arXiv subjects

Yoshiaki Sofue

Publications and source records attributed to Yoshiaki Sofue.

At least 127 records · Page 7Linked to original sources

arXiv1999

Accurate Rotation Curves and Distribution of Dark Matter in Galaxies

We present high-accuracy rotation curves, which show a steep nuclear rise and high-velocity central rotation, followed by a broad maximum in the disk and flat part. We use the rotation curves to directly calculate the radial distribution of surface mass density, and obtain radial variations of the mass-to-luminosity ratio (M/L). The M/L ratio and, therefore, the dark mass fraction (DMF) is not constant at all, but varies within the bulge, increases already within the disk, and rapidly from the disk toward halo. In some galaxies, the DMF within the bulge increases inward toward the center, indicating a massive dark core.

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arXiv1999

Compatibility of the CO and HI Linewidths Considering the Gas Distribution and Rotation Curves

We have examined the trend between CO and HI linewidths by compiling the linewidths of 219 nearby galaxies. The trend is that the CO linewidths of fast rotating galaxies tend to be larger than the HI linewidths, and that the HI linewidths of slow rotating galaxies tend to be larger than the CO linewidths, whereas the intermediately rotating galaxies have almost equal values. We have explained the trend using the synthetic rotation curve model, which provides the linewidth - absolute magnitude relations at any radii, and concluded that the trend has been occurred by the different distributions of CO and HI in a galaxy and the gradient of the rotation curves. The exact linewidth should be corrected for the effects of the gradient of the rotation curve and the gas distribution.

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arXiv1999

Dark Bulge, Exponential Disk, and Massive Halo in the Large Magellanic Cloud

The rotation curve of the Large Magellanic Cloud, which we have derived from high-resolution HI position-velocity diagrams observed by Kim et al (1998), shows a steep central rise and flat rotation with a gradual rise toward the edge. Using the rotation curve, we have calculated the distribution of surface mass density, and show that the LMC has a dark compact bulge, an exponential disk, and a massive halo. The bulge is 1.2 kpc away from the center of the stellar bar, and is not associated with an optical counterpart. This indicates that the "dark bulge" has a large fraction of dark matter, with an anomalously high mass-to-luminosity (M/L) ratio. On the contrary, the stellar bar has a smaller M/L ratio compared to the surrounding regions. (http://www.ioa.s.u-tokyo.ac.jp/~sofue/99lmc.htm)

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arXiv1998

Nuclear-to-Disk Rotation Curves and Mass-to-Luminosity Ratio in Galaxies

High-resolution nuclear-to-outer rotation curves for Sb, SBb, Sc, and SBc galaxies show generally a steep nuclear rise and flat rotation from the disk to halo. The high-velocity central rotation indicates massive core within bulges. Since this characteristics is common to most galaxies, the high-velocity central rotation cannot be due to a particular orientation of non-circular motion. Using these rotation curves, we derive the distributions of surface-mass density, and compare directly with observed surface-luminosity distributions. The mass-to-luminosity ratio (M/L) remains constant in the outer bulge and disk, while it increases toward the halo, indicating the massive halo. In the central region, the M/L also increases steeply toward the center, reaching by an order of magnitude greater value than the disk value, which may indicate a massive core of radius about 100 parsecs and mass of the order of E9 Msun. The core may be an object linking a bulge and a massive black hole. (Related data at http://www.ioa.s.u-tokyo.ac.jp/~sofue/)

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arXiv1998

Nuclear-to-Disk Rotation Curves of Galaxies in the H_alpha and [NII] Emission Lines

We have obtained optical CCD spectroscopy along the major axes of 22 nearby spiral galaxies of Sb and Sc types in order to analyze their rotation curves. By subtracting the stellar continuum emission, we have obtained position velocity (PV) diagrams of the H alpha and [NII] lines. We point out that the H alphaline is often superposed by a broad stellar absorption feature (Balmer wind) in the nuclear regions, and, therefore, the [NII] line is a better tracer of kinematics in the central a few hundred pc regions. By applying the envelope-tracing technique to the H alpha and [NII] PV diagrams, we have derived nucleus-to-disk rotation curves of the observed galaxies. The rotation curves rise steeply within the central a few hundred parsecs, indicating rapidly rotating nuclear disk and mass concentration near the nucleus. [For more rotation curves : http://www.ioa.s.u-tokyo.ac.jp/]

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arXiv1998

Is M82 a Disk-Truncated Bulge by a Close Encounter with M81?

