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 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^-1 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 ~2×10⁹ M_solar it shows a steep velocity gradient and a velocity range (~750 km s^-1) 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^-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 x₁ orbit at the bar's leading edge; it is falling into the nucleus with a large noncircular velocity and will eventually contribute ~2×10⁸ M_solar 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 ~50 M_solar yr^-1. We associate the disk with 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 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.