arXiv subjects

Y. Sofue

Publications and source records attributed to Y. Sofue.

At least 19 recordsLinked to original sources

arXiv2025

The inner rotation curve of the Milky Way

We derived the inner rotation curve (RC) of the Milky Way by applying the terminal velocity method (TVM) to the longitude-velocity diagrams (LVD) made from the large-scale survey data of the Galactic plane in the HI (HI4PI whole sky survey) and CO lines (CfA-Chile 1.2-m Galactic plane survey, Nobeyama 45-m Galactic plane and Galactic Center surveys, and Mopra 22-m southern Galactic plane survey). The derived RC agrees well with the RCs derived from the astrometric measurements of the maser sources by very long baseline interferometer (VLBI) observations and the GAIA result. We combined them to construct a unified RC from $R=0$ to $\sim 25$ kpc and decomposed the curve into bulge, disc and dark halo components with high precision. The dark matter density near the Sun is estimated to be $0.107 \pm 0.003$ GeV cm$^{-3}$. We present the RC as ascii tables for the solar constants of $(R_0,V_0)=(8.178 {\rm kpc}, 235.1 {\rm km/s})$, We also obtained a detailed comparison of the eastern ($l\ge 0^\circ$) and western ($< 0^\circ$) RCs in the HI and CO lines, which allowed the creation of an E/W asymmetry curve of the velocity difference. The E/W asymmetry is fitted by a sinusoidal function of the radius with the amplitude increasing toward the Galactic Center. We consider the possibility of the origin due to a weak bar inside $\sim 4$ kpc.

astro-ph.GA↗

arXiv2025

The Galactic-Centre Arms inferred from ACES (ALMA CMZ Exploration Survey)

Analyzing longitude-velocity diagrams (LVDs) in the CS(J=2-1) and H13CN(J=1-0) molecular lines from the internal release data of the ALMA Central-Molecular-Zone Exploration Survey (ACES) and in the 13CO (J=1-0) line from the Nobeyama Galactic-Centre (GC) survey, we identify six GC Arms as prominent straight LV ridges. In addition to the currently known Arms I to IV, we identify a new inner arm, Arm V, and further highlight the circum-nuclear disc (CND) as Arm VI. Integrated intensity maps of the Arms on the sky suggest that most of the Arms compose ring-like structures inclined from the Galactic plane. We determine the radii (curvatures) of the Arms using the velocity-gradient ($dv/dl$) method, assuming that the arms are rotating on circular orbits at a constant velocity of $\sim 150$ km/s. We show that Arms I and II compose the main ring structure of the CMZ with radii $\sim 100$--120 pc; Arm III is a dense arm 42 pc from the GC; Arm IV is a clear and narrow arm 20 pc from the GC; and Arm V is a faint, long arm of 8.2 pc radius. We show that the circum-nuclear disc (CND) composes the sixth arm, Arm VI, of radius $\sim 2.3$ pc associated with bifurcated spiral fins. We also discuss the association of the 20- and 50-km/s clouds with these Arms. The radii of the arms fall on an empirical relation $R\sim 630 (2/5)^N$ for $N=1$ (Arm I) to 6 (VI), suggesting either discrete rings or a logarithmic spiral with pitch angle $\sim 22^\circ$. The vertical full extent of the arm increases with radius and is represented by $z\sim 0.7 (R/1 {\rm pc})^{0.7}$ pc. The tilt angle of the arms from the Galactic plane, or the warping, increases rapidly toward the GC.

astro-ph.GA↗

arXiv2024

Disruption of a massive molecular cloud by a supernova in the Galactic Centre: Initial results from the ACES project

