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

Abstract

Abstract We report on high-resolution interferometer observations of the ${{12\atop} \mathrm{CO}}$ ($J = 1 \hbox{--} 0$) 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 $r\sim 5^{\prime\prime}$ (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 $1.3 \times 10^8 \,{{{M}_{\odot}}}$, which corresponds to $\sim 3.5\%$ of the dynamical mass, and shows a slight non-circular motion. The double peaks are separated by $\sim {4\rlap {.}{}^{\mathrm {\prime \prime }}7}$ ($370 \:\mathrm{pc}$), 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.