arXiv subjects

Yoshiaki Sofue

Publications and source records attributed to Yoshiaki Sofue.

At least 109 records · Page 6Linked to original sources

arXiv2006

ASTE observations of nearby galaxies: A tight correlation between CO(J=3-2) emission and Halpha

Star formation rates (SFRs) obtained via extinction corrected H alpha are compared to dense gas as traced by CO(J=3-2) emission at the centers of nearby galaxies, observed with the ASTE telescope. It is found that, although many of the observed positions are dusty and therefore heavily absorbed at H alpha, the SFR shows a striking correlation with dense gas in the form of the Schmidt law with an index 1.0. The correlation is also compared between gas traced by CO(J=1-0) and application of H alpha extinction correction. We find that dense gas produces a far better correlation with SFR in view of surface density values.

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arXiv2006

Threshold between Spontaneous and Cloud-Collisional Star Formation

Based on simple physical and geometric assumptions, we have calculated the mean surface molecular density of spiral galaxies at the threshold between star formation induced by cloud-cloud collision and spontaneous gravitational collapse. The calculated threshold is approximately $\log Σ_\mathrm{crit} \sim 2.5$, where $ Σ\quad \mathrm{M_{\solar}}\cdot \mathrm{pc}^{-2}$ is the observed surface mass density of an assumed flat gas disk. Above this limit, the rate of molecular cloud collisions dominates over spontaneous molecular cloud collapse. This model may explain the apparent discontinuity in the Schmidt law found recently at $2 \lesssim \log Σ\lesssim 3$.

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arXiv2005

Virgo High-Resolution CO Survey :V. Circumnuclear Elliptical Ring in NGC 4569

We present high-resolution ($1\farcs8$ -- $4\farcs5$) CO data of the Virgo spiral galaxy NGC 4569, obtained using the Nobeyama Millimeter Array. We found that the molecular gas is highly concentrated in the circumnuclear region with two off-center peaks. A CO image with the highest angular resolution of $2\farcs0 \times 1\farcs8$ shows that six blobs likely form a circumnuclear elliptical ring (CER) with a semi-major axis radius of 720 pc. The CER shows a strongly twisted velocity field, and the position--velocity (PV) diagram shows significant forbidden velocity components. These kinetic features are understood as being non-circular motion due to a bar-potential. We found that the CER coincides with the H$α$ bright central core and that the mass ratio of the molecular gas to the dynamical mass is about 18%. These results support a gaseous inflow scenario induced by a weak bar potential and self-gravity of the gas.

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arXiv2005

The Schmidt Law at High Molecular Densities

We have combined Halpha and recent high resolution CO(J=1-0) data to consider the quantitative relation between gas mass and star formation rate, or the so-called Schmidt law in nearby spiral galaxies at regions of high molecular density. The relation between gas quantity and star formation rate has not been previously studied for high density regions, but using high resolution CO data obtained at the NMA(Nobeyama Millimeter Array), we have found that the Schmidt law is valid at densities as high as $10^3 \mathrm{M_\odot} \mathrm{pc}^{-2}$ for the sample spiral galaxies, which is an order of magnitude denser than what has been known to be the maximum density at which the empirical law holds for non-starburst galaxies. Furthermore, we obtain a Schmidt law index of $N=1.33\pm0.09$ and roughly constant star formation efficiency over the entire disk, even within the several hundred parsecs of the nucleus. These results imply that the physics of star formation does not change in the central regions of spiral galaxies. Comparisons with starburst galaxies are also given. We find a possible discontinuity in the Schmidt law between normal and starburst galaxies.

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arXiv2005

Magnetic-Reconnection and Current-Sheet Model for the Radio Arc and Threads in the Galactic Center

We propose a new mechanism to explain the radio Arc and threads in the Galactic Center by current sheets produced by local magnetic shears due to interaction of a moving cloud and the vertical field based on three-dimensional magneto-hydrodynamical simulations. Magnetic reconnetion and acceleration of cosmic-ray electrons in the current sheet will result in high contrast of radio emissivity inside and outside the Arc and threads.