The Milky Way's Central Molecular Zone (CMZ) differs dramatically from our local solar neighbourhood, both in the extreme interstellar medium conditions it exhibits (e.g. high gas, stellar, and feedback density) and in the strong dynamics at play (e.g. due to shear and gas influx along the bar). Consequently, it is likely that there are large-scale physical structures within the CMZ that cannot form elsewhere in the Milky Way. In this paper, we present new results from the Atacama Large Millimeter/submillimeter Array (ALMA) large programme ACES (ALMA CMZ Exploration Survey) and conduct a multi-wavelength and kinematic analysis to determine the origin of the M0.8$-$0.2 ring, a molecular cloud with a distinct ring-like morphology. We estimate the projected inner and outer radii of the M0.8$-$0.2 ring to be 79" and 154", respectively (3.1 pc and 6.1 pc at an assumed Galactic Centre distance of 8.2 kpc) and calculate a mean gas density $> 10^{4}$ cm$^{-3}$, a mass of $\sim$ $10^6$ M$_\odot$, and an expansion speed of $\sim$ 20 km s$^{-1}$, resulting in a high estimated kinetic energy ($> 10^{51}$ erg) and momentum ($> 10^7$ M$_\odot$ km s$^{-1}$). We discuss several possible causes for the existence and expansion of the structure, including stellar feedback and large-scale dynamics. We propose that the most likely cause of the M0.8$-$0.2 ring is a single high-energy hypernova explosion. To viably explain the observed morphology and kinematics, such an explosion would need to have taken place inside a dense, very massive molecular cloud, the remnants of which we now see as the M0.8$-$0.2 ring. In this case, the structure provides an extreme example of how supernovae can affect molecular clouds.

astro-ph.GA↗

arXiv2021

Interaction of the Galactic-Centre Super Bubbles with the Gaseous Disc

The interaction of Galactic-Centre (GC) super bubbles (GSB) with the gaseous disc and halo of the Milky Way is investigated using radio continuum, X-ray, HI and CO line surveys. The radio North Polar Spur (NPS) constitutes the brightest eastern ridge of GSB, brightening towards the galactic plane and reaching $ l = 22°, \ b = + 2°$ at the sharpest end, where it intersects the tangential direction of the 3-kpc expanding ring and crater. Examination of the spur ridges reveals that the entire GSB, including the NPS and its counter spurs, constitutes a GC-symmetrical $Ω/$\rotatebox[origin=c]{180}{$Ω$} shape. The thickness and gas density of the HI and CO discs are shown to increase sharply from the inside (lower longitude) to the outside of the 3-kpc crater. Formation of crater is explained by the sweeping of the upper layer of disc gas by the shock wave from the GC by the explosion $ \sim 10 $ My ago with the emitted energy of several $10 ^ {55} $ ergs. Based on the discussion, a unified view on the structure and formation mechanism of GSB is presented.

astro-ph.GA↗

arXiv2019

Magnetic field and ISM in the local Galactic disc

Correlation analysis is obtained among Faraday rotation measure, HI column density, thermal and synchrotron radio brightness using archival all-sky maps of the Galaxy. A method is presented to calculate the magnetic strength and its line-of-sight (LOS) component, volume gas densities, effective LOS depth, effective scale height of the disk) from these data in a hybrid way. Applying the method to archival data, all-sky maps of the local magnetic field strength and its parallel component are obtained, which reveal details of local field orientation.

astro-ph.GA↗

arXiv2019

FOREST Unbiased Galactic Plane Imaging Survey with the Nobeyama 45-m Telescope (FUGIN) IV: Galactic Shock Wave and Molecular Bow Shock in the 4-kpc Arm of the Galaxy

The FUGIN CO survey with the Nobeyama 45-m Telescope revealed the 3D structure of a galactic shock wave in the tangential direction of the 4-kpc molecular arm. The shock front is located at G30.5+00.0+95 km/s on the up-stream (lower longitude) side of the star-forming complex W43 (G30.8-0.03), and composes a molecular bow shock (MBS) concave to W43, exhibiting an arc-shaped molecular ridge perpendicular to the galactic plane with width $\sim 0^\circ.1$ (10 pc) and vertical length $\sim 1^\circ \ (100\ {\rm pc})$. The MBS is coincident with the radio continuum bow of thermal origin, indicating association of ionized gas and similarity to a cometary bright-rimmed cloud. The up-stream edge of the bow is sharp with a growth width of $\sim 0.5$ pc indicative of shock front property. The velocity width is $\sim 10$ km/s, and the center velocity decreases by $\sim 15$ \kms from bottom to top of the bow. The total mass of molecular gas in MBS is estimated to be $\sim 1.2\times 10^6 <_\odot$ and ionized gas $\sim 2\times 10^4 M_\odot$. The vertical disk thickness increases step like at the MBS by $\sim 2$ times from lower to upper longitude, which indicates hydraulic-jump in the gaseous disk. We argue that the MBS was formed by the galactic shock compression of an accelerated flow in the spiral-arm potential encountering the W43 molecular complex. A bow-shock theory can well reproduce the bow morphology. We argue that molecular bows are common in galactic shock waves not only in the Galaxy but also in galaxies, where MBS are associated with giant cometary HII regions. We also analyzed the HI data in the same region to obtain a map of HI optical depth and molecular fraction. We found a firm evidence of HI-to-H$_{2}$ transition in the galactic shock as revealed by a sharp molecular front at the MBS front.

