Spiral Galaxies and Tracers of Mass Accretion

Can the present dynamics of spiral galaxies tell us something about the merging history, the formation and evolution of disks? Galaxy interactions thicken or destroy disks; the simultaneous presence of thick and thin disks is a tracer of past accretion, and also of recent disk re-formation. Observation of a large number of counter-rotating disks is also evidence of past mergers, as well as the frequency of polar-ring galaxies. Finally, the ubiquitous presence of warps in the outer parts of HI disks might also provide a clue of how frequently disks accrete mass with different angular momentum.