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arXiv2004

Offsets between H-alpha and CO arms of a spiral galaxy NGC 4254: A New Method for Determining the Pattern Speed of Spiral Galaxies

We examined offsets between HII regions and molecular clouds belonging to spiral arms of a late type spiral galaxy NGC 4254 (M99). We used a high resolution CO(1-0) image obtained by Nobeyama Millimeter Array (NMA) and an H-alpha image. We derived angular offsets (theta) in the galactic disk, and found that these offsets show a linear dependence on the angular rotation velocity of gas (Omega_G). This linear relation can be expressed by an equation: theta =(Omega_G - Omega_P) * t_{H-alpha}, where Omega_P and t_{H-alpha} are constant. Here, Omega_P corresponds to the pattern speed of spiral arms and t_{H-alpha} is interpreted as the timescale between the peak compression of the molecular gas in spiral arms and the peak of massive star formation. We may thus determine Omega_P and t_{H-alpha} simultaneously by fitting a line to our theta - Omega_G plot, if we assumed they are constant. From our plot, we obtained t_{H-alpha} =4.8 (+/- 1.2) Myr and Omega_P = 26 (+10/-6) km/s/kpc, which are consistent with previous studies. We suggest that this theta - Omega_G plot can be a new tool to determine the pattern speed and the typical timescale needed for star formations.

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arXiv2004

Virgo High-Resolution CO Survey IV. Spiral-Driven Gas Dynamics in the Non-Barred Seyfert Galaxy NGC 4501

We report on high-resolution interferometer observations of the 12CO(J=1-0) emission in the central 5 kpc region of the Seyfert 2 galaxy NGC 4501. The observations were made using the Nobeyama Millimeter Array during a long-term CO line survey of Virgo spirals. The major features are: (1) a nuclear concentration with a radius of r~5'' (390 pc), which is resolved into double peaks, and (2) spiral arms which extend out from the nuclear region. The nuclear component has a mass of 1.3x10^8 Mo, which corresponds to ~3.5% of the dynamical mass, and shows a slight non-circular motion. The double peaks are separated by ~4''.7. (370 pc), and located on the root of optical spiral arms in a HST image. The gas arms are associated with the spiral dust lanes, and are linked to the central double peaks. The non-circular motions along the molecular arms indicate the fact that the gas is driven by the density wave, rather than the stochastic processes. We calculated the gas cloud orbits in a stellar spiral potential, and explained the observed CO spiral arms and non-circular motions. We suggest that the central gas condensation arises from spiral-driven gas transfer. We estimated and compared the effect of two possible mechanisms of angular-momentum transfer: galactic shock, and gravitational torques. We discuss that the galactic shock is dominant.

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arXiv2003

Faint 6.7um Galaxies and their Contributions to the Stellar Mass Density in the Universe

We discuss the nature of faint 6.7um galaxies detected with the mid-infrared camera ISOCAM on board the Infrared Space Observatory (ISO). The 23 hour integration on the Hawaii Deep Field SSA13 has provided a sample of 65 sources down to 6uJy at 6.7um. For 57 sources, optical or near-infrared counterparts were found with a statistical method. All four Chandra sources, three SCUBA sources, and one VLA/FIRST source in this field were detected at 6.7um with high significance. Using their optical to mid-infrared colors, we divided the 6.7um sample into three categories: low redshift galaxies with past histories of rapid star formation, high redshift ancestors of these, and other star forming galaxies. Rapidly star forming systems at high redshifts dominate the faintest end. Spectroscopically calibrated photometric redshifts were derived from fits to a limited set of template SEDs. They show a high redshift tail in their distribution with faint (<30uJy) galaxies at z>1. The 6.7um galaxies tend to have brighter K magnitudes and redder I-K colors than the blue dwarf population at intermediate redshifts. Stellar masses of the 6.7um galaxies were estimated from their rest-frame near-infrared luminosities. Massive galaxies (M_star~10e11M_sun) were found in the redshift range of z=0.2-3. Epoch dependent stellar mass functions indicate a decline of massive galaxies' comoving space densities with redshift. Even with such a decrease, the contributions of the 6.7um galaxies to the stellar mass density in the universe are found to be comparable to those expected from UV bright galaxies detected in deep optical surveys.

