-density clumps. As a result, we find that at an elapsed time of 1000 yr-a typical age for a young SNR-the forward shock can evolve consistently with the fast shock velocity measured in RX J1713.7-3946, provided that the clump volume filling factor is approximately 10% or less. Considering that hadronic gamma-ray emission originates exclusively from the clumpy, high-density gas, our findings suggest that the total energy of cosmic-ray protons in RX J1713.7-3946 is higher than previously estimated, amounting to at least several percent of the typical supernova explosion energy.

astro-ph.HE↗

arXiv2026

An ALMA view of the Jet-Arc CO clouds toward the TeV $\gamma$-ray source HESS J1023-575 and Westerlund 2; Evidence for the footprints of microquasar jets, the very powerful cosmic-ray accelerator in the Galactic disk

The TeV $\gamma$-ray source HESS J1023-575 (HESSJ 1023 hereafter) is one of the brightest H.E.S.S. sources near the young massive cluster Westerlund 2. HESS J1023 shows a remarkable positional alignment with the Jet and Arc CO clouds on its eastern and western sides over 170 pc length. We have carried out sub-pc scale observations of the CO clouds with ALMA and have discovered that the clouds consist of numerous thin filamentary features of $\sim$0.5 pc width and 10--20 pc length at distance of 7.5 kpc, which are well aligned with the Jet-Arc axis. Based on the magneto-hydrodynamical model of microquasar jets launched from {the center of the $\gamma$-ray source} HESS J1023-575, we present an interpretation that the thin filamentary clouds are the footprints of the microquasar jets on the HI gas. The model also explains the dissimilar Jet vs. Arc clouds in terms of HI density difference on each side. By using the density of the CO and HI gas and the $\gamma$-ray luminosity, we have calculated the cosmic ray proton energy $W_{\rm p}$ to be 7$\times$10$^{48}$ erg under the hadronic scheme, which is ten times larger than those derived in the TeV $\gamma$-ray SNRs RX J1713.7-3946 and RX J0852.0-4622. It is likely that HESS~J1023 has been active over 1-10 Myr, which is significantly longer than the duration of cosmic ray acceleration of the SNRs. HESS~J1023 is therefore an outstanding source of cosmic rays equivalent to at least 1000 SNRs, and is possibly the most powerful CR accelerator in the Galactic disk. A high energy compact source in HESS~J1023, which is likely a Myr-old black hole or neutron star, remains veiled due to heavy extinction.

astro-ph.HE↗

arXiv2026

Evolution of compressed clouds formed by filament coalescence. I. Oblique collisions

Stars are thought to form predominantly within filamentary molecular clouds. Recent studies have suggested that active star formation, including the formation of stellar clusters and massive stars, occurs within so-called "hub" structures, where multiple filaments converge. Understanding the formation and evolution of such hub-filament systems is therefore essential for unveiling the physical processes responsible for cluster and massive star formation, although the full picture remains incomplete. To address this, we have focused on filament-filament collisions as a potential formation mechanism of the hubs. In this study, we investigate the fundamental evolutionary processes of oblique collisions between two magnetized filaments using three-dimensional ideal magnetohydrodynamical simulations. As a model of initial filaments, we consider two identical finite-length magnetized filaments, varying the collision angle between their long axes, the collision velocity, which is set perpendicular to the long axes, and the initial line mass. We find that as the collision angle decreases from orthogonal to parallel, the compressed cloud becomes more prone to gravitational collapse. In addition, the instability of the post-collision compressed cloud can be explained by its energy balance. Specifically, if the absolute value of the gravitational energy exceeds the sum of the kinetic, thermal, and magnetic energies immediately after the collision, the cloud undergoes gravitational collapse. Conversely, if the gravitational energy is smaller, the cloud expands. In addition, we estimate the upper limit of the collision velocity that enables hub-filament formation and identify the collision conditions favorable for massive star formation.

astro-ph.GA↗

arXiv2025

Bell Instability and Cosmic-Ray Acceleration in Active Galactic Nuclei Ultrafast Outflow Shocks