The rotation curve of the small-mass starburst galaxy M82 has a steep nuclear rise, peaking at 200 pc radius, and then, declines in a Keplerian fashion. This rotation curve mimics that for a central bulge of spiral galaxies with high concentration of stellar mass. The declining rotation indicates that its extended disk mass is missing. In order to explain this peculiar rotation characteristics, we propose a hypothesis that M82 is a surviving central bulge of a much larger disk galaxy, whose outer disk was truncated during a close encounter with M81. We simulate a tidal truncation of the disk of a companion galaxy by a tidal penetration through its more massive parent galaxy. The model can well reproduce the observed peculiar feature of M82.

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arXiv1997

Why Can We Detect Quasar BR1202$-$0725 in Co ?

We present CO luminosity evolution of both elliptical and spiral galaxies based on a galactic wind model and a bulge-disk model, respectively. We have found that the CO luminosity peaks around the epoch of galactic wind caused by collective supernovae $\sim$ 0.85 Gyr after the birth of the elliptical with $M = 2 \cdot 10^{12} M_\odot$ while $\sim$ 0.36 Gyr after the birth of the bulge with $M = 2 \cdot 10^{11} M_\odot$. After these epochs, the CO luminosity decreases abruptly because the majority of molecular gas was expelled from the galaxy system as the wind. Taking account of typical masses of elliptical galaxies and bulges of spiral galaxies, we suggest that CO emission can be hardly detected from galaxies with redshift $z \sim 1 - 4$ unless some amplification either by galaxy mergers and/or by gravitational lensing is working. Therefore, our study explains reasonably why CO emission was detected from the high-redshift quasar BR 1202$-$0725 at $z =4.7$ while not detected from the powerful radio galaxies with $1 < z < 4$.

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arXiv1997

On the Keplerian Rotation Curves of Galaxies

We propose a criterion for examining whether or not the uncertainty of the outer rotation curves is sufficiently small to distinguish a Keplerian rotation curve from flat ones. We have applied this criterion to both Keplerian and non-Keplerian rotation curves so far obtained, and investigated their relative fraction. We also studied the minimum extent of the dark halos indicated by non-Keplerian rotation curves using the criterion. We have found that one cannot rule out the possibility that a significant fraction of rotation curves become Keplerian within 10-times the disk scale length. If the Keplerian rotation curves so far observed trace the mass truncation, several galaxies may have rather small halos, the extent of which is not larger than twice that of the optical disk.

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arXiv1997

Nuclear Rotation Curves of Galaxies in the CO Line Emission

We have obtained high-resolution position-velocity (PV) diagrams along the major axes of the central regions of nearby galaxies in the CO-line emission using the Nobeyama 45-m telescope and the Millimeter Array. Nuclear rotation curves for 14 galaxies have been derived based on the PV diagrams using the envelope-tracing method. The nuclear rotation curves for most of the galaxies show a steep rise within a few hundred pc, which indicates a high-density concentration of mass. Keywords: Galaxies: general - Galaxies: structure - ISM: molecular line

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arXiv1997

Effects of Galaxy Interaction on the Tully-Fisher Relation: CO vs HI Linewidths

We investigate effects of galaxy-galaxy interaction on the Tully-Fisher relation. The HI linewidth in interacting galaxies is significantly broader than the CO linewidth, and the HI to CO linewidth ratio is proportional to the strength of interaction. This provides different distances to galaxies measured by the Tully-Fisher relation with the CO and HI linewidths. Distances derived from the HI linewidths are 62% larger than those derived from the CO linewidths for strongly interacting galaxies, and 25% larger for weakly interacting galaxies. We argue that the CO-line Tully-Fisher relation will be more reliable to measure the distances of interacting galaxies as well as galaxies in rich clusters.