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arXiv2003

Three-Dimensional Distribution of the ISM in the Milky Way Galaxy: I. The HI Disk

We derived the three-dimensional distribution of HI gas in the Milky Way Galaxy using the latest HI survey data cubes and rotation curves. The distance of the HI gas was determined by the kinematic distance using a rotation curve. We solved the near-far problem in the inner Galaxy by a fitting method which involves introducing a model of vertical HI distribution. In our resultant maps we could trace three prominent arms: the Sagittarius-Carina arm, the Perseus arm, and the Outer arm. These three arms were found to be logarithmic spiral arms. The pitch angles of the Sagittarius-Carina, Perseus, and Outer arms were estimated to be about $11\arcdeg$, $18\arcdeg$, and $7\arcdeg$, respectively. The Sun is located in a region rich in HI gas between the Sagittarius-Carina arm and the Perseus arm. The HI disk shows large and asymmetric warping in the outer disk: the HI disk goes up to about 1.5 kpc above the Galactic plane in the northern hemisphere, and down to about 1 kpc in the southern hemisphere, which means asymmetric warping. The inner HI disk is also found to be tilting. The radius of the HI disk is about 17 kpc and the HI mass within this radius is estimated to be $2.5 \times 10^9$ M$_\odot$, which corresponds to 1.5 % of the dynamical mass predicted from the rotation curve. We also found that the HI outskirt is largely swelling in the fourth quadrant, and hence the Galaxy is significantly lopsided. The scale-height of the HI layer increases with the radius, and is correlated with the HI volume density at the centroid of the HI layer.

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arXiv2003

Galactic Center Shells and a Recurrent Starburst Model

By applying filtering techniques to remove straight filaments in the 20-cm VLA radio image of the Galactic Center Arc region, we have shown that numerous concentric radio shells of radii 5 to 20 pc are surrounding the Pistol and Sickle region, which we call Galactic Center Shells (GCS). Each shell has thermal energy of the order of $10^{49-50}$ erg. Several CO-line shells are associated, whose kinetic energies are of the order of $10^{49-50}$ erg. Summing up the energies of recognized GCSs, the total energy amounts to $\sim 10^{51}$ erg. The GCSs show an excellent correlation with the FIR shells observed at 16--26 microns with the MSX. We propose a model in which GCSs were produced by recurrent and/or intermittent starbursts in the Pistol area during the last million yr. The most recent burst occurred some $10^5$ years ago, producing an inner round-shaped shell (GCS I); earlier ones a million years ago produced outer shells (GCS II and III), which are more deformed by interactions with the surrounding ISM and Sgr A halo. We argue that recurrent starbursts had also occurred in the past, which produced larger scale hyper shell structures as well. A burst some million years ago produced the Galactic Center Lobe, and a much stronger one 15 million years ago produced the North Polar Spur.