We investigate magnetic-field amplification driven by the nonresonant hybrid (NRH or Bell) instability and its impact on cosmic-ray (CR) acceleration at reverse shocks of ultrafast outflows (UFOs) from active galactic nuclei (AGN). Previous kinetic studies by particle-in-cell simulations have demonstrated that when maximum CR energy is near the injection scale, NRH instability efficiently amplifies magnetic field up to the saturation level. However, the efficiency of NRH instability goes down as maximum energy increase since CR current is carried by escaping CRs near the maximum energy. We employ a one-dimensional MHD--CR framework solving telegraph-type diffusion--convection equations to trace the coupled evolution of CRs, magnetic fields, and shock dynamics under realistic parameters. We find a distinct transition with magnetic field strength: for weak background fields ($B_{0}\!\lesssim\!10^{-4}\,\mathrm{G}$), NRH instability efficiently amplifies upstream turbulence, driving a self-regulated state where $E_{\max}$ becomes independent of initial strength of magnetic turbulence. In contrast, for stronger background fields ($B_{0}\!\gtrsim\!10^{-3}\,\mathrm{G}$), the escaping CR current is too weak to drive NRH instability, and magnetic turbulence further decays through parametric instabilities, potentially reducing the acceleration efficiency. We give the physical interpretation for the transition and discuss conditions for PeV--EeV acceleration at UFO reverse shocks.

astro-ph.HE↗

arXiv2025

Infrared bubble recognition in the Milky Way and beyond using deep learning

We propose a deep learning model that can detect Spitzer bubbles accurately using two-wavelength near-infrared data acquired by the Spitzer Space Telescope and JWST. The model is based on the Single Shot MultiBox Detector as an object detection model, trained and validated using Spitzer bubbles identified by the Milky Way Project (MWP-Bubble). We found that using only MWP-Bubbles with clear structures, along with normalization and data augmentation, significantly improved performance. To reduce the dataset bias, we also use the data without bubbles in the dataset selected by combining two techniques: negative sampling and clustering. The model was optimized by hyperparameter tuning using Bayesian optimization. Applying this model to a test region of the Galactic plane resulted in a 98 $\%$ detection rate for MWP-Bubbles with 8 $\mu$ m emission clearly encompassing 24 $\mu$ m emission. Additionally, we applied the model to a broader area of $1^\circ \leq |l| \leq 65^\circ$, $|b| \leq 1^\circ$, including both training and validation regions, and the model detected 3,006 bubbles, of which 1,413 were newly detected. We also attempted to detect bubbles in the high-mass star-forming region Cygnus $X$, as well as in the external galaxies Large Magellanic Cloud (LMC) and NGC 628. The model successfully detected Spitzer bubbles in these external galaxies, though it also detected Mira-type variable stars and other compact sources that can be difficult to distinguish from Spitzer bubbles. The detection process takes only a few hours, demonstrating the efficiency in detecting bubble structures. Furthermore, the method used for detecting Spitzer bubbles was applied to detect shell-like structures observable only in the 8 $\mu$ m emission band, leading to the detection of 469 shell-like structures in the LMC and 143 in NGC 628.

astro-ph.GA↗

arXiv2025

ALMA 0.1 pc View of Molecular Clouds Associated with High-Mass Protostellar Systems in the Small Magellanic Cloud: Are Low-Metallicity Clouds Filamentary or Not?

Filamentary molecular clouds are an essential intermediate stage in the star formation process. To test whether these structures are universal throughout cosmic star formation history, it is crucial to study low-metallicity environments within the Local Group. We present an ALMA analysis of the ALMA archival data at the spatial resolution of $\sim$0.1 pc for 17 massive young stellar objects (YSOs) in the Small Magellanic Cloud (SMC; Z $\sim$0.2 $Z_{\odot}$). This sample represents approximately 30% of the YSOs confirmed by Spitzer spectroscopy. Early ALMA studies of the SMC have shown that the CO emission line traces an H$_2$ number density of $\gtrsim$10$^4$ cm$^{-3}$, an order of magnitude higher than in the typical Galactic environments. Using the CO($J$ = 3-2) data, we investigated the spatial and velocity distribution of molecular clouds. Our analysis shows that about 60% of the clouds have steep radial profiles from the spine of the elongated structures, while the remaining clouds have a smooth distribution and are characterized by lower brightness temperatures. We categorized the former as filaments and the latter as non-filaments. Some of the filamentary clouds are associated with YSOs with outflows and exhibit higher temperatures, likely reflecting their formation conditions, suggesting that these clouds are younger than non-filamentary ones. This indicates that even if filaments form during star formation, their steep structures may become less prominent and transit to a lower-temperature state. Such transitions in structure and temperature have not been reported in metal-rich regions, highlighting a key behavior for characterizing the evolution of the interstellar medium and star formation in low-metallicity environments.

