Smoothing Rotation Curves and Mass Profiles
  We show that spiral activity can erase pronounced features in disk galaxy
rotation curves. We present simulations of growing disks, in which the added
material has a physically motivated distribution, as well as other examples of
physically less realistic accretion. In all cases, attempts to create
unrealistic rotation curves were unsuccessful because spiral activity rapidly
smoothed away features in the disk mass profile. The added material was
redistributed radially by the spiral activity, which was itself provoked by the
density feature. In the case of a ridge-like feature in the surface density
profile, we show that two unstable spiral modes develop, and the associated
angular momentum changes in horseshoe orbits remove particles from the ridge
and spread them both inwards and outwards. This process rapidly erases the
density feature from the disk. We also find that the lack of a feature when
transitioning from disk to halo dominance in the rotation curves of disk
galaxies, the so called "disk-halo conspiracy", could also be accounted for by
this mechanism. We do not create perfectly exponential mass profiles in the
disk, but suggest that this mechanism contributes to their creation.
