Gasdynamics in the LINER Galaxy NGC 5005: Episodic Fueling of a Nuclear Disk

Abstract

We report high-resolution CO(1¨0) observations in the central 6 kpc (1@) of the LINER galaxy NGC 5005 with the Owens Valley Radio Observatory millimeter array. Molecular gas is distributed in three componentsa ring at a radius of about 3 kpc, a strong central condensation, and a stream to the northwest of the nucleus but inside the 3 kpc ring. The ring shows systematic noncircular motions, with apparent inward velocities of D50 km s~1 on the minor axis. The central condensation is a disk of D1 kpc radius with a central depression of D50 pc radius. This disk has a molecular gas mass of D2 ) 109 it shows a steep velocity gradient and a velocity range (D750 km s~1) 30% larger than the velocity M _ ; width of the rest of the galaxy. The stream between the 3 kpc ring and the nuclear disk lies on a straight dust lane seen in the optical. If this material moves in the plane of the galaxy, it lies at a radius of D1 kpc but has a velocity oUset by up to D150 km s~1 from galactic rotation. We suggest that an optically inconspicuous stellar bar lying within the 3 kpc ring can explain the observed gasdynamics. This bar is expected to connect the nuclear disk and the ring along the position angle of the northwest stream. A position-velocity cut in this direction reveals features that match the characteristic motions of gas in a barred potential. Our model indicates that gas in the northwest stream is on an orbit at the bars x 1 leading edge; it is falling into the nucleus with a large noncircular velocity and will eventually contribute D2 ) 108 to the nuclear disk. If most of this material merges with the disk on its —rst passage of M _ pericenter, the gas accretion rate during the collision will be D50 yr~1. We associate the disk with M _ an inner 2:1 Lindblad resonance and attribute its large line width to favorably oriented elliptical orbits rather than (necessarily) to a large central mass. The 3 kpc ring is likely an inner 4:1 Lindblad resonance ringor a pair of tightly wound spiral armsarising at the bar ends. Both scenarios can explain the apparent noncircular motions in the ring. The high rate of bar-driven in—ow and the irregular appear- ance of the northwest stream suggest that a major fueling event is in progress in NGC 5005. Such epi- sodic (rather than continuous) gas supply can regulate the triggering of starburst and accretion activity in galactic nuclei. Subject headings: galaxies: activegalaxies: ISMgalaxies: kinematics and dynamics ¨ galaxies: spiralgalaxies: starburst