arXiv subjects

Yoshiaki Sofue

Publications and source records attributed to Yoshiaki Sofue.

At least 37 records · Page 2Linked to original sources

arXiv2024

Dark supernova remnant buried in the Galactic-Centre "Brick" G0.253+0.016 revealed by an expanding CO-line bubble

We performed a 12CO and 13CO-line study of the "Brick" (G0.253+0.016) in the Galactic Centre (GC) by analyzing archival data obtained with the Nobeyama 45-m telescope. We present kinematics and molecular gas distributions in the longitude-velocity diagram, and suggest that the Brick is located along the GC Arm I in the central molecular zone (CMZ), which yields a distance from the Sun of 8 kpc and galacto-centric distance of 0.2 kpc. The major and minor-axis diameters of the Brick are $D_x\times D_y=8.4 {\rm pc} \times 4.1 {\rm epc}$ at position angle of $40°$ and $130°$, respectively, and the scale radius is $r_{\rm brick}=\sqrt{D_x D_y}=2.96$ pc. The molecular mass inferred from the 12CO-line integrated intensity is $M_{\rm bri,xco}\sim 5.1\times 10^4 M_\odot$ for a conversion factor $X_{\rm CO}=1.0\times 10^{20}$ H$_2$ cm$^{-2}$ [K km/s]$^{-1}$. On the other hand, the dynamical (Virial) mass for the measured velocity dispersion of $σ_v=10.0 $ km/s is calculated to be $M_{\rm bri,vir}\sim 6.8 \times 10^4M_\odot$, which yields a new conversion factor of $X_{\rm CO} =1.3\times 10^{20}$ H$_2$ cm$^{-2}$ [K km/s]${-1}$. The Brick's center has a cavity surrounded by a spherical molecular bubble of radius $r_{\rm bub}=1.85$ pc and mass $\sim 1.7\times 10^4M_\odot$ expanding at $v_{\rm exp}\simeq 10 $ km/s with kinetic energy of $E_0\sim 1.7\times 10^{49}$ erg. If the bubble is approximated by an adiabatic spherical shock wave, the age is estimated to be $t\sim 2/5 r_{\rm bub}/v_{\rm exp}\sim 7.2\times 10^4$ y. Neither non-thermal radio structures nor thermal radio emission indicative of HII region are found in the archival data from the MeerKAT. We suggest that the molecular bubble will be a dark supernova remnant buried in the Brick, which, therefore, has experienced in the past ($\sim 0.1$ Myr ago) massive-star formation with a supernova explosion.

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arXiv2024

A broad linewidth, compact, millimeter-bright molecular emission line source near the Galactic Center

A compact source, G0.02467-0.0727, was detected in ALMA \threemm observations in continuum and very broad line emission. The continuum emission has a spectral index $α\approx3.3$, suggesting that the emission is from dust. The line emission is detected in several transitions of CS, SO, and SO$_2$ and exhibits a line width FWHM $\approx160$ \kms. The line profile appears Gaussian. The emission is weakly spatially resolved, coming from an area on the sky $\lesssim1"$ in diameter ($\lesssim10^4$ AU at the distance of the Galactic Center; GC). The centroid velocity is $v_{LSR}\approx40$-$50$ \kms, which is consistent with a location in the Galactic Center. With multiple SO lines detected, and assuming local thermodynamic equilibrium (LTE) conditions, $T_\mathrm{LTE} = 13$ K, which is colder than seen in typical GC clouds, though we cannot rule out low-density, subthermally excited, warmer gas. Despite the high velocity dispersion, no emission is observed from SiO, suggesting that there are no strong ($\gtrsim10~\mathrm{km~s}^{-1}$) shocks in the molecular gas. There are no detections at other wavelengths, including X-ray, infrared, and radio. We consider several explanations for the Millimeter Ultra-Broad Line Object (MUBLO), including protostellar outflow, explosive outflow, collapsing cloud, evolved star, stellar merger, high-velocity compact cloud, intermediate mass black hole, and background galaxy. Most of these conceptual models are either inconsistent with the data or do not fully explain it. The MUBLO is, at present, an observationally unique object.

