Kinematic Evidence of Minor Mergers in Normal Sa Galaxies
  A detailed study of nearby, morphologically normal Sa galaxies reveals that about half show some spectrocopic evidence for kinematically distinct components despite their undisturbed optical appearance. In eight of nine objects mapped in the 21 cm HI line, the HI distribution extends far outside the optical disk and warps of the HI disk are complex. The multiwavelength evidence can be interpreted in terms of the kinematic "memory" of past minor mergers in objects that otherwise exhibit no morphological signs of interaction. 
Martha P. Haynes