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Table 2.

Comparison of rotation in regions of M 51, showing the results for both samples.

Property S/N = 7 S/N = 10 Comments
Whole Galaxy

Prograde 435 257
Retrograde 247 119

Regions of Fig. 6

0.0084 0.0080 Ave cloud grad km s−1 pc−1
−0.0035 −0.0052 Ave cloud grad km s−1 pc−1
−0.0096 −0.0137 Ave cloud grad km s−1 pc−1
−0.0217 −0.0252 Ave cloud grad km s−1 pc−1

Effect of spiral arms

Arm 143 66 Retrograde
Arm 201 116 Prograde
Not arm 104 53 Retrograde
Not arm 234 141 Prograde

Effect of location within spiral arms

Leading 98 42 Retrograde
Leading 69 37 Prograde
Inner 45 24 Retrograde
Inner 132 79 Prograde
Not arm 104 53 Retrograde
Not arm 234 141 Prograde

Notes. Top part shows that rotation is generally prograde. The second part shows that cloud rotation follows galactic shear. The third part shows that the fraction of retrograde rotators is higher in the spiral arms. The last part shows that the retrograde rotators are mostly found in and actually dominate the leading edge of the arms. The fraction of pro- and retrograde rotators does not vary between the red and blue shifted parts of the “not arm” regions.

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