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arXiv2024

Stochastic star formation in the Milky Way inferred from the unity index of KS law

We performed a correlation analysis between the brightness temperature of the CO line and number density of HII regions in the longitude--velocity diagram (LVD) of the Milky Way in order to investigate the volumetric star-formation law. We determined the index $α$ of the Kennicutt-Schmidt (KS) law for the molecular gas defined by $ρ_{\rm SFR} \propto ρ_{\rm H_2}^α$, where $ρ_{\rm SFR}$ is the SFR (star-formation rate) density and $ρ_{\rm H_2}$ is the molecular-gas density. We obtained $α= 1.053 \pm 0.075$ and $1.031 \pm 0.067$ for the CO-line data from the Nobeyama 45-m and Columbia 1.2-m telescope Galactic plane surveys, respectively. This result is consistent with the KS indices currently determined for the molecular gas in the Milky Way as well as in spiral and starburst galaxies. We argue that an index close to 1 is universal in favour of stochastic (spontaneous) star formation, but is inconsistent with cloud-collision model, which predicts a steeper index of $α=2$. We also suggest that the efficiency of star formation in the Galactic Centre is an order of magnitude lower than that in the disc.

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arXiv2023

The CO-to-H$_2$ Conversion Factor of Galactic Giant Molecular Clouds using CO isotopologues: High-resolution $X_{\rm CO}$ maps

We investigated the correlation between intensities of the $^{12}$CO and $^{13}$CO ($J=1$-0) lines toward the Galactic giant molecular clouds (GMCs) W51A, W33, N35-N36 complex, W49A, M17SW, G12.02-00.03, W43, and M16 using the FUGIN (FOREST Unbiased Galactic plane Imaging survey with the Nobeyama 45-m telescope) CO line data. All the GMCs show intensity saturation in the $^{12}$CO line when the brightness temperature of $^{13}$CO is higher than a threshold temperature of about $\sim 5$ K. We obtained high-resolution ($\sim 20"$) distribution maps of the $X_{\rm CO}$ factor ($X_{\rm CO, iso}$) in individual GMCs using correlation diagrams of the CO isotopologues. It is shown that $X_{\rm CO, iso}$ is variable in each GMC within the range of $X_{\rm CO, iso} \sim (0.9 {\rm -} 5) \times 10^{20}$ cm$^{-2}$ (K km s$^{-1})^{-1}$. Despite the variability in the GMCs, the average value among the GMCs is found to be nearly constant at $X_{\rm CO, iso} = (2.17 \pm 0.27) \times 10^{20}$ cm$^{-2}$ (K km s$^{-1})^{-1}$, which is consistent with that from previous studies in the Milky Way.

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arXiv2023

Face-on Map of the Molecular Disc and 3-kpc Expanding Ring of the Galaxy based on a High-Accuracy Rotation Curve

We analyze the longitude-velocity diagram (LVD) of the CO-line emission from archival data and use the most accurate rotation curve (RC) of the Milky Way to transform radial velocity to face-on position in the Galactic plane. We point out that the face-on transformation is highly sensitive to the adopted RC, especially in the inner Milky Way, in the sense that deviations of the RC from the true rotation velocity yield an artifact hole or overcrowded concentration along the tangent circle for over- or under-estimated RC. Even if the RC is sufficiently accurate, non-circular motion such as with the 3 kpc expanding ring causes significant artifacts in the resulting face-on-map as long as a circular rotation is assumed. On the other hand, if we properly take into account the non-circular motion, it can be used to solve the near-far degeneracy problem of determination of kinematic distance. We thus propose a new method to solve the degeneracy by incorporating the expanding motion of a ring or arms. We apply the method to the LVD of the 3-kpc expanding ring and present its face-on map projected onto the galactic plane for the first time.

