NGC 6946: Molecular Spiral Arms Masquerading as a Bar?

Details NGC 6946: Molecular Spiral Arms Masquerading as a Bar? NGC 6946: Molecular Spiral Arms Masquerading as a Bar?

Oct 1995

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995apj...452l..21r&link_type=abstract

Astrophysical Journal Letters v.452, p.L21

Astronomy and Astrophysics

Astronomy

39

Galaxies: Individual Ngc Number: Ngc 6946, Galaxies: Ism, Galaxies: Spiral

Scientific paper

NGC 6946 is an example of a galaxy with a barlike molecular gas distribution from which the presence of a stellar bar has been inferred. We present high dynamic range Berkeley-Illinois-Maryland Array (BIMA) observations of CO 1--0 emission at a resolution of 4" and a deep IR mosaic of K'-band emission. The CO distribution shows two strong offset ridges of emission curving out from a central CO peak; however, the CO distribution is complex, with evidence for an m = 4 structure. A central oval distortion in the stellar distribution has radius of 3."5, too small to have caused the barlike molecular distribution. There is evidence in the K'-band image for a weak bar with a diameter of 120", although the barlike structure can also be attributed to blending of the spiral arms. The main CO ridges lie on the trailing side of the IR ridges 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 compressed gas in a bar would be located on the leading side of the bar. On the other hand, if the IR ridge enhancements are due entirely to K supergiants (which trace recent star formation), the CO must trace the dust 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 is weakly barred from the CO and IR morphology.

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