COUNTER-ROTATING POPULATIONS IN A DISK GALAXY

Galactic absorption-line spectra can now yield valuable information ab out the internal kinematics beyond the classical measures of mean velo city and (Gaussian) velocity dispersion. An application of a new algor ithm to the photometrically normal disk galaxy NGC 7217 has revealed t hat around 30% of the stars circulate in the opposite sense to the maj ority. The subtle, serendipitous nature of the discovery raises the po ssibility that this situation may not be uncommon. NGC 7217 and the pr eviously discovered case of NGC 4550 (in which both directions of circ ulation are equally populated) point towards substantial secondary inf all of material with variable angular momentum, and suggest a possible explanation of the decrease of bulge-to-disk ratio with strengthening spiral structure along the Hubble sequence.