astro-ph.GA↗

arXiv2016

Galactic-Center Hyper-Shell Model for the North Polar Spurs

The bipolar-hyper shell (BHS) model for the North Polar Spurs (NPS-E, -W, and Loop I) and counter southern spurs (SPS-E and -W) is revisited based on numerical hydrodynamical simulations. Propagations of shock waves produced by energetic explosive events in the Galactic Center are examined. Distributions of soft X-ray brightness on the sky at 0.25, 0.7, and 1.5 keV in a +/-50 deg x +/-50 deg region around the Galactic Center are modeled by thermal emission from high-temperature plasma in the shock-compressed shell considering shadowing by the interstellar HI and H2 gases. The result is compared with the ROSAT wide field X-ray images in R2, 4 and 6 bands. The NPS and southern spurs are well reproduced by the simulation as shadowed dumbbell-shaped shock waves. We discuss the origin and energetics of the event in relation to the starburst and/or AGN activities in the Galactic Center. [ High resolution pdf is available at http://www.ioa.s.u-tokyo.ac.jp/~sofue/htdocs/2016bhs/ ]

astro-ph.GA↗

arXiv2015

Global structure of isothermal X-ray emission along the Fermi bubbles

In our previous works (Kataoka et al. 2013, Tahara et al. 2015), we found absorbed thermal X-ray plasma with kT ~ 0.3 keV observed ubiquitously near the edges of the Fermi bubbles and interpreted this emission as weakly shock-heated Galactic halo (GH) gas. Here we present a systematic and uniform analysis of archival Suzaku (29 pointings; 6 newly presented) and Swift (68 pointings; 49 newly presented) data within Galactic longitudes |l| < 20 deg and latitude 5 deg < |b| < 60 deg, covering the whole extent of the Fermi bubbles. We show that the plasma temperature is constant at kT = 0.30+-0.07 keV, while the emission measure (EM) varies by an order of magnitude, increasing toward the Galactic center (i.e., low |b|) with enhancements at the north polar spur (NPS), SE-claw and NW-clump features. Moreover, the EM distribution of kT ~ 0.30 keV plasma is highly asymmetric in the northern and southern bubbles. Although the association of the X-ray emission with the bubbles is not conclusive, we compare the observed EM properties with simple models assuming (i) a filled halo without bubbles, whose gas density follows a hydrostatic isothermal model (King profile) and (ii) a bubble-in-halo in which two identical bubbles expand into the halo forming thick shells of swept halo gas. We argue that the EM profile in the north (b > 0 deg) favors (ii), whereas that of the south (b < 0 deg) is rather close to (i), but weak excess signature is clearly detected also in the south like NPS (South Polar Spur; SPS). Such an asymmetry, if due to the bubbles, cannot be fully understood only by the inclination of bubbles' axis against the Galactic disk normal, thus suggesting asymmetric outflow due to different environmental/initial condition.

astro-ph.HE↗

arXiv2015

Suzaku X-ray Observations of the Fermi Bubbles: Northernmost Cap and Southeast Claw Discovered with MAXI-SSC