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arXiv1996

Nuclear-to-Outer Rotation Curves of Galaxies in the CO and HI Lines

We have derived rotation curves of nearby galaxies, which are almost completely sampled from the inner to outer regions. We used high-resolution position-velocity diagrams in the CO line along the major axes for the inner regions, and HI-line data for the outer regions. We combined the CO and HI data to obtain total rotation curves from the nuclear to outer regions. The rotation curves show a steep nuclear rise within a radius smaller than the resolutions of CO-line observations, indicating a compact massive concentration near the nucleus. We show that the nuclear steep rise is general and universal, and a rigid body-like gentle rise is exceptional. Keywords: Galaxies: kinematics - Galaxies: rotation - Galaxies: structure - Galaxies: general - ISM: CO emission - ISM: HI gas

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arXiv1996

Mass of the Galaxy Inferred from Outer Rotation Curve

Using an outer rotation curve of the Galaxy, we explore the galactic constants and the mass of the Galaxy. We show that Θ_0 of 200 km/s is more favorable than the IAU standard value of 220 km/s, and also show that if Θ_0 is smaller than 207 km/s the rotation curve beyond 2R_0 is declining in Keplerian fashion. In the case of Θ_0= 200 km/s and R_0= 7.6 kpc, the total mass and the extent of the Galaxy inferred from the rotation curve are 2.0+/-0.3x 10^{11} M_\odot and 15 kpc, respectively. These results may significantly change the previous view of the Galaxy, that its outer region is dominated by a massive dark halo extending out to several tens of kpc.

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arXiv1996

CO vs HI Rotations of Galaxies

Using the fact that molecular gas is distributed in the inner disk while HI in the outer disk, we have derived most-completely sampled rotation curves by combining HI and CO position-velocity diagrams. We show that inner rotation curves have a steep increase within the central few hundred-pc region. The inner behavior of rotation can be fitted by the Miyamoto-Nagai potential, only if a central mass component with scale radius of 100 -- 200 pc and a mass of $\sim 3-5\times 10^{9}\Msun$ is assumed in addition to the usual bulge. This implies that the central $\sim 100$-pc region has a stellar mass density as high as several hundred $\Msun{\rm pc}^{-3}$, orders-of-magnitudes higher than that for a single bulge and disk (several $\Msun {\rm pc}^{-3}$). Such a concentric multi-component structure within a bulge (``bulge in bulge'') may have crucial implication for the dynamical evolution of the central regions of galaxies. Since the molecular gas distribution is closely correlated with optical disk, CO line can be used for the line width-luminosity (Tully Fisher) relation in addition to (instead of) the HI line. We show that the CO-line width-luminosity relation can be a powerful routine method to derive distances of cosmologically-remote galaxies and the Hubble parameter for a significantly large volume of the universe. By applying the CO-line width-luminosity relation to CO-line data by large-aperture mm-wave telescope, we have derived distances to galaxies at $cz \sim 20,000$. In order to measure the inclination and magnitudes, we performed optical imaging observations of thus CO-detected galaxies using the CFHT 3.6-m telescope at high resolution. The radio and optical data have been combined to derive the distances to the galaxies. We further derived the Hubble ratio for these galaxies by applying the CO-line

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arXiv1996

Radio Continuum and Molecular Gas in the Galactic Center

Nonthermal radio emission in the galactic center reveals a number of vertical structures across the galactic plane, which are attributed to poloidal magnetic field and/or energetic outflow. Thermal radio emission comprises star forming regions distributed in a thin, dense thermal gas disk. The thermal region is associated with dense molecular gas disk, in which the majority of gas is concentrated in a rotating molecular ring. Outflow structures like the radio lobe is associated with rotating molecular gas at high speed, consistent with a twisted magnetic cylinder driven by accretion of a rotating gas disk.

