Gas Shepherding by an Infalling Satellite

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, couples 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 due to the action of dynamical friction on the satellite. 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 a few pc before completely fragmenting into stars. Finally, I briefly discuss various candidates for shepherds and how gas shepherding may help fuel active galaxies and central starbursts. Subject headings: galaxies: nuclei – galaxies: starburst – galaxies: star clusters – accretion, accretion disks