Nobeyama millimeter array CO (J = 1-0) observations of the Hα/radio lobe galaxy NGC 3079: Gas dynamics in a weak bar potential and central massive core

We present ¹²CO (1-0) observations in the central 4.5 kpc (1′) of the Hα/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 along the galaxy major axis. We detected its central 2 kpc radius, while its full extent is beyond our spatial coverage. Molecular gas is smoothly distributed in the main disk, having a gas mass of 5 × 10⁹ M⊙ within the central ∼2 kpc radius. The spiral arms are superimposed on the main disk. Abrupt velocity changes of up to ∼200 km s^-1 are observed along the spiral arms in S-shaped twists of isovelocity contours and double velocity-peaked features on the spectra. The nuclear disk with ∼600 pc radius appears in position-velocity (PV) diagrams, having an intense concentration of molecular gas. Its appearance on PV diagrams is indicative of oval motions of the gas, rather than circular. The nuclear disk and spiral arms form the so-called figure-of-eight pattern on a PV diagram. The nuclear core is more compact than our current resolution (2″ = 150 pc) and has a gas mass of 3 × 10⁸ M⊙ within the central 150 pc. Although it is unresolved, the nuclear core shows a very high velocity ∼200 km s ^-1 even at the radius of ∼100 pc on the PV diagram. We propose a model that NGC 3079 contains a weak bar. The weak bar model explains the observed features of the main disk, spiral arms, and nuclear disk. The main disk and spiral arms result from gaseous x₁-orbits and associated crowding, respectively. The nuclear disk arises from gaseous x ₂-orbits. The gas concentration in the nuclear disk could be explained by the expected gas-fueling mechanism: the gas on x ₁-orbits flows along spiral arms (or offset shocks), colliding with the gas on x₂-orbits, and accumulating onto the nuclear disk. Assuming that the gas moves nearly along the spiral arms that run perpendicular to the line of sight, the pattern speed of the bar is estimated to be 55 ± 10 km s^-1 kpc^-1. 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 10⁹ M⊙ within the central 100 pc. This mass is 3 orders of magnitude more massive than that of a central black hole in this galaxy.