COUNTERROTATING STARS IN THE DISK OF THE SAB GALAXY NGC-7217

We have analyzed high signal-to-noise spectra of the disk galaxy NGC 7 217 in order to extract the full line-of-sight velocity distribution a long both its major and minor axes. The data reveal that 20%-30% of th e stars in this galaxy are in a distinct component on retrograde orbit s. This counterrotating population cannot be explained away as a syste matic error, and it does not seem to be caused by the bulge's contribu tion to the velocity distribution. We have developed a new technique f or fitting dynamical disk models directly to the galaxy spectra, and a pplication of this method confirms the presence of the distinct counte rrotating disk population. NGC 7217 is only the second disk galaxy kno wn to contain counterrotating stars, but we argue that similar compone nts in other regular disk systems would not have been detected by trad itional techniques, and so there could exist many such systems. The ex istence of disk stars on retrograde orbits provides a new clue as to t he manner in which the galaxy formed: it favors a scenario in which ma tter continues to accrete onto the galaxy over a long period of time, with rapid, substantial changes occurring in the angular momentum of t he infalling material. The observable consequences of this evolutionar y history include a large bulge-to-disk ratio and the absence of stron g spiral structure, and so the presence or absence of a counterrotatin g component may go some way toward explaining the Hubble sequence for disk galaxies.