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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., 14 pages, 11 figures

We found that loosely wound spiral shocks in an isothermal gas disk caused by a non-axisymmetric potential are hydrodynamically unstable, if the shocks are strong enough. High resolution, global hydrodynamical simulations using three different numerical schemes, i.e. AUSM, CIP, and SPH, show similarly that trailing spiral shocks with the pitch angle of larger than ~10 deg wiggle, and clumps are developed in the shock-compressed layer. The numerical simulations also show clear wave crests that are associated with ripples of the spiral shocks. The spiral shocks tend to be more unstable in a rigidly rotating disk than in a flat rotation. This instability could be an origin of the secondary structures of spiral arms, i.e. the spurs/fins, observed in spiral galaxies. In spite of this local instability, the global spiral morphology of the gas is maintained over many rotational periods. The Kelvin-Helmholtz (K-H) instability in a shear layer behind the shock is a possible mechanism for the wiggle instability. The Richardson criterion for the K-H stability is expressed as a function of the Mach number, the pitch angle, and strength of the background spiral potential. The criterion suggests that spiral shocks with smaller pitch angles and smaller Mach numbers would be more stable, and this is consistent with the numerical results., Comment: 11 pages, 14 figures, to be published in MNRAS, high quality figures can be downloaded from http://th.nao.ac.jp/~wada/paperlist.html

4192 12 13 48.29 +14 54 01.9 1 4212 12 15 39.40 +13 54 04.6 2 4254 12 18 49.61 +14 24 59.6 1 4303 12 21 54.94 +04 28 25.6 1 4402 12 26 07.45 +13 06 44.7 1 4419 12 26 56.40 +15 02 50.2 1 4501 12 31 59.12 +14 25 13.3 1 4535 12 34 20.35 +08 11 52.2 1 4536 12 34 27.08 +02 11 17.1 1 4548 12 35 26.44 +14 29 47.4 1 4569 12 36 49.82 +13 09 45.8 3 4579 12 37 43.53 +11 49 05.5 4 4654 12 43 56.67 +13 07 36.1 1 4689 12 47 45.53 +13 45 46.5 1