The motions in the central region of NGC 4736: evidence for an expanding ring

Extensive spectroscopic observations of the central part of NCC 4736 are presented. The analysis shows that there are noncircular motions and that a Lindblad resonance dispersion ring as the interpretation must be rejected on geometrical grounds. The most likely model is one in which the bright central ring of H ii regions is expanding with a velocity of about 30km 1, but zmotions cannot be ruled out. Discrepant velocities are established in the direction of the triple radio source. It is proposed that the central part of the disk is disturbed by nuclear activity and the accompanying expulsion of gas clouds. Although no detailed model could be constructed, it is argued that further observational data on NGC 4736 and comparison with a few other galaxies do not contradict such a model. Subject headings: galactic nuclei - galaxies, individual - galaxies, motions in - nebulae