arXiv subjects

J. Koda

Publications and source records attributed to J. Koda.

60 records · Page 4Linked to original sources

arXiv2003

The Virgo High-Resolution CO Survey. II. Rotation Curves and Dynamical Mass Distributions

Based on a high-resolution CO survey of Virgo spirals with the Nobeyama Millimeter-wave Array, we determined the dynamical centers using velocity fields, and derived position-velocity diagrams (PVDs) along the major axes of the galaxies across their dynamical centers. We applied a new iteration method to derive rotation curves (RCs), which reproduce the observed PVDs. The obtained high-accuracy RCs generally show steep rise in the central 100 to 200 pc regions, followed by flat rotation in the disk. We applied a deconvolution method to calculate the surface-mass density (SMD) using the RCs based on two extreme assumptions that the mass distribution is either spherical or thin-disk shaped. Both assumptions give nearly identical results, agreeing with each other within a factor of two at any radii. The SMD distributions revealed central massive cores with peak SMD of 10^4 - 10^5 Msun pc^-2 and total mass within 200 pc radius of the order of about 10^9 Msun Correlation analysis among the derived parameters show that the central CO-line intensity is positively correlated with the central SMD, which suggests that the deeper is the gravitational potential, the higher is the molecular gas concentration in the nuclei regardless morphological types.

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arXiv2002

NMA CO (J=1-0) Observations of the Halpha/Radio Lobe Galaxy NGC 3079: Gas Dynamics in a Weak Bar Potential and Central Massive Core

within 24 lines with 80 characters) We present ^12CO (1-0) observations in the central 4.5 kpc (1 arcmin) of the Halpha/Radio lobe galaxy NGC 3079 with the Nobeyama Millimeter Array. The molecular gas shows four components: a main disk, spiral arms, a nuclear disk, and a nuclear core. The main disk extends beyond our spatial coverage. The spiral arms are superimposed on the main disk. The nuclear disk with about 600 pc radius has an intense concentration of molecular gas. Its appearance on PV diagrams is indicative of oval motions of the gas, rather than circular. The nuclear core is more compact than our resolution. Though it is unresolved, the nuclear core shows a very high velocity about 200 km/s even at the radius of about 100 pc on the PV diagram. We propose a model that NGC 3079 contains a weak bar. This model explains the observed features: the main disk and spiral arms result from gaseous x1-orbits and associated crowding respectively. The nuclear disk arises from gaseous x2-orbits. From the appearance of the spiral arms on the PV diagram, the pattern speed of the bar is estimated to be 55+-10 km/s/kpc. The high velocity of the nuclear core cannot be explained by our model for a bar. Thus we attribute it to a central massive core with a dynamical mass of 10^9 Msun within the central 100 pc. This mass is three orders of magnitude more massive than that of a central black hole in this galaxy.

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arXiv2001

Multi-Phase Gas Dynamics in a Weak Barred Potential

The structure of the interstellar medium in the central kpc region of a galaxy with a weak bar-like potential is investigated taking into account realistic cooling and heating processes and the self-gravity of the gas. Using high resolution hydrodynamical simulations, it is revealed that the resonant structures (e.g. smooth spiral shocks and a nuclear ring) are very different from those seen in past numerical models where simple models of the ISM, i.e. non-self-gravitating, isothermal gas were assumed. We find that the pc-scale filaments and clumps form large scale spirals, which resemble those seen in real galaxies. The fine structures are different between the arms and in the nuclear region. The next generation millimeter interferometer (ALMA) may reveal the fine structures of the cold gas in nearby galaxies. We also find a large scale anisotropy in the gas temperature, which is caused due to non-circular velocity field of the gas.The damped orbit model based on the epicyclic approximation explains the distribution of the hot (> 10^4 K) and cold (< 100 K) gases appearing alternately around the galactic center. Because of the temperature anisotropy, cold gases observed by molecular lines do not necessarily represent the real gas distribution in galaxies. Position-Velocity diagrams depend strongly on the viewing angles. As a result, the rotational velocity inferred from the PV maps could be two times larger or smaller than the true circular velocity.

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arXiv2000

Ultra-High-Density Molecular Core and Warped Nuclear Disk in the Deep Potential of Radio-Lobe Galaxy NGC 3079

We have performed high-resolution synthesis observations of the 12CO (J=1-0) line emission from the radio lobe edge-on spiral NGC 3079 using a 7-element mm-wave interferometer at Nobeyama, which consisted of the 45-m telescope and 6-element Array. The molecular nuclear disk (NMD) of 750 pc radius is found to be inclined by 20 degrees from the optical disk, and the NMD has spiral arms. An ultra-high-density molecular core (UHC) was found at the nucleus. The gaseous mass of the UHC within 125 pc radius is as large as about 3 x 10^8 Msun, an order of magnitude more massive than that in the same area of the Galactic Center, and the mean density is as high as approximately 3 x 10^3 H2 cm^-3. A position-velocity diagram along the major axis indicates that the rotation curve starts at a finite velocity exceeding 300 \kms already from the nucleus. The surface mass density in the central region is estimated to be as high as about 10^5 Msun pc^-2, producing a very deep gravitational potential. We argue that the very large differential rotation in such a deep potential will keep the UHC gravitationally stable from current star formation.

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arXiv1999

On the origin of the Tully-Fisher relation

We discuss the origin of the Tully-Fisher (TF) relation using the $N$-body/SPH method, which includes cooling, star formation and stellar feedback of energy, mass and metals. We consider initially rotating overdense spheres, and trace formation processes of disk galaxies from $z=25$ to $z=0$ in the Cold Dark Matter (CDM) cosmology. To clarify the origin of the TF relation, we simulate formation of 14 galaxies with different masses and spin parameters, and compute observable values, such as the total magnitude and the line-width. We find that the simulated galaxies reproduce the slope and scatter of the TF relation: the slope is originated in the difference of total galactic masses, and the scatter is produced by the difference of initial spin parameters. As well as the TF relation, observed features of spiral galaxies, such as the exponential light-profile and the flat rotation curve, are reproduced in our simulations, which were assumed {\it a priori} in past semi-analytical approaches.

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arXiv1999

Central Rotation Curves of Spiral Galaxies

We present high-resolution central-to-outer rotation curves for Sb, SBb, Sc, and SBc galaxies. We discuss their general characteristics, particularly their central behavior, as well as dependencies on morphological types, activity, and peculiarity. The rotation curves generally show a steep nuclear rise, and high-velocity central rotation, followed by a broad maximum in the disk and then by a flat rotation due to the massive halo. Since the central high velocity and steep rise are common to all massive galaxies, it cannot be due to non-circular motions. Disk rotation curves of barred galaxies show larger dispersion than those of normal galaxies which is probably due to non-circular motions. Interacting galaxies show often perturbed outer rotation curves, while their central rotation shows no particular peculiarity. Also, central activities, such as starbursts and AGN, appear to show no particular correlation with the property of rotation curves. This would suggest that the central activities are triggered by a more local effect than the global dynamical property.

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