6-m telescope spectroscopic observations of the bubble complex in NGC 6946

We describe the results of a long-slit spectroscopic study of an unusual star complex in the nearby spiral galaxy NGC 6946 using the SAO 6 m telescope and the Keck 10 m telescope. The complex resembles a circular bubble 600 pc in diameter with a young super star cluster (SSC) near the center. The kinematics of ionized gas is studied through Halpha emission with several slit positions. Position-velocity diagrams show two distinct features with high speed motions. One is an irregularly shaped region to the east of the SSC, 270 pc in size, in which most of the Halpha emission is blue shifted by 120 km s(-1), and another is a 350 pc shell centered on the SSC with positive and negative velocity shifts of 60 km s(-1). Balmer and He I absorption lines in the SSC give an age of 12-13 Myr, which is consistent with the photometric age but significantly older than the kinematic ages of the high speed regions. The energetics of the SSC and its interaction with the environment are considered. The expansion energies exceed 10(52) ergs, but the power outputs from winds and supernova in the SSC are large enough to account for this. The intensities of Balmer, [N II], and [S II] emission lines within and around the complex indicate that shock excitation makes a significant contribution to the emission from the most energetic region.