The Distribution of the Molecular Gas in NGC 6946

Extract

We present CO(2-1) and (1-0) maps of the inner disk of the nearby Scd spiral NGC 6946 (D= 5 Mpc). These maps have been obtained using the IRAM 30m telescope (beamsizes = 22″ and 14″, map sampling 10″= 250 pc). The CO(2-1) map contains about 1500 points and covers approximately the inner disk out to R = 3.5′. The overlay of CO(2-1) line area and Hα emission (figure 1) shows that spiral arms are seen in CO and is a very good correspondence between the optical and molecular arms. However there is CO emission in between the arms; the contrast in both lines is about 4. Note also the oval distribution of CO emission in the nucleus, revealing the presence of a bar. The CO(2-1)/CO(1-0) emissivity ratio is low (around 0.5 in the disk) suggesting subthermal excitation.