NGC-6946 - MOLECULAR SPIRAL ARMS MASQUERADING AS A BAR

NGC 6946 is an example of a galaxy with a barlike molecular gas distri bution from which the presence of a stellar bar has been inferred. We present high dynamic range Berkeley-Illinois-Maryland Array (BIMA) obs ervations of CO 1-0 emission at a resolution of 4'' and a deep IR mosa ic of K'-band emission. The CO distribution shows two strong offset ri dges of emission curving out from a central CO peak; however, the CO d istribution is complex, with evidence for an m = 4 structure. A centra l oval distortion in the stellar distribution has radius of 31''.5, to o small to have caused the barlike molecular distribution. There is ev idence in the K'-band image for a weak bar with a diameter of 120'', a lthough the barlike structure can also be attributed to blending of th e spiral arms. The main CO ridges lie on the trailing side of the IR r idges both north and south of the nucleus. The IR ridges appear to be an enhancement in the old stellar population. If this is the case, the offset implies that the CO and IR trace spiral arms since the compres sed gas in a bar would be located on the leading side of the bar. On t he other hand, if the IR ridge enhancements are due entirely to K supe rgiants (which trace recent star formation), the CO must trace the dus t lanes of a bar. If NGC 6946 has a bar, it is larger but weaker than previously inferred, and it has characteristics atypical of known bars . We conclude that it can be difficult to determine whether a galaxy i s weakly barred from the CO and IR morphology.