ABSTRACT

Radial gas flows can be induced in a galactic disk by a bar potential, and its implied gravitational torques. All gas inside corotation is driven towards the center, and forms a nuclear ring at the inner Lindblad resonance (ILR). When the mass concentration is high enough, there exists two ILRs, and the existence of periodic orbits perpendicular to the bar makes the gas response to shift in phase with respect to the stellar main bar. This produces a strong torque on the gas, and drives a rapid nuclear gas flow inside ILR. With a more viscous gas, however, a second bar of stars and gas can decouple from the primary one, with a higher pattern speed. In this “bar within bar” configuration, the gas is in phase with the stellar component, and the gravity torques are minimised. The gas inside the second corotation flows slowly inwards. The nuclear bar is relatively long- lived, which explains its frequent occurence in observed barred spirals.

NUCLEAR BARS AND THEIR POSSIBLE INTERPRETATIONS

Bars are the way to redistribute angular momentum in a galaxy, and to reshape the mass distribution. The induced gas flow towards the center is the cause of star-bursts, hot spots in nuclear rings, and may be of nuclear activity. A clue to the detailed mechanisms of the central gas flow is the observation of nuclear bars and central isophote twists. This phenomenon in barred spiral galaxies has been observed for a long time. Already de Vaucouleurs (1974) had noticed bars within bars, and Sandage and Brucato (1979) high surface brightness nuclear bars, as independent entities.