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arXiv2023

Giant molecular cloud G18.1-0.3+51 associated with HII regions and supernova remnant in the 3-kpc expanding ring

Analyzing the high-resolution CO-line survey of the Galactic plane with the Nobeyama 45-m telescope (FUGIN), we show that the star-forming complex G18.15-0.30+51 (G18) at radial velocity of 51 \kms is {a tight triple association of a giant molecular cloud (GMC), HII regions, and a supernova remnant (SNR).} The kinematical distance of G18 is determined to be $d=3.9\pm 0.2$ kpc for near solution for circular rotation, $12\pm 0.2$ kpc for far solution, or $d=6.1\pm 0.1$, if it is in the 3-kpc expanding ring. The HI-line absorption of radio continuum from the HII regions constrains the distance to $5.6 \lesssim d_{\rm SNR}\le 7.6$ kpc. The $Σ-D$ (radio brightness-diameter) relation yields the distance to the SNR of $d_{\rm SNR}=10.1^{+11.5}_{-4.7}$ kpc, allowing for a minimum distance of 5.4 kpc. From these we uniquely determined the distance of G18 to be $6.07\pm 0.13$ kpc in the 3-kpc expanding ring with the SNR being physically associated. The molecular mass of the GMC is estimated to be $M_{\rm mol}\sim 3\times 10^5 M_\odot$. The ratio of Virial to luminous molecular masses is greater than unity in the central region and decreases outward to $\lesssim 0.2$ at the cloud edge, indicating that the central region is dynamic, while the entire cloud is stable. We discuss the origin of the G18 triple system and propose a sustainable GMC model with continuous star formation.

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arXiv2023

Cosmic-ray electrons and the magnetic field of the North Polar Spur

The recent confirmation of the bipolarity of the eROSITA bubbles suggests that the well-known North Polar Spur (NPS)/Loop I probably is a 10 kpc sized relic in the Galactic halo and not a small local structure near the Sun. By virtue of multiwavelength observations of the NPS, unprecedentedly precise parameter constraints on the cosmic-ray electrons (CRes) and magnetic field in the post-shock halo medium can be provided. The parameters of the CRes and the magnetic field can be derived independently by modeling the gamma-ray and the radio data of the NPS via inverse Compton scattering and synchrotron emission, respectively. Our main results are (1) that the energy density of the CRe is (3-6)$\times 10^{-14}$ erg cm$^{-3}$, and the spectral index is $p\simeq 2.0\pm 0.1, $ below the cooling break energy of about 5 GeV; (2) that the magnetic field strength is 3 $μ$G; and (3) that the shock acceleration efficiency of the CRe is (1-2)%. Given the Mach number of 1.5, the high acceleration efficiency and flat spectrum of the CRe suggest that preexisting relativistic electrons may be reaccelerated in the NPS. Alternatively, these CRes could be accelerated by an evolving shock in the early epoch when its Mach number is high, and efficiently diffuse throughout the post-shock halo gas. In addition, the cooling break energy suggests that the cooling timescale is $10^7$ yr, which agrees with the age of the eROSITA bubbles.

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arXiv2023

On the Hα faintness of the North Polar Spur

The ratio of H$α$ intensity to 1.4 GHz radio continuum intensity in the North Polar Spur (NPS) is measured to be $\lesssim 50$, two orders of magnitude smaller than the values of $\sim 10^4$ observed in the typical shell-type old supernova remnants, Cygnus Loop and S147. The extremely low H$α$-to-radio intensity ratio favours the Galactic-Centre explosion model for NPS, which postulates a giant shock wave at a distance of several kilo parsecs in the hot and low-density Galactic halo with low hydrogen recombination rate, over the local supernova(e) remnant model.