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arXiv2003

The Virgo High-Resolution CO Survey. III. NGC 4254

We present high-angular-resolution (1".5 - 5") interferometer observations of the \co emission in the central region of the SA(s)c galaxy NGC 4254. The observations were obtained using the Nobeyama Millimeter-wave Array (NMA) during the course of a long-term CO line survey of Virgo spirals. We present the spectra, channel maps, integrated intensity distributions, velocity fields, position--velocity diagrams, and compare the data with various optical images. The rotation velocity is already finite at the nucleus, or at least it rises steeply to 80 km/s within the central 1", indicating the existence of a massive core of 10^8 Msun within 1" (80 pc) radius. The CO intensity maps show that the inner disk has well-developed multiple spiral arms, winding out from a bar-shaped elongated molecular complex. In addition to the bisymmetric spiral arms, an asymmetric tightly wound arm with high molecular gas density is found to wind out from the molecular bar. The molecular spiral arms, particularly the tightly wound arm, well traces optical dark lanes, and are associated with H$α$ arms having many H {\sc ii} regions. The inner asymmetric spiral structures can be explained by ram-pressure distortion of inter-arm low density regions of the inner disk by the intra-cluster gas wind, and is indeed reproduced by a hydrodynamical simulation.

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arXiv2002

Spiral Molecular Front in Galaxies: Quick Transition from Atomic to Molecular Hydrogen in Spiral Arms

We derived a two-dimensional map of the molecular fraction, fmol, (the ratio of molecular-gas density to that of total gas) in the spiral galaxy M51, and examined the behavior of molecular fronts (MF), where MF represents the place where fmol changes drastically from nearly zero to unity and vice versa. We show that the MF phenomenon occurs not only radial, but also in the azimuth direction through the spiral arms, and fmol changes rapidly in the arm-to-inter-arm transition regions. The existence of the azimuthal MF indicates that the atomic gas (HI) is quicklytransformed to molecular gas (H_2) during the passage through spiral arms. We performed a numerical simulation of MF based on an HI-to-H_2 phase transition theory, and reproduced the observations. We estimated a azimuthal scale length of the transition to be less than 200 pc, corresponding to a time scale of about 2 Myr for H_2 gas formation. The azimuthal width of a molecular arm is estimated to be at most 2.5 kpc, where the gas can remain in molecular phase for about 25 Myr.

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arXiv2002

Ram-Ppressure Effects on Dense Molecular Arms in the Central Regions of Spiral Galaxies by Intracluster Medium

We investigated ram-pressure effects by an intracluster wind on an inner disk of spiral galaxies by a hydrodynamical simulation. Even if the wind is mild and not strong enough to strip the gas disk, the ram pressure disturbs orbits of the inter-arm gas significantly. This results in asymmetric dense molecular arms in the inner few kpc region of a galaxy. This mechanism would explain the asymmetric CO gas distributions in the central regions often observed in Virgo spirals.

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arXiv2001

Bipolar hypershell models of the extended hot interstellar medium in spiral galaxies

We simulated the million degree interstellar medium and its soft X-ray images in the disk and halo of spiral galaxies using the bipolar hypershell (BHS) model. In this model hourglass-shaped expanding shells of several kpc radii are produced by a sudden energy release in the central region. We then applied our model to a mini-sample of starburst galaxies seen under different inclinations suhc as the nearly edge-on galaxies NGC 253. Our results reproduce the X-ray characteristics observed in the 0.1--2.4 keV ROSAT energy band: The bipolar hypershell morphology, the spectral energy distribution of the diffuse disk and halo emission as well as absorption gaps in the diffuse X-ray emission caused by a shadowing of soft X-rays due to cold intervening gas in the disks of the galaxies. The required total energy for the starburst is estimated to be of the order of $10^{55}$ ergs, corresponding to the overall kinetic energy generated in $\sim 10^4$ type-II supernova explosions. The expansion velocity of the shells is estimated to be $\sim 200$ \kms, which is necessary to heat the gas to $\sim 0.2$ keV (2.3 million K), and the age to be of the order of $3 \times 10^7$ years. In the case of the very nearby galaxy NGC 253 all characteristics of the BHS model can be studied with high spatial resolution. Using the property that the shell morphology is sensitive to the ambient density distribution, we propose using soft X-ray data to probe the gas distributions in the disk, halo and intergalactic space. The application of our model to images at higher spatial and spectral resolution, as provided by Chandra and XMM, will help us to further disentangle the ISM density distributions and will lead to a better understanding of the disk halo interface.

