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Date and Time of the Query: 2019-06-18 T20:10:07 PDT
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For refcode 1990A&A...233..357C:
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Copyright by European Southern Observatory (ESO). Reproduced by permission
1990A&A...233..357C ^12^CO (1-0) and ^12^CO (2-1) mapping of gas complexes in the spiral galaxy NGC 6946: comparison with H I and H II regions F. Casoli, E. Clausset, E. Viallefond, F. Combes, and F. Boulanger Radioastronomie Millimetrique, ENS, 24 Rue Lhomond, F-75231 Paris Cedex 05, France DEMIRM, Observatoire de Paris, Section de Meudon, F-92195 Meudon Principal Cedex, France IPAC, California Institute of Technology, Mail Stop 100-22, Pasadena, CA 91109, USA Received June 13, accepted November 27, 1989 Abstract. We present ^12^CO (1-0) and ^12^CO (2-1) observations in the disk of the Scd galaxy NGC 6946, made with the IRAM 30 m telescope with respective resolutions of 23" and 14". We have observed two circular fields 110" in diameter with 10" sampling. In the eastern field, we have resolved the spiral structure and found CO concentrations which coincide in space and velocity with one of the optical spiral arms as defined by the H II regions and the maxima of H I and radio-continuum emission. The arm is just resolved at the resolution of the ^12^CO (1-0) observations with a width at half-maximum around 500 pc. The CO arm-intarm contrast averaged over a region more than 2 kpc in size, is around four in the two lines. This is larger than the contrast observed in the old stellar disk and in H I. In those regions of the galaxy that we have observed, the gas is mostly molecular. In the eastern field, the portion of the arm that we have mapped can be decomposed in two gaseous complexes. The first one is associated with a giant H I, region whose ionization requires about 100 O5 stars and is mostly molecular (M(H_2_) = 5 l0^7^ M_sun_ M (H I) = 2 10^7^ M_sun_, M (H II) ~ 10^7^ ). The second one shows less star-formation activity and has M(H_2_) = M(H I) 7 10^6^ M_sun_. The maps are quite similar in the two CO lines but there are small- scale differences suggesting some variations of the excitation state, possibly due to the presence of H II regions. The ratio of CO (2-1) to CO (1 -0) emissivities, around 0.4, is typical of a low excitation state. We suggest that the CO lines observed at this scale (500 pc) are mainly emitted in the low-density, optically thick envelopes of the molecular clouds. Key words: galaxies: individual: NGC 6946 - galaxies: kinematics and dynamics of - galaxies: spiral - interstellar medium: clouds
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