astro-ph.GA↗

arXiv2024

A Tale of Three: Magnetic Fields along the Orion Integral-Shaped Filament as Revealed by JCMT BISTRO survey

As part of the BISTRO survey, we present JCMT 850 $\mu$m polarimetric observations towards the Orion Integral-Shaped Filament (ISF) that covers three portions known as OMC-1, OMC-2, and OMC-3. The magnetic field threading the ISF seen in the JCMT POL-2 map appears as a tale of three: pinched for OMC-1, twisted for OMC-2, and nearly uniform for OMC-3. A multi-scale analysis shows that the magnetic field structure in OMC-3 is very consistent at all the scales, whereas the field structure in OMC-2 shows no correlation across different scales. In OMC-1, the field retains its mean orientation from large to small scales, but shows some deviations at small scales. Histograms of relative orientations between the magnetic field and filaments reveal a bimodal distribution for OMC-1, a relatively random distribution for OMC-2, and a distribution with a predominant peak at 90$^\circ$ for OMC-3. Furthermore, the magnetic fields in OMC-1 and OMC-3 both appear to be aligned perpendicular to the fibers, which are denser structures within the filament, but the field in OMC-2 is aligned along with the fibers. All these suggest that gravity, turbulence, and magnetic field are each playing a leading role in OMC-1, 2, and 3, respectively. While OMC-2 and 3 have almost the same gas mass, density, and non-thermal velocity dispersion, there are on average younger and fewer young stellar objects in OMC-3, providing evidence that a stronger magnetic field will induce slower and less efficient star formation in molecular clouds.

astro-ph.GA↗

arXiv2024

The JCMT BISTRO Survey: The Magnetic Fields of the IC 348 Star-forming Region

We present 850 $\mu$m polarization observations of the IC 348 star-forming region in the Perseus molecular cloud as part of the B-fields In STar-forming Region Observation (BISTRO) survey. We study the magnetic properties of two cores (HH 211 MMS and IC 348 MMS) and a filamentary structure of IC 348. We find that the overall field tends to be more perpendicular than parallel to the filamentary structure of the region. The polarization fraction decreases with intensity, and we estimate the trend by power-law and the mean of the Rice distribution fittings. The power indices for the cores are much smaller than 1, indicative of possible grain growth to micron size in the cores. We also measure the magnetic field strengths of the two cores and the filamentary area separately by applying the Davis-Chandrasekhar-Fermi method and its alternative version for compressed medium. The estimated mass-to-flux ratios are 0.45-2.20 and 0.63-2.76 for HH 211 MMS and IC 348 MMS, respectively, while the ratios for the filament is 0.33-1.50. This result may suggest that the transition from subcritical to supercritical conditions occurs at the core scale ($\sim$ 0.05 pc) in the region. In addition, we study the energy balance of the cores and find that the relative strength of turbulence to the magnetic field tends to be stronger for IC 348 MMS than HH 211 MMS. The result could potentially explain the different configurations inside the two cores: a single protostellar system in HH 211 MMS and multiple protostars in IC 348 MMS.

astro-ph.GA↗

arXiv2024

Growth of Massive Molecular Cloud Filament by Accretion Flows. II. New Mechanism to Support a Supercritical Filament against Radial Collapse

Observations indicate that dense molecular filamentary clouds are sites of star formation. The filament width determines the most unstable scale for self-gravitational fragmentation and influences the stellar mass. Therefore, constraining the evolution of filaments and the origin of their properties are important for understanding star formation. Although some observations show a universal width of 0.1 pc, many theoretical studies predict the contraction of thermally supercritical filaments (> 17 Msun pc-1) due to radial collapse. Through non-ideal magnetohydrodynamics simulations with ambipolar diffusion, we explore the formation and evolution of filaments via slow-shock instability at the front of accretion flows. We reveal that ambipolar diffusion allows the gas in the filament to flow across the magnetic fields around the shock front, forming dense blobs behind the concave points of the shock front. The blobs transfer momentum that drives internal turbulence. We name this mechanism the "STORM" (Slow-shock-mediated Turbulent flOw Reinforced by Magnetic diffusion). The persistence and efficiency of the turbulence inside the filament are driven by the magnetic field and the ambipolar diffusion effect, respectively. The STORM mechanism sustains the width even when the filament reaches very large line masses (~ 100 Msun pc-1).