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arXiv2000

A Unified Scaling Law in Spiral Galaxies

We investigate the origin of a unified scaling relation in spiral galaxies. Observed spiral galaxies are spread on a plane in the three-dimensionallogarithmic space of luminosity L, radius R and rotation velocity V. The plane is expressed as $L \propto (V R)^α$ in I-passband, where $α$ is a constant. On the plane, observed galaxies are distributed in an elongated region which looks like the shape of a surfboard. The well-known scaling relations, L-V (Tully-Fisher relation), V-R (also the Tully-Fisher relation) and R-L (Freeman's law), can be understood as oblique projections of the surfboard-like plane into 2-D spaces. This unified interpretation of the known scaling relations should be a clue to understand the physical origin of all the relations consistently. Furthermore, this interpretation can also explain why previous studies could not find any correlation between TF residuals and radius. In order to clarify the origin of this plane, we simulate formation and evolution of spiral galaxies with the N-body/SPH method, including cooling, star formation and stellar feedback. Initial conditions are set to isolated 14 spheres with two free parameters, such as mass and angular momentum. The CDM (h=0.5, $Ω_0=1$) cosmology is considered as a test case. The simulations provide the following two conclusions: (a) The slope of the plane is well reproduced but the zero-point is not. This zero-point discrepancy could be solved in a low density ($Ω_0<1$) and high expansion (h>0.5) cosmology. (b) The surfboard-shaped plane can be explained by the control of galactic mass and angular momentum.

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arXiv1999

Bipolar-Hyper-Shell Galactic Center Statrburst Model: Further Evidence from ROSAT Data and New Radio and X-ray Simulations

Using the all-sky ROSAT soft X-ray and 408-MHz radio continuum data, we show that the North Polar Spur and its western and southern counter-spurs draw a giant dumbbell-shape necked at the galactic plane. We interpret these features as due to a shock front originating from a starburst 15 million years ago with a total energy of the order of $\sim 10^{56}$ ergs or $10^5$ type II supernovae. We simulate all-sky distributions of radio continuum and soft X-ray intensities based on the bipolar-hyper-shell galactic center starburst model. The simulations can well reproduce the radio NPS and related spurs, as well as radio spurs in the tangential directions of spiral arms. Simulated X-ray maps in 0.25, 0.75 and 1.5 keV bands reproduce the ROSAT X-ray NPS, its western and southern counter-spurs, and the absorption layer along the galactic plane. We propose to use the ROSAT all-sky maps to probe the physics of gas in the halo-intergalactic interface, and to directly date and measure the energy of a recent Galactic Center starburst.

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arXiv1999

Dark-Matter Distribution inferred from High-accuracy Rotation Curves

Using high-accuracy rotation curves of spiral galaxies, we derive distributions of the surface mass density. Comparing with the luminosity profiles, we show that the dark mass in disk and bulge distributions follows those of luminous (stellar) mass, while luminous mass in halos does not follow dark mass. The dark-mass fraction (DMF) increases with the radius in the disk, and rapidly toward edge in the halo. In some galaxies, DMF increases toward the center,

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arXiv1999

Distribution of Dark Matter in Bulge, Disk and Halo inferred from High-accuracy Rotation Curves

High-accuracy rotation curves of spiral galaxies show a steep rise in the central few hundred pc region, indicating high concentration of mass toward the center. Using the rotation curves, we idrectly calculate radial distributions of the surface-mass density (SMD), and obtain radial profiles of the mass-to-luminosity ratio (M/L). The M/L ratio is found to vary significantly not only in the massive halo but also within the bulge and disk regions. It increases with the radius within the disk, and rapidly toward halo. In some galaxies, the M/L increases inward toward the center within the bulge, indicating a massive dark core. On these bases, we discuss the radial distribution of dark matter in spiral galaxies.

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