| Abstract | HI observations of the face-on ScI galaxy NGC 628 with the VLA reveal new details in the kinematics for an isolated, extended HI disk. On a global scale, the analysis of the velocity field shows: (1) clear regression of the line-of-the-nodes in the well-known warp, (2) two symmetrically placed high velocity complexes (HVCs) that do not fit into a warped disk model of inclined, circular rings, and (3) an extended arm or tail of HI that arcs over one third of the circumference of NGC 628 and extends outwards to include the most distant HI associated with the galaxy. The extended arc does not have a symmetrically located counter- part, and it is speculated that either the arc is from primordial origin or it represents a tidally dispersed remnant of an HI-rich object (M_HI_ ~ 9 10^8^ M_sun_) that is merging into the outer disk. The presence of the added mass in the outer disk distorts NGC 628 itself, creating the HVCs as tidal by-products. Superimposed on the large scale symmetries and systematic behavior the galaxy as a whole shows a trend of increasingly chaotic motion toward larger radii. We are probably witnessing a stage in the evolution of a large isolated system as it settles toward a flattened disk after enduring numerous accretion events. The evidence for recent accretion helps to solve the problem of warp longevity since the warp is excited by the acquired mass. |