We report on Suzaku observations of large-scale X-ray structures possibly related with the Fermi Bubbles obtained in 2013 with a total duration of ~ 80 ks. The observed regions were the: (i) northern cap (N-cap; l ~ 0 deg, 45 deg < b < 55 deg) seen in the Mid-band (1.7-4.0 keV) map recently provided by MAXI-SSC and (ii) southeast claw (SE-claw; l ~ 10 deg, -20 deg < b < -10 deg) seen in the ROSAT all-sky map and MAXI-SSC Low-band (0.7-1.7 keV) map. In each region, we detected diffuse X-ray emissions which are represented by a three component plasma model consisting of an unabsorbed thermal component (kT ~ 0.1 keV) from the Local Bubble, absorbed kT = 0.30+/-0.05 keV emission representing the Galactic Halo, and a power-law component due to the isotropic cosmic X-ray background radiation. The emission measure of the GH component in the SE-claw shows an excess by a factor of ~ 2.5 over the surrounding emission at 2 deg away. We also found a broad excess in the 1.7-4.0 keV count rates across the N-cap after compiling other archival data from Suzaku and Swift. The spectral stacking analysis of the N-cap data indicates the presence of another thermal component with kT = 0.70 (+0.22,-0.11) keV. The temperature of kT ~ 0.3 keV of the Galactic Halo is higher than the ubiquitous value of kT ~ 0.2 keV near the Fermi Bubbles, and can be even higher (~ 0.7 keV). We discuss our findings in the context of bubble-halo interaction.

astro-ph.HE↗

arXiv2014

Suzaku observations of the diffuse x-ray emission across the fermi bubbles' edges

We present Suzaku X-ray observations along two edge regions of the Fermi Bubbles, with eight ~20 ksec pointings across the northern part of the North Polar Spur (NPS) surrounding the north bubble and six across the southernmost edge of the south bubble. After removing compact X-ray features, diffuse X-ray emission is clearly detected and is well reproduced by a three-component spectral model consisting of unabsorbed thermal emission (temperature kT ~0.1 keV from the Local Bubble (LB), absorbed kT ~0.3 keV thermal emission related to the NPS and/or Galactic Halo (GH), and a power-law component at a level consistent with the cosmic X-ray background. The emission measure (EM) of the 0.3 keV plasma decreases by ~50% toward the inner regions of the north-east bubble, with no accompanying temperature change. However, such a jump in the EM is not clearly seen in the south bubble data. While it is unclear if the NPS originates from a nearby supernova remnant or is related to previous activity within/around the Galactic Center, our Suzaku observations provide evidence suggestive of the latter scenario. In the latter framework, the presence of a large amount of neutral matter absorbing the X-ray emission as well as the existence of the kT ~ 0.3 keV gas can be naturally interpreted as a weak shock driven by the bubbles' expansion in the surrounding medium, with velocity v_exp ~300 km/s (corresponding to shock Mach number M ~1.5), compressing the GH gas to form the NPS feature. We also derived an upper limit for any non-thermal X-ray emission component associated with the bubbles and demonstrate, that in agreement with the findings above, the non-thermal pressure and energy estimated from a one-zone leptonic model of its broad-band spectrum, are in rough equilibrium with that of the surrounding thermal plasma.

astro-ph.HE↗

arXiv2011

Near-Solar-Circle Method for Determination of the Galactic Constants

We propose a method to determine the galactic constants R_0 (distance to the Galactic Center) and V_0 (rotation velocity of the Sun) from measurements of distances, radial velocities and proper motions of objects near the solar circle. This is a modification of the solar-circle method to a more practical observational method. We apply the method to determine R_0 using data from the literature with known distances and radial velocities, and obtain R_0 = 7.54 +/- 0.77 kpc.

astro-ph.GA↗

arXiv2008

Unified Rotation Curve of the Galaxy -- Decomposition into de Vaucouleurs Bulge, Disk, Dark Halo, and the 9-kpc Rotation Dip --

We present a unified rotation curve of the Galaxy re-constructed from the existing data by re-calculating the distances and velocities for a set of galactic constants R_0=8 kpc and V_0=200 km/s. We decompose it into a bulge with de Vaucouleurs-law profile of half-mass scale radius 0.5 kpc and mass 1.8 x 10^{10}M_{sun}, an exponential disk of scale radius 3.5 kpc of 6.5 x 10^{10}M_{sun}, and an isothermal dark halo of terminal velocity 200 km/s. The r^{1/4}-law fit was obtained for the first time for the Milky Way's rotation curve. After fitting by these fundamental structures, two local minima, or the dips, of rotation velocity are prominent at radii 3 and 9 kpc. The 3-kpc dip is consistent with the observed bar. It is alternatively explained by a massive ring with the density maximum at radius 4 kpc. The 9-kpc dip is clearly exhibited as the most peculiar feature in the galactic rotation curve. We explain it by a massive ring of amplitude as large as 0.3 to 0.4 times the disk density with the density peak at radius 11 kpc. This great ring may be related to the Perseus arm, while no peculiar feature of HI-gas is associated.