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arXiv2023

Asymmetric eROSITA bubbles as the evidence of a circumgalactic medium wind

The eROSITA bubbles are detected via the instrument with the same name. The northern bubble shows noticeable asymmetric features, including distortion to the west and enhancement in the eastern edge, while the southern counterpart is significantly dimmer. Their origins are debated. Here, we performed hydrodynamic simulations showing that asymmetric eROSITA bubbles favor a dynamic, circumgalactic medium wind model, but disfavor other mechanisms such as a non-axisymmetric halo gas or a tilted nuclear outflow. The wind from the east by north direction in Galactic coordinates blows across the northern halo with a velocity of about 200 km s$^{-1}$, and part of it enters the southern halo. This creates a dynamic halo medium and redistributes both density and metallicity within. This naturally explains the asymmetric bubbles in both the morphology and surface brightness. Our results suggest that our Galaxy is accreting low-abundance circumgalactic medium from one side while providing outflow feedback.

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arXiv2022

Supernova-remnant origin of the Galactic-Centre filaments

The mechanism to produce the numerous Galactic-Centre filaments (GCF) that vertically penetrate the Galactic plane without clear evidence of connection to the disc remains a mystery . Here we show that the GCFs are explained by relics of supernova remnants (rSNR) driven by hundreds of supernovae (SNe) exploded in the star-forming ring of the central molecular zone (CMZ) at an SN rate of $\sim 2\times 10^{-4}$ y$^{-1}$ in the past $\sim 0.5$ My. The evolution of rSNRs is simulated by the propagation of fast-mode magneto-hydrodynamic (MHD) waves, which are shown to converge around the Galactic rotation axis by the focusing effect. Tangential projection of the cylindrical wave fronts on the sky constitutes the vertical filaments. The SNR model explains not only the morphology, but also the non-thermal radio spectrum, smoothed brightness over the distribution area consistent with the $Σ-D$ relation of SNR,and the heating mechanism of hot plasma in the GC. We also discuss the implication of the model on the study of the interstellar physics and star-formation activity in the Galactic Centre.

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arXiv2022

Molecular Fraction in the Galactic Center: The Central Molecular and HI Zones

By mapping the molecular fraction of the Galactic Center (GC), we quantitatively address the question of how much molecular and central the CMZ (Central Molecular Zone) is. For this purpose we analyze the CO and HI-line archival data, and determine the column- (surface-) and volume-molecular fractions, $f_{\rm mol}^Σ$ and $f_{\rm mol}^ρ$, which are the ratio of column-mass density of H$_2$ projected on the sky to that of total gas (H$_2$ + HI) from the line intensities, and the ratio of volume-mass densities of \Htwo\ to total gas from the brightness temperature, respectively. It is shown that $f_{\rm mol}^Σ$ is as high as $ \sim 0.9-0.95$ in the CMZ, and $f_{\rm mol}^ρ$ is $0.93-0.98$ in the GC Arms I and II attaining the highest value of $\sim 0.98$ toward Sgr B2. The expanding molecular ring (EMR, or the parallelogram) has a slightly smaller $f_{\rm mol}^Σ$ as $\sim 0.9-0.93$. We define the CMZ as the region with $f_{\rm mol}^Σ\ge 0.8-0.9$ between the shoulders of plateau-like distribution of H$_2$ column density from $l=-1^\circ.1$ to $+1^\circ.8$ having Gaussian vertical distribution with a half thickness of $\pm 0^\circ.2$. The CMZ is embedded in the Central HI Zone (CHZ), which is defined as an HI disc between $l\sim -2^\circ$ and $+2^\circ.5$, $b=-0^\circ.5$ and $+0^\circ.5$. Based on the analysis, we discuss the origin of CMZ and interstellar physics such as the volume filling factors of molecular and HI gases inferred from the difference between $ f_{\rm mol}^Σ$ and $f_{\rm mol}^ρ$.