astro-ph.GA↗

arXiv2024

The Impact of Stellar Radiative Feedback on Formation of Young Massive Clusters via Fast HI Gas Collisions

Young massive clusters (YMCs) are dense aggregates of young stars and are often speculated as potential precursors to globular clusters. However, the formation mechanism of massive and compact gas clumps that precede YMCs remains unknown. In this paper, we study the formation of such massive clumps via fast HI gas collisions (~100 km/s) as suggested by recent observations and their subsequent evolution into YMCs by using three-dimensional magnetohydrodynamics simulations involving self-gravity and detailed thermal/chemical processes. In particular, the impact of ionization feedback from stellar radiation is included in an approximate fashion where the temperature within the HII regions is elevated to 10,000 K, while supernova feedback is not included. We examine whether the resulting massive clumps can survive this ionization feedback and evolve into YMCs. Our simulations reveal the emergence of gas clumps that do not only possess substantial mass (~10^5 M_sun) but also sufficient compactness (~5 pc). Notably, these clumps exhibit significantly higher escape velocities compared to the sound speed of the HII region, indicating effective gravitational retention of gas against feedback-induced evaporation. Consequently, these conditions foster efficient star formation within the massive gas clumps, ultimately leading to their evolution into YMCs. We also perform simulations involving lower-velocity gas collisions, approximately 15 km/s, typical shock velocities induced by galactic superbubbles.

astro-ph.GA↗

arXiv2024

Internal 1000 AU-scale Structures of the R CrA Cluster-forming Cloud -- I: Filamentary Structures

We report on ALMA ACA observations of a high-density region of the Corona Australis cloud forming a young star cluster, and the results of resolving internal structures. In addition to embedded Class 0/I protostars in continuum, a number of complex dense filamentary structures are detected in the C18O and SO lines by the 7m array. These are sub-structures of the molecular clump that are detected by the TP array as the extended emission. We identify 101 and 37 filamentary structures with a few thousand AU widths in C18O and SO, respectively, called as feathers. The typical column density of the feathers in C18O is about 10^{22} cm^{-2}, and the volume density and line mass are ~ 10^5 cm^{-3}, and a few times M_{sun} pc^{-1}, respectively. This line mass is significantly smaller than the critical line mass expected for cold and dense gas. These structures have complex velocity fields, indicating a turbulent internal property. The number of feathers associated with Class 0/I protostars is only ~ 10, indicating that most of them do not form stars but rather being transient structures. The formation of feathers can be interpreted as a result of colliding gas flow as the morphology well reproduced by MHD simulations, supported by the the presence of HI shells in the vicinity. The colliding gas flows may accumulate gas and form filaments and feathers, and trigger the active star formation of the R CrA cluster.

astro-ph.GA↗

arXiv2024

Bell Instability-Mediated Diffusive Shock Acceleration at Supernova Blast Wave Shock Propagating in the ISM

Supernova blast wave shock is a very important site of cosmic-ray acceleration. However, the detailed physical process of acceleration, in particular, non-linear interplay between cosmic-ray streaming and magnetic field amplification has not been studied under a realistic environment. In this paper, using a unique and novel numerical method, we study cosmic-ray acceleration at supernova blast wave shock propagating in the interstellar medium with well-resolved magnetic field amplification by non-resonant hybrid instability (or Bell instability). We find that the magnetic field is mildly amplified under typical ISM conditions that leads to maximum cosmic-ray energy ~30 TeV for supernova remnants with age ~1000 years consistent with gamma-ray observations. The strength of the amplified magnetic field does not reach so-called saturation level, because cosmic-ray electric current towards the shock upstream has finite spatial extent, by which Bell instability cannot experience many e-folding times.

astro-ph.HE↗

arXiv2024

Filamentary Network and Magnetic Field Structures Revealed with BISTRO in the High-Mass Star-Forming Region NGC2264 : Global Properties and Local Magnetogravitational Configurations

