18 February 2020Molecular cloud rotation in M33 and M51

Systematic rotation of molecular clouds was detected for the first time in the Local Group spiral Messier 33, where a sample of 566 molecular clouds is available. There is a clear preference for prograde rotation (i.e. same direction as the galaxy as a whole). This preference is even stronger when the high signal-to-noise sample is used, reducing the effect of noise. We have now analyzed the results for Messier 51, observed at high resolution with the Plateau de Bure interferometer as part of the PAWS project. M33 is a rather flocculent galaxy (without strong spiral arms) whereas M51 has very strong spiral arms. As for M33, the clouds rotate mostly prograde. However, the fraction of prograde rotators is lower in the arms than outside of the arms (i.e. more retrograde rotators in the arms). The PAWS observations cover the region within corotation, so the clouds “catch up” to the arms and then leave them. Dividing the arm clouds into “entering” and “leaving” clouds, we find that while a large majority of the clouds enter the arms prograde, the majority leave with retrograde rotation!! Rotation direction has thus been reversed within the arms… And much more …