astro-ph↗

arXiv2008

Molecular Gas Distribution in Barred and Non-Barred Galaxies along the Hubble Sequence

We present results from a survey of 12CO(J=1-0) spectra obtained for the central regions of 68 nearby galaxies at an angular resolution of 16 arcseconds using the Nobeyama Radio Observatory 45m telescope, aimed at characterizing the properties of star forming molecular gas. Combined with similar resolution observations in the literature, the compiled sample set of 166 galaxies span a wide range of galactic properties. NGC 4380, which was previously undetected in CO, was detected. This initial paper of a series will focus on the data and the gaseous properties of the samples, and particularly on the degree of central concentration of molecular gas in a range of morphological types, from early (S0/Sa) to late (Sd/Sm) galaxies with and without bars. The degree of molecular central concentration in the central kiloparsec, compared to the central several kiloparsecs of galaxies, is found to vary smoothly with Hubble type, so that early type galaxies show larger central concentration. The comparison of barred and non-barred galaxies within early and late type galaxies suggest that difference in Hubble type, representing the effect of bulges, is the more important factor in concentrating gas into the central regions than bars.

astro-ph↗

arXiv2006

The Dog on the Ship: The "Canis Major Dwarf Galaxy" as an Outlying Part of the Argo Star System

Overdensities in the distribution of low latitude, 2MASS giant stars are revealed by systematically peeling away from sky maps the bulk of the giant stars conforming to ``isotropic'' density laws generally accounting for known Milky Way components. This procedure, combined with a higher resolution treatment of the sky density of both giants and dust allows us to probe to lower Galactic latitudes than previous 2MASS giant star studies. While the results show the swath of excess giants previously associated with the Monoceros ring system in the second and third Galactic quadrants at distances of 6-20 kpc, we also find a several times larger overdensity of giants in the same distance range concentrated in the direction of the ancient constellation Argo. Isodensity contours of the large structure suggest that it is highly elongated and inclined by about 3 deg to the disk, although details of the structure -- including the actual location of highest density, overall extent, true shape -- and its origin, remain unknown because only a fraction of it lies outside highly dust-obscured, low latitude regions. Nevertheless, our results suggest that the 2MASS M giant overdensity previously claimed to represent the core of a dwarf galaxy in Canis Major (l ~ 240 deg) is an artifact of a dust extinction window opening to the overall density rise to the more significant Argo structure centered at larger longitude (l ~ 290 +- 10 deg, b ~ -4 +- 2 deg).

astro-ph↗

arXiv2006

The Virgo CO Survey: VI. Gas Dynamics and Star Formation Along the Bar in NGC 4303

We present CO interferometer observations of the barred galaxy NGC 4303 (M61). This galaxy has a gas concentration at the central region and offset ridges in the bar. Sharp velocity gradients are apparent across the ridges. Analyses of the CO data and the newborn stellar clusters revealed in HST images indicate the existence of unresolved molecular clouds with masses of 10^4-6Msun. The observed shear velocity gradient across the ridges is too small to break up giant molecular clouds. Therefore, the clouds are likely to survive passage through the ridges. We discuss a cloud orbit model in a bar potential. The model reproduces the narrow offset ridges and sharp velocity gradients across the ridges in NGC 4303. We discuss cloud-cloud collisions (and close interactions) as a possible triggering mechanism for star formation. The newborn stellar clusters in NGC 4303 are located predominantly at the leading sides of the offset ridges, where cloud orbits are densely populated and suggest a high collisional frequency and possibly a high rate of triggered star formation. Cloud-based dynamics is less dissipative than smooth hydrodynamic models, possibly extending the timescales of gas dynamical evolution and gas fueling to central regions in barred galaxies.

