CO Observations of M51

Abstract

We present observations, which are part of an ongoing investigation, of the CO (J=1-0) emission in the spiral galaxy M51. The spectra were obtained in a beamswitched on-on mode with the Onsala 20 m antenna (beam size ∿33″), equipped with a cooled mixer and a 512·1 MHz multichannel receiver, and are shown in Figure 1. The inset diagram shows the observed positions superposed on the optical outline of the galaxy. With the present signal-to-noise ratio there is no evidence for an arm-interarm intensity contrast. This is even more apparent in integrated intensity. This result agrees with the lower resolution findings of Rickard et al. (1981). We have observed ¹³CO in one position (22″ south of the center). The ¹³CO to ¹²CO ratio, ∿0.1, agrees with Bell results from observations with a 1!7 beam (Encrenaz et al. {dy1979}).