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arXiv2022

Three-Dimensional Structure of the Central Molecular Zone

A detailed comparison of HI and CO line cube data of the Galactic Center (GC) region from the archives is obtained. The central molecular zone (CMZ) is shown to be embedded in the HI disc (central HI zone, CHZ) of radius $\sim 320$ pc and vertical scale height $\sim 70$ pc. A radio continuum belt is shown to run parallel to molecular Arms I and II. The belt draws a double-infinity shape on the sky, connecting Sgr E ($l\sim -1^\circ.2)$, C, B1, B2 and Sgr D ($+1^\circ.2$), and is interpreted as a warping star-forming ring. The molecular Arms are closely associated with the HI arms on the longitude-velocity diagram (LVD), showing coherent rigid-body ridges. Due to the close relationship between HI and CO, the HI line absorption can be used to determine the Arms' position relative to Sgr A, B1, B2 and C.Combining the trigonometric data of proper motions of Sgr A$^*$ and maser sources of Sgr B2 as well as radial velocities, the 3D velocity vector of Sgr B2 is determined. From these analyses, the molecular Arm I with Sgr B2 is shown to be located in the near side of Sgr A$^*$, and Arm II with Sgr C in the other side, both composing a pair of symmetrical Arms around the GC. We present a possible 3D view of Sgr A through E and Arms I and II along with a parameter list.

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arXiv2022

One-kpc Expanding Cylinder of HI-Gas around the Galactic Center

We report the discovery of an expanding cylinder of HI gas of radius 1 kpc and vertical extent 800 pc by analyzing the 21-cm line survey data from the literature. The cylinder is expanding at 150 km s$^{-1}$ and rotating at 100 km s$^{-1}$, and is interpreted as due to a high-velocity conical wind at $\sim 180$ km s$^{-1}$ from the Galactic Center. The total mass of the cylinder is estimated to be $\sim 8.5\times 10^5 M_\odot$ and kinetic energy $\sim 3\times 10^{53}$ ergs.

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arXiv2022

Discovery of non-equilibrium ionization plasma associated with the North Polar Spur and Loop I

We investigated the detailed plasma condition of the North Polar Spur (NPS)/Loop I using archival $Suzaku$ data. In previous research collisional ionization equilibrium (CIE) have been assumed for X-ray plasma state, but we also assume non-equilibrium ionization (NEI) to check the plasma condition in more detail. We found that most of the plasma in the NPS/Loop I favors the state of NEI, and has the density-weighted ionization timescale of $n_e t\sim10^{11-12}$ s cm$^{-3}$ and the electron number density $n_e\sim$ a few $\times$ 10$^{-3}$ cm$^{-3}$. The plasma shock age, $t$, or the time elapsed after the shock front passed through the plasma, is estimated to be on the order of a few $\rm{Myr}$ for the NPS/Loop I, which puts a strict lower limit to the age of the whole NPS/Loop I structure. We found that NEI results in significantly higher temperature and lower emission measure than those currently derived under CIE assumption. The electron temperature under NEI is estimated to be as high as 0.5 keV toward the brightest X-ray NPS ridge at $Δθ=-20^\circ$, which decreases to 0.3 keV at $-10^\circ$, and again increases to $\sim 0.6$ keV towards the outer edge of Loop I at $Δθ\sim0^\circ$, about twice the currently estimated temperatures. Here, $Δθ$ is the angular distance from the outer edge of Loop I. We discuss the implication of introducing NEI for the research in plasma states in astrophysical phenomena.

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arXiv2021

Bipolar Molecular Outflow from M17

Kinematics of the molecular clouds in the star forming complex M17 is studied using the high-resolution CO-line mapping data at resolution ($20" \sim 0.2$ pc) with the Nobeyama 45-m telescope. The northern molecular cloud of M17, which we call the molecular "lobe", is shown to have an elongated shell structure around a top-covered cylindrical cavity. The lobe is expanding at $\sim 5$ \kms in the minor axis direction, and at $ \sim 3/\cos \ i$ km s$^{-1}$ in the major axis direction, where $i$ is the inclination of the major axis. The kinetic energy of the expanding motion is on the order of $\sim 3\times 10^{49}$ ergs. We show that the lobe is a backyard structure having the common origin to the denser molecular "horn" flowing out from NGC 6618 toward the south, so that the lobe and horn compose a bipolar outflow. Intensity distributions across the lobe and horn show a double-peak profile typical for a cylinder around a cavity. Position-velocity diagrams (PVD) across the lobe and horn exhibit open ring structure with the higher- and/or lower-velocity side(s) being lacking or faded. This particular PVD behavior can be attributed to outflow in a conical cylinder with the flow velocity increasing toward the lobe and horn axes.

