Gas Dynamics in the LINER Galaxy NGC5005: Episodic Fueling of a Nuclear Disk

We report high-resolution CO(1–0) observations in the central 6 kpc (1) of the LINER galaxy NGC5005 with the Owens Valley Radio Observatory millimeter array. Molecular gas is distributed in three components — a 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 ∼ 50 km s on the minor axis. The central condensation is a disk of ∼ 1 kpc radius with a central depression of ∼ 50 pc radius. This disk has a molecular gas mass of ∼ 2× 10 M⊙; it shows a steep velocity gradient and a velocity range (∼ 750 km s ) 30% larger than the velocity 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 ∼ 1 kpc but has a velocity offset by up to ∼ 150 km s from galactic rotation. We suggest that an optically inconspicuous stellar bar lying within the 3 kpc ring can explain the observed gas dynamics. 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 which match the characteristic motions of gas in a barred potential. Our model indicates that gas in the northwest stream is on an x1 orbit at the bar’s leading edge; it is falling into the nucleus with a large noncircular velocity, and will eventually contribute ∼ 2 × 10 M⊙ to the nuclear disk. If most of this material merges with the disk on its first passage of pericenter, the gas accretion rate during the collision will be ∼ 50M⊙ yr . We associate the disk with an inner 2:1 Lindblad resonance and attribute its large linewidth 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 ring — or a pair of tightly wound spiral arms — arising at the bar ends. Both scenarios can explain the apparent noncircular motions in the ring. The high rate of bar-driven inflow and the irregular appearance of the northwest stream suggest that a major fueling event is in progress in NGC 5005. Such episodic (rather than continuous) gas supply can regulate the triggering of starburst and accretion activity in galactic nuclei. Subject headings: galaxies: dynamics and kinematics — galaxies: ISM — galaxies: spiral — galaxies: active — galaxies: starburst To appear in ApJ, 530, #1 (Feb. 10, 2000). Owens Valley Radio Observatory, Caltech 105-24, Pasadena, CA 91125 Nobeyama Radio Observatory, Minamisaku, Nagano, 384-1305, Japan; E-mail(KS): [email protected] Guest investigator of the RGO Astronomy Data Center