We report 850 $\mu$m continuum polarization observations toward the filamentary high-mass star-forming region NGC 2264, taken as part of the B-fields In STar forming Regions Observations (BISTRO) large program on the James Clerk Maxwell Telescope (JCMT). These data reveal a well-structured non-uniform magnetic field in the NGC 2264C and 2264D regions with a prevailing orientation around 30 deg from north to east. Field strengths estimates and a virial analysis for the major clumps indicate that NGC 2264C is globally dominated by gravity while in 2264D magnetic, gravitational, and kinetic energies are roughly balanced. We present an analysis scheme that utilizes the locally resolved magnetic field structures, together with the locally measured gravitational vector field and the extracted filamentary network. From this, we infer statistical trends showing that this network consists of two main groups of filaments oriented approximately perpendicular to one another. Additionally, gravity shows one dominating converging direction that is roughly perpendicular to one of the filament orientations, which is suggestive of mass accretion along this direction. Beyond these statistical trends, we identify two types of filaments. The type-I filament is perpendicular to the magnetic field with local gravity transitioning from parallel to perpendicular to the magnetic field from the outside to the filament ridge. The type-II filament is parallel to the magnetic field and local gravity. We interpret these two types of filaments as originating from the competition between radial collapsing, driven by filament self-gravity, and the longitudinal collapsing, driven by the region's global gravity.

astro-ph.SR↗

arXiv2023

The Gamma Ray Origin in RXJ0852.0-4622 Quantifying the Hadronic and Leptonic Components: Further Evidence for the Cosmic Ray Acceleration in Young Shell-type SNRs

Fukui et al. (2021) quantified the hadronic and leptonic gamma rays in the young TeV gamma ray shell-type supernova remnant (SNR) RXJ1713.7-3946 (RXJ1713), and demonstrated that the gamma rays are a combination of the hadronic and leptonic gamma ray components with a ratio of $\sim 6:4$ in gamma ray counts $N_\mathrm{g}$. This discovery, which adopted a new methodology of multiple-linear gamma-ray decomposition, was the first quantification of the two gamma ray components. In the present work, we applied the same methodology to another TeV gamma ray shell-type SNR RX~J0852.0$-$4622 (RXJ0852) in the 3D space characterized by [the interstellar proton column density $N_{\mathrm{p}}$]-[the nonthermal X-ray count $N_{\mathrm{x}}$]-[$N_{\mathrm{g}}$], and quantified the hadronic and leptonic gamma ray components to have a ratio of $\sim 5:5$ in $N_{\mathrm{g}}$. The present work adopted fitting of two/three flat planes in the 3D space instead of a single flat plane, which allowed to suppress fitting errors. The quantification indicates that the hadronic and leptonic gamma rays are in the same order of magnitude in these two core-collapse SNRs, verifying the significant hadronic gamma ray components. We argue that the target interstellar protons, in particular their spatial distribution, are essential in any attempts to identify type of particles responsible for the gamma-ray emission. The present results confirm that the CR energy $\lesssim 100$\,TeV is compatible with a scheme that SNRs are the dominant source of these Galactic CRs.

astro-ph.HE↗

arXiv2023

Growth of Massive Molecular Cloud Filament by Accretion Flows I: Slow Shock Instability v.s. Ambipolar Diffusion

The Herschel Gould Belt Survey showed that stars form in dense filaments in nearby molecular clouds. Recent studies suggest that massive filaments are bound by the slow shocks caused by accretion flows onto the filaments. The slow shock is known to be unstable to the corrugation deformation of the shock front. The corrugation instability could convert the accretion flow's ram pressure into turbulent pressure that influences the width of the filament, which, according to theory, determines the self-gravitational fragmentation scale and core mass. In spite of its importance, the effect of slow shock instability on star-forming filaments has not been investigated. In addition, the linear dispersion relation obtained from the ideal magnetohydrodynamics (MHD) analysis shows that the most unstable wavelength of shock corrugation is infinitesimally small (or mean free path). In the scale of dense filaments, the effect of ambipolar diffusion can suppress the instability at small scales. This study investigates the influence of ambipolar diffusion on the instability of the slow shock. We perform two-dimensional MHD simulations to examine the linear growth of the slow shock instability, considering the effect of ambipolar diffusion. The results demonstrate that the most unstable scale of slow shock instability is approximately five times the length scale of ambipolar diffusion l_AD calculated using post-shock variables, where, l_AD corresponds to the scale where the magnetic Reynolds number for ambipolar diffusivity is unity.

