Gas Shepherding by an Infalling Satellite

Abstract

I calculate the action of a satellite, infalling through dynamical friction, on a coplanar gaseous disk of finite radial extent. The disk tides, raised by the infalling satellite, couple the satellite and disk. Dynamical friction acting on the satellite then shrinks the radius of the coupled satellite-disk system. Thus, the gas is shepherded to smaller radii. In addition, gas shepherding produces a large surface density enhancement at the disk edge. If the disk edge then becomes gravitationally unstable and fragments, it may give rise to enhanced star formation. On the other hand, if the satellite is sufficiently massive and dense, the gas may be transported from ~100 pc to inside of 10 to tens of parsecs before completely fragmenting into stars. I argue that gas shepherding may drive the fueling of active galaxies and central starbursts, and I compare this scenario to competing scenarios. I argue that sufficiently large and dense super star clusters (acting as the shepherding satellites) can shepherd a gas disk down to 10 to tens of parsecs. Inside of 10 to tens of parsecs, another mechanism may operate, i.e., cloud-cloud collisions or a marginally (gravitationally) stable disk, that drives the gas 1 pc, where it can be viscously accreted, feeding a central engine.