astro-ph↗

arXiv2005

New CO observations and simulations of the NGC 4438/NGC 4435 system

NGC 4438 is a highly perturbed spiral with a stellar tidal tail and extraplanar molecular gas, now very HI deficient, crossing the center of the Virgo cluster at high speed. Various authors have attributed the perturbed appearance to the ram pressure of the intracluster medium, the tidal interaction with NGC 4435, and an ISM-ISM collision between the ISM of NGC 4438 and NGC 4435. We present new CO observations covering virtually all of NGC 4438 and the center of NGC 4435 and detailed simulations including all of the above effects. For the first time CO is detected in NGC 4435. In NGC 4438 we find double line profiles at distances up to 40" to the west and south-west and redshifted lines with respect to galactic rotation in the south of the center. The lack of gas to the North and East coupled with the large gaseous extent to the West and the redshifted and double line profiles can only be reproduced with a ram pressure wind. NGC 4438 is most probably on its first passage through the cluster center and has been stripped of its HI only over the past 100 Myr. While an ISM-ISM collision between NGC 4435 and NGC 4438 may occur, the effect is not significant compared to ram pressure and tidal forces, not surprising for the passage of an S0 galaxy 5-10 kpc from the center of NGC 4438. We also detect narrow CO lines, in the absence of detected HI, in the northern tidal arm some 15 kpc from the center of NGC 4438. This can be understood from the simulations assuming a few percent of the gas is too dense to experience the ram pressure wind. NGC 4438 has changed greatly over the past 100 Myr due to its plunge through the center of the Virgo cluster and the interaction with the S0 NGC 4435.

astro-ph↗

arXiv2005

Optical identification of ISO far-infrared sources in the Lockman Hole using a deep VLA 1.4 GHz continuum survey

By exploiting the far-infrared(FIR) and radio correlation, we have performed the Likelihood-Ratio analysis to identify optical counterparts to the far-infrared sources in the Lockman Hole. Using the likelihood ratio analysis and the associated reliability, 44 FIR sources have been identified with radio sources. Redshifts have been obtained for 29 out of 44 identified sources. One hyper-luminous infrared galaxy (HyLIRG) with and four ultraluminous infrared galaxies (ULIRGs) are identified in our sample. The space density of the FIR sources at z = 0.3-0.6 is 4.6\times 10^{-5}Mpc^{-3}, implying a rapid evolution of the ULIRG population. Most of \ISO FIR sources have their FIR-radio ratios similar to star-forming galaxies ARP 220 and M82. At least seven of our FIR sources show evidence for the presence of an active galactic nucleus (AGN) in optical emission lines, radio continuum excess, or X-ray activity. Three out of five (60%) of the ULIRG/HyLIRGs are AGN galaxies. Five of the seven AGN galaxies are within the ROSAT X-ray survey field, and two are within the XMM-Newton survey fields. X-ray emission has been detected in only one source, 1EX030, which is optically classified as a quasar. The non-detection in the XMM-Newton 2-10 keV band suggests a very thick absorption obscuring the central source of the two AGN galaxies. Several sources have an extreme FIR luminosity relative to the optical R-band, L(90μ\mathrm{m})/L(R) > 500, which is rare even among the local ULIRG population. While source confusion or blending might offer an explanation in some cases, they may represent a new population of galaxies with an extreme activity of star formation in an undeveloped stellar system -- i.e., formation of bulges or young ellipticals.

astro-ph↗

arXiv2004

Nobeyama Millimeter Array Observations of GRB 030329: a Decay of Afterglow with Bumps and Molecular Gas in the Host Galaxy

Nobeyama Millimeter Array was used to observe millimeter-wave afterglow of GRB 030329 at 93 GHz and 141 GHz from 2003 April 6 to 2003 May 30. A sensitive search for CO(J=1-0) emission/absorption from the host galaxy of GRB 030329 was also carried out. Unresolved millimeter continuum emission at the position of GRB 030329 was detected until 2003 April 21. We found a steep decline of continuum flux (\propto t^{-2.0}) during this period, in accord with a previous report. Moreover, our data implies that the decay was accompanied by possible plateaus phases, or bumps, on a time scale of several days. From an integrated spectrum, produced by summing up the data from 2003 April 10 to 2003 May 30, we found a possible emission feature, which could be a redshifted CO(J=1-0) line. Its position and redshift coincide well with those of GRB 030329, though further observations are required to confirm the detection. If the emission feature is real, the observed CO flux is 1.4 +/- 0.52 Jy km/s, corresponding to a large molecular gas mass of M(H_2) > 10^9 Mo. This implies that the host galaxy, which is optically faint, is highly obscured due to a rich interstellar medium.

astro-ph↗