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arXiv1995

High-Velocity Molecular Gas in the Galactic Center Radio Lobe

We point out a possible association of high-velocity molecular gas with the Galactic Center Radio Lobe (GCL). A molecular spur in the eastern GCL ridge is receding at $\Vlsr \sim +100$ \kms, and the western spur approaching at $\Vlsr \sim -150$ \kms, suggesting a high-velocity rotation of the GCL. We study the kinematics of the GCL based on these molecular line data.

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arXiv1995

Galactic Center Molecular Arms, Ring and Expanding Shells. II - Expanding Molecular Shell -

We investigate the three-dimensional structure of the so-called ``200-pc expanding molecular ring" in the Galactic Center by analyzing (b, Vlsr) diagrams of the 13CO(J=1-0) line emission from the Bell-Telephone Laboratory survey. We show that the b-v features can be fitted by a spheroidal shell pinched at the equator (dumbbell-shaped shell), which we call the expanding molecular shell (EMS). The radius is about 180 pc and the vertical extent is more than +/-50 pc. The shell is expanding at 160 km/s, and is rotating at 70 km/s. The association of the radio continuum emission indicates that the EMS is a mixture of molecular and ionized hydrogen gases. An extended hot (X-ray) plasma appears to fill the interior of the shell. The estimated molecular mass of the shell is approximately 1.E7 Msun and its kinetic energy 2.E54 ergs.

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arXiv1995

The Molecular Front in Galaxies II : Galactic-scale Gas Phase Transition of HI and H2

We have examined the distribution of HI and H$_2$ gases in four face-on galaxies by using the observed dat a of CO and HI line emissions from the literatures. We demonstrate that the gas phase transition of HI and H$_2$ occurs suddenly within a narrow range of radi us, which we call the molecular front. We have tried to explain such phase transition in galactic scale with a help of the phase transition theory proposed by Elmegreen. The crucial parameters for determinating the molecular fraction $f_{\rm mol}$ are interstellar pressure $P $, UV radiation field $U$, and metallicity $Z$, and we have constructed a model galaxy in which $P$, $U$ a nd $Z$ obey an exponential function of the galacto-centric radius. The model shows that the molecular front must be a fundamental feature of galaxies which has an exponentia l disk, and that the metallicity gradient is most crucial for the formation of the molecular front. We have also tried to reproduce the observed molecular fraction $f_{\rm mol}$ by giving the set of ($P$, $ U$, $Z$) observationally, and show that the model can describe the variation of the molecular fraction $f_ {\rm mol}$ in galaxies quite well. We discuss the implication of the molecular front for the chemical evolution of galaxies.

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arXiv1995

The Most Completely Sampled Rotation Curves for Galaxies

We have compiled high-resolution position-velocity diagrams observed along the major axes of nearby spiral galaxies in the CO-line emission, and derived rotation curves for the inner regions of the galaxies. We have combined the inner rotation curves with the outer HI and optical rotation curves to obtain the total rotation curves. The inner rotation curves are characterized by a steep increase within a few hundred pc radius, indicating a compact massive concentration near the nucleus. We fit the obtained rotation curves for individual galaxies by a modified Miyamoto-Nagai's potential by assuming existence of four mass components; a nuclear mass component with a scale radius of 100-150 pc and a mass of $\sim 3-5\times 10^{9}Msun$; a central bulge of 0.5 to 1 kpc radius of a mass $\sim 10^{10}Msun$; a disk with scale radius 5 to 7 kpc and thickness 0.5 kpc of a mass $\sim 1-2 Mgal$; and a massive halo of scale radius 15 to 20 kpc with a mass $\sim 2-3 Mgal$. We discuss the implication of the nuclear compact mass component for the formation mechanism of multiple structures within the galactic bulge during its formation.

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