astro-ph.GA↗

arXiv2023

An improved dynamical Poisson equation solver for self-gravity

Since self-gravity is crucial in the structure formation of the universe, many hydrodynamics simulations with the effect of self-gravity have been conducted. The multigrid method is widely used as a solver for the Poisson equation of the self-gravity; however, the parallelization efficiency of the multigrid method becomes worse when we use a massively parallel computer, and it becomes inefficient with more than $10^4$ cores, even for highly tuned codes. To perform large-scale parallel simulations ($> 10^4$ cores), developing a new gravity solver with good parallelization efficiency is beneficial. In this article, we develop a new self-gravity solver using the telegraph equation with a damping coefficient, $\kappa$. Parallelization is much easier than the case of the elliptic Poisson equation since the telegraph equation is a hyperbolic partial differential equation. We analyze convergence tests of our telegraph equations solver and determine that the best non-dimensional damping coefficient of the telegraph equations is $\tilde{\kappa} \simeq 2.5$. We also show that our method can maintain high parallelization efficiency even for massively parallel computations due to the hyperbolic nature of the telegraphic equation by weak-scaling tests. If the time step of the calculation is determined by heating/cooling or chemical reactions, rather than the CFL condition, our method may provide the method for calculating self-gravity faster than other previously known methods such as the fast Fourier transform and multigrid iteration solvers because gravitational phase velocity determined by the CFL condition using these timescales is much larger than the fluid velocity plus sound speed.

astro-ph.GA↗

arXiv2023

Global deceleration and inward movements of X-ray knots and rims of RCW 103

Kinematics of shocks, ejecta knots, and the compact remnant of a supernova remnant gives an insight into the nature of the progenitor and surrounding environment. We report on a proper motion measurement of X-ray knots and rims of the magnetar-hosting supernova remnant RCW 103. Chandra data obtained in three epochs, 1999, 2010, and 2016 are used. We find a global deceleration of 12 knots and rims both in northern and southern regions within the last $\sim 24$ yrs, even though its age is thought to be larger than 2 kyr. Some of them even changed their moving directions from outward ($\sim 1,000$ km s$^{-1}$) to inward ($\sim -2,000$ km s$^{-1}$). Our findings can be explained with a collision with a high-density medium both in the northern and southern edges of the remnant, although the remnant may still be expanding in the wind-blown cavity. The proper motion of the associated magnetar 1E161348$-$5055 is possibly detected with a velocity of $\approx 500$ km s$^{-1}$.

astro-ph.HE↗

arXiv2023

Predicting reliable H$_2$ column density maps from molecular line data using machine learning

The total mass estimate of molecular clouds suffers from the uncertainty in the H$_2$-CO conversion factor, the so-called $X_{\rm CO}$ factor, which is used to convert the $^{12}$CO (1--0) integrated intensity to the H$_2$ column density. We demonstrate the machine learning's ability to predict the H$_2$ column density from the $^{12}$CO, $^{13}$CO, and C$^{18}$O (1--0) data set of four star-forming molecular clouds; Orion A, Orion B, Aquila, and M17. When the training is performed on a subset of each cloud, the overall distribution of the predicted column density is consistent with that of the Herschel column density. The total column density predicted and observed is consistent within 10\%, suggesting that the machine learning prediction provides a reasonable total mass estimate of each cloud. However, the distribution of the column density for values $> \sim 2 \times 10^{22}$ cm$^{-2}$, which corresponds to the dense gas, could not be predicted well. This indicates that molecular line observations tracing the dense gas are required for the training. We also found a significant difference between the predicted and observed column density when we created the model after training the data on different clouds. This highlights the presence of different $X_{\rm CO}$ factors between the clouds, and further training in various clouds is required to correct for these variations. We also demonstrated that this method could predict the column density toward the area not observed by Herschel if the molecular line and column density maps are available for the small portion, and the molecular line data are available for the larger areas.

astro-ph.GA↗