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arXiv2021

Highest-Resolution Rotation Curve of the Inner Milky Way proving the Galactic Shock Wave

We present a rotation curve (RC) of the inner Galaxy of the 1st quadrant at $10°\le l \le 50°~ (R=1.3-6.2~{\rm kpc})$ with the highest spatial (2 pc) and velocity (1.3 km/s) resolutions. We used the ${^{12}}$CO(J=1-0)-line survey data observed with the Nobeyama 45-m telescope at an effective angular resolution of $20"$ (originally $15")$, and applied the tangent-velocity method to the longitude-velocity diagrams by employing the Gaussian deconvolution of the individual CO-line profiles. A number of RC bumps, or local variation of rotation velocity, with velocity amplitudes $\pm \sim 9$ km/s and radial scale length $\sim 0.5-1$ kpc are superposed on the mean rotation velocity. The prominent velocity bump and corresponding density variation around $R\sim 4$ kpc in the tangential direction of the Scutum arm (4-kpc molecular arm) is naturally explained by an ordinary galactic shock wave in a spiral arm with small pitch angle, not necessarily requiring a bar-induced strong shock. Tables of RC are available at the PASJ supplementary data site and http://www.ioa.s.u-tokyo.ac.jp/~sofue/h-rot.htm.

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arXiv2021

Dark Supernova Remnants revealed by CO-line Bubbles in the W43 Molecular Complex along the 4-kpc Arm

Fine structure of the density distribution in giant molecular clouds (GMC) around W43 (G31+00+90 km/s at $\sim 5.5$ kpc) was analyzed using the FUGIN$^*$ CO-line survey at high-angular ($20''\sim 0.5$ pc) and velocity (1.3 km/s) resolutions ($^*$Four-receiver-system Unbiased Galactic Imaging survey with the Nobeyama 45-m telescope). The GMCs show highly turbulent structures, and the eddies are found to exhibit spherical bubble morphology appearing in narrow ranges of velocity channels. The bubbles are dark in radio continuum emission, unlike usual supernova remnants (SNR) or HII regions, and in infrared dust emission, unlike molecular bubbles around young stellar objects. The CO bubbles are interpreted as due to fully evolved buried SNRs in molecular clouds after rapid exhaustion of the released energy in dense molecular clouds. The CO bubbles may be a direct evidence for exciting and maintaining the turbulence in GMCs by SN origin. Search for CO bubbles as "dark SNRs" (dSNR) will have implication to estimate the supernova rate more accurately, and hence the star formation activity in the Milky Way.

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arXiv2021

Origin of Galactic Spurs: New Insight from Radio/X-ray All-sky Maps

In this study, we analyze giant Galactic spurs seen in both radio and X-ray all-sky maps to reveal their origins. We discuss two types of giant spurs: one is the brightest diffuse emission near the map's center, which is likely to be related to Fermi bubbles (NPSs/SPSs, north/south polar spurs, respectively), and the other is weaker spurs that coincide positionally with local spiral arms in our Galaxy (LAS, local arm spur). Our analysis finds that the X-ray emissions, not only from the NPS but from the SPS are closer to the Galactic center by ~5 deg compared with the corresponding radio emission. Furthermore, larger offsets of 10-20 deg are observed in the LASs; however, they are attributed to different physical origins. Moreover, the temperature of the X-ray emission is kT ~ 0.2 keV for the LAS, which is systematically lower than those of the NPS and SPS (kT ~ 0.3 keV) but consistent with the typical temperature of Galactic halo gas. We argue that the radio/X-ray offset and the slightly higher temperature of the NPS/SPS X-ray gas are due to the shock compression/heating of halo gas during a significant Galactic explosion in the past, whereas the enhanced X-ray emission from the LAS may be due to the weak condensation of halo gas in the arm potential or star